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Sun noise
Quiet sun radio noise is often used for measurements of radio receiving systems. The radio flux from the quiet sun is a known signal in that it is measured and reported at observatories around the earth, and therefore provides a basis of assessment of radio receiving systems. I have been playing around with a little online calculator to calculate interpolated recent solar flux data for amateur radio UHF and microwave bands from published observations. It is at http://www.vk1od.net/qsrf for those who may find it useful. Owen |
Sun noise
Owen Duffy wrote in
: Quiet sun radio noise is often used for measurements of radio receiving systems. The radio flux from the quiet sun is a known signal in that it is measured and reported at observatories around the earth, and therefore provides a basis of assessment of radio receiving systems. I have been playing around with a little online calculator to calculate interpolated recent solar flux data for amateur radio UHF and microwave bands from published observations. It is at http://www.vk1od.net/qsrf for those who may find it useful. There was a time once.... I used to have an FT-221 tricked out with a hot front end. Solar noise would run the S meter up to well over the S9 mark and you could even see the galactic plane passing through the antenna pattern. Needless to say, it heard well on terrestrial 2m SSB. -- Dave Oldridge+ ICQ 1800667 |
Sun noise
Dave Oldridge wrote in
: .... I used to have an FT-221 tricked out with a hot front end. Solar noise would run the S meter up to well over the S9 mark and you could even see the galactic plane passing through the antenna pattern. Needless to say, it heard well on terrestrial 2m SSB. That is no mean feat! I think ambient noise temperature at 144MHz for an antenna pointed at cold sky is somewhere around 200K to 250K, when you add a pretty good receiver at say 30K, you are talking 230K to 280K total system noise, and the sun is probably around 800K with a low end 4 bay EME antenna setup (Gain~22dBi), for a noise rise of 10*log((800+255)/255) or 16dB. A single yagi of gain around 15dBi is much poorer, not only is the sun noise reduced proportionately to the gain reduction, but the ambient noise increases with higher gain in the side and back area of the antenna, but it still should be possible to reliably 'see' the sun with a very good receiver. Ambient noise temperature for a beam at zero elevation here in suburbia varies from 1000K to 6000K depending on the day and time... so a very low temperature receiver is wasted for terrestrial contacts. Owen Owen |
Sun noise
I used to have an FT-221 tricked out with a hot front end. Solar noise would run the S meter up to well over the S9 mark and you could even see the galactic plane passing through the antenna pattern. Needless to say, it heard well on terrestrial 2m SSB. That is no mean feat! You better believe no mean feat! I worked off the moon on 432 MHz back in the late 70's. Sixteen 16 element Yagis (calculated gain ~26 dB) and an STA from the FCC to use 5 KW as long as the antennas were pointed above 25 degrees elevation. (Protecting Eglin radars some 200 plus miles away.) I could consistently manage some 4, 4-1/2 dB of sun noise off a quiet sun and not one smidge more even with a mast mounted GaAs Fet preamp supposedly with some 0.8 dB NF. I DID however, at around the same time, own a 2 meter Jap all-mode transceiver that I happened to measure the "S" meter accuracy with an HP signal generator. It turned out that 2 uVolts was "S"-1. THREE uVolts was "S"-9. W4ZCB |
Sun noise
I DID however, at around the same time, own a 2 meter Jap all-mode
transceiver that I happened to measure the "S" meter accuracy with an HP signal generator. It turned out that 2 uVolts was "S"-1. THREE uVolts was "S"-9. ============================== Is it correct that for frequencies up to 30 MHz a S9 signal is 50 microvolt into 50 Ohms (or -73 dBm) but that for higher frequencies a S9 signal is 5 microvolts into 50 ohms (or -93 dBm). If that is (the agreed) norm ,was it ever formally sanctioned by IARU ? I can hardly believe that any of the far eastern rice boxes have a properly calibrated S-meter. Also the top end of the S-meter scale is usually rather 'compressed', which surprises me since ICs with a log type input/output relationship must be readily available. Time to attempt calibrating the S-meter of my (almost vintage) TenTec Paragon TRX. Frank GM0CSZ / KN6WH |
Sun noise
I DID however, at around the same time, own a 2 meter Jap all-mode transceiver that I happened to measure the "S" meter accuracy with an HP signal generator. It turned out that 2 uVolts was "S"-1. THREE uVolts was "S"-9. ============================== Is it correct that for frequencies up to 30 MHz a S9 signal is 50 microvolt into 50 Ohms (or -73 dBm) but that for higher frequencies a S9 signal is 5 microvolts into 50 ohms (or -93 dBm). If that is (the agreed) norm ,was it ever formally sanctioned by IARU ? I've never heard of a convention like that. Can't imagine a reason to make it different for HF and VHF. I can hardly believe that any of the far eastern rice boxes have a properly calibrated S-meter. Also the top end of the S-meter scale is usually rather 'compressed', which surprises me since ICs with a log type input/output relationship must be readily available. They are now. (AD 600, 603 and MOT chips with RSSI etc.) VERY expensive compared to a couple of 2N2222 type IF transistors and mass production will go to the ends of the earth to save a couple pennies. Time to attempt calibrating the S-meter of my (almost vintage) TenTec Paragon TRX. Frank GM0CSZ / KN6WH If I had a Paragon, I'd try very hard to get rid of it. The worst radio Ten Rec ever made (And the only up conversion one I remember) WRT LO phase noise. Horrible on of course, both RX and TX. I had to put up with a neighbors for several years. (And he-me when he was receiving. My transceiver improved a whole lot when he bought a new radio.) W4ZCB |
Sun noise
Highland Ham wrote in
: I DID however, at around the same time, own a 2 meter Jap all-mode transceiver that I happened to measure the "S" meter accuracy with an HP signal generator. It turned out that 2 uVolts was "S"-1. THREE uVolts was "S"-9. ============================== Is it correct that for frequencies up to 30 MHz a S9 signal is 50 microvolt into 50 Ohms (or -73 dBm) but that for higher frequencies a S9 signal is 5 microvolts into 50 ohms (or -93 dBm). If that is (the agreed) norm ,was it ever formally sanctioned by IARU ? I can hardly believe that any of the far eastern rice boxes have a properly calibrated S-meter. Also the top end of the S-meter scale is usually rather 'compressed', which surprises me since ICs with a log type input/output relationship must be readily available. Time to attempt calibrating the S-meter of my (almost vintage) TenTec Paragon TRX. Frank GM0CSZ / KN6WH Frank, leaving aside your apparent prejudice about the country of origin of a radio... It seems that IARU Region 1 Technical Recommendation 1 lays down the Region 1 view of S meter equivalence in power, but that does not seem to have wider acceptance more than 25 years later. Talking about SSB telephony receivers... The next problem that occurs is that many radios are loosely calibrated with the selectable internal preamp OFF, a good idea for lower HF bands, but questionable on VHF and above. The user manuals don't often state the correct configuration for calibrated S meter response. Apparently, the technology hasn't advanced enough to have the S meter calibrated whether or not the internal preamp on in use. (It is quite possible that the example that Harold gave is one of those radios that is calibrated with internal preamp OFF and he measured it with preamp ON.) The radio's S meter may often be fairly roughly calibrated between about S6 and S9+20 (with the preamp OFF if that is the case for the particular radio), but is unlikely to be very accurate below and above that range. Considering the design of a transceiver, if the S meter is calibrated with preamp OFF, the receiver could have a noise figure around 15dB, which means its noise floor is around -126dBm. Typically, the AGC is deferred until the signal reaches about 20dB above the noise floor, so that would be -106dBm+ which is about S3 and a half+. So the AGC will not operate, and the S meter not deflect until the signal is above S3+, yet the meters will typically be scaled from S0 or S1. Next, these radios may be used with an external preamp, or a transverter, both with substantial gain. So, a transceiver that is roughly calibrated with the preamp OFF, is used with preamp ON for another 20dB or so of gain, then an external 25dB preamp offset by a few dB of line loss, so the S meter is now reading some 35dB high. The increased gain is usually much higher for a LAN-transverter-transceiver cascade. In my experience, quantitative signal reports handed out on VHF and above are commonly nonsense, which is interesting given the greater focus on weak signal working and understanding path characteristics. The other thing that contributes to this nonsense is that people cheating on power often give the other station the same report that they received to disguise the lack of symmetry due to their own excessive tx power. FM receiver S meter calibration is a whole 'nother thing. Lacking an AGC system, it is typically limiter current that is used to drive the S meter and it usually takes less than 10uV to drive them to full scale, S9 might be indicated by just a few uV. Mind you, a 5uV FM signal has very good S/N, but that doesn't make it a strong signal. Owen |
Sun noise
Talking about SSB telephony receivers...
The next problem that occurs is that many radios are loosely calibrated with the selectable internal preamp OFF, a good idea for lower HF bands, but questionable on VHF and above. The user manuals don't often state the correct configuration for calibrated S meter response. Apparently, the technology hasn't advanced enough to have the S meter calibrated whether or not the internal preamp on in use. (It is quite possible that the example that Harold gave is one of those radios that is calibrated with internal preamp OFF and he measured it with preamp ON.) Owen GM Owen The transceiver I had back then didn't have a switcheable preamp. It was just always on. Don't even remember the mfg, but it was a little flat radio. 3 inches high (if that) and spread all over the desktop. The "S" meter was indeed driven from the limiters Anyway, "S" meters that are inaccurate or change the value of the received signal when you turn a preamp on or off (Or an attenuator on or off) are an abomination. In this day and age of microprocessors, it's childs play to adjust the "S" meter to accomodate those changes. (Take a look at the "PicaStar" which does exactly that. It also yields 6 dB per "S" unit and 10 dB for every 10 dB over "S" 9 as well. Makes the radio into virtually a piece of lab test equipment. Short of that use, the "S" meter report IS rather rediculous. (You're 5-9, what was the call and name again?) W4ZCB |
Sun noise
Highland Ham wrote:
I DID however, at around the same time, own a 2 meter Jap all-mode transceiver that I happened to measure the "S" meter accuracy with an HP signal generator. It turned out that 2 uVolts was "S"-1. THREE uVolts was "S"-9. ============================== Is it correct that for frequencies up to 30 MHz a S9 signal is 50 microvolt into 50 Ohms (or -73 dBm) but that for higher frequencies a S9 signal is 5 microvolts into 50 ohms (or -93 dBm). If that is (the agreed) norm ,was it ever formally sanctioned by IARU ? Wrong way around: those standards (along with 6dB per S-point) were formally sanctioned by IARU, but almost no amateur receiver has ever met those standards. I can hardly believe that any of the far eastern rice boxes have a properly calibrated S-meter. Also the top end of the S-meter scale is usually rather 'compressed', which surprises me since ICs with a log type input/output relationship must be readily available. Conventional S-meters don't actually measure signal strength - they measure the AGC voltage. The S-meter could only be accurate if the voltage/gain characteristic of the AGC-controlled stages happened to be accurately logarithmic across the entire dynamic range of the receiver; which is almost never true. An accurate S-meter will also need some compensation for variations in gain across the HF bands. Above all, a true reading of 'signal strength' should NOT change when you switch in a preamp or an attenuator, or vary the RF gain. For the long story, see 'S Meter Blues' by W8WWV: http://tinyurl.com/8nme6 -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Sun noise
"Owen Duffy" wrote in message ... Dave Oldridge wrote in : ... I used to have an FT-221 tricked out with a hot front end. Solar noise would run the S meter up to well over the S9 mark and you could even see the galactic plane passing through the antenna pattern. Needless to say, it heard well on terrestrial 2m SSB. That is no mean feat! I think ambient noise temperature at 144MHz for an antenna pointed at cold sky is somewhere around 200K to 250K, when you add a pretty good receiver at say 30K, you are talking 230K to 280K total system noise, and the sun is probably around 800K with a low end 4 bay EME antenna setup (Gain~22dBi), for a noise rise of 10*log((800+255)/255) or 16dB. A single yagi of gain around 15dBi is much poorer, not only is the sun noise reduced proportionately to the gain reduction, but the ambient noise increases with higher gain in the side and back area of the antenna, but it still should be possible to reliably 'see' the sun with a very good receiver. Ambient noise temperature for a beam at zero elevation here in suburbia varies from 1000K to 6000K depending on the day and time... so a very low temperature receiver is wasted for terrestrial contacts. Owen Owen I would like to see what mods are made to the 221 to do that and also what kind of antenna system. I have a 221 I am using with a gasfet preamp in the shack that should be less than 1 db of noise fugure and about 20 db of gain. The antenna is a klm 22c and 75 feet of 9913 type of coax. I can just see some sun noise with the antenna aimed at the sun. It sure does not deflect the smeter several sunits. The antenna is on an azel mount. I am sure the system is working as I compaired it to an Icom 706 and another antenna that is mounted on a tower and I am getting about the differance in signal levels I would expect at the horizon. |
Sun noise
I would like to see what mods are made to the 221 to do that and also what
kind of antenna system. I have a 221 I am using with a gasfet preamp in the shack that should be less than 1 db of noise fugure and about 20 db of gain. The antenna is a klm 22c and 75 feet of 9913 type of coax. I can just see some sun noise with the antenna aimed at the sun. It sure does not deflect the smeter several sunits. The antenna is on an azel mount. I am sure the system is working as I compaired it to an Icom 706 and another antenna that is mounted on a tower and I am getting about the differance in signal levels I would expect at the horizon. The 75 feet of coax is your problem!!! What is the point of a 1dB NF preamp with all that loss ahead of it? To see the benefit of the low noise figure the amp must be before all that cable loss!! 73 Jeff G8HUL |
Sun noise
"Jeff" wrote in message . com... I would like to see what mods are made to the 221 to do that and also what kind of antenna system. I have a 221 I am using with a gasfet preamp in the shack that should be less than 1 db of noise fugure and about 20 db of gain. The antenna is a klm 22c and 75 feet of 9913 type of coax. I can just see some sun noise with the antenna aimed at the sun. It sure does not deflect the smeter several sunits. The antenna is on an azel mount. I am sure the system is working as I compaired it to an Icom 706 and another antenna that is mounted on a tower and I am getting about the differance in signal levels I would expect at the horizon. The 75 feet of coax is your problem!!! What is the point of a 1dB NF preamp with all that loss ahead of it? To see the benefit of the low noise figure the amp must be before all that cable loss!! 73 Jeff G8HUL The coax only has a loss of 1 db for the length I am running it. I doubt that at 2 meters I would see any benift of putting the preamp at the antenna. It is as easy to get a 1 db or less noise figure at 2 meters as it is to make any preamp and the 221 does need some help on the receiving side. From the articals I have read the beam width of most any single antennas at 2 meters is wide enough the ground noise will override that ammount of loss. |
Sun noise
The coax only has a loss of 1 db for the length I am running it. I doubt
that at 2 meters I would see any benift of putting the preamp at the antenna. It is as easy to get a 1 db or less noise figure at 2 meters as it is to make any preamp and the 221 does need some help on the receiving side. From the articals I have read the beam width of most any single antennas at 2 meters is wide enough the ground noise will override that ammount of loss. Agreed. Your 1 dB preamp just became a 2 dB preamp and the visible ground temperature likely overrode either figure and by a good margin. I suspect the original poster with his S9 sun noise and "seeing" the Galactic plane was being a bit optimistic. W4ZCB |
Sun noise
" The coax only has a loss of 1 db for the length I am running it. I doubt that at 2 meters I would see any benift of putting the preamp at the antenna. It is as easy to get a 1 db or less noise figure at 2 meters as it is to make any preamp and the 221 does need some help on the receiving side. From the articals I have read the beam width of most any single antennas at 2 meters is wide enough the ground noise will override that ammount of loss. I think 1dB is a little optimistic for 75' of 9913 coax, it is in reality, I suspect, nearer 1.5dB. Any loss ahead of your preamp adds directly to the noise figure of the system so the best NF that you could ever have is 2.5dB plus a little form the 221. Also 20dB of gain in the preamp sounds like a sure way to produce intermods in your radio. Regards Jeff |
Sun noise
I think 1dB is a little optimistic for 75' of 9913 coax, it is in reality, I suspect, nearer 1.5dB. Any loss ahead of your preamp adds directly to the noise figure of the system so the best NF that you could ever have is 2.5dB plus a little form the 221. Also 20dB of gain in the preamp sounds like a sure way to produce intermods in your radio. Regards Jeff Well of course it is Jeff, but in moonbounce, noise figure is the holy grail. There weren't as many folks on 432 and moonbounce in particular back in 1979, but I NEVER was plagued with intermod off the moon, and the directivity of the array kept me from having any from anywhere else. W4ZCB |
Sun noise
"Ralph Mowery" wrote in
: "Owen Duffy" wrote in message ... Dave Oldridge wrote in : ... I used to have an FT-221 tricked out with a hot front end. Solar noise would run the S meter up to well over the S9 mark and you could even see the galactic plane passing through the antenna pattern. Needless to say, it heard well on terrestrial 2m SSB. Firstly, this was actually Howard's statement. That is no mean feat! And my comment. I think ambient noise temperature at 144MHz for an antenna pointed at cold sky is somewhere around 200K to 250K, when you add a pretty good receiver at say 30K, you are talking 230K to 280K total system noise, and the sun is probably around 800K with a low end 4 bay EME antenna setup (Gain~22dBi), for a noise rise of 10*log((800+255)/255) or 16dB. A single yagi of gain around 15dBi is much poorer, not only is the sun noise reduced proportionately to the gain reduction, but the ambient noise increases with higher gain in the side and back area of the antenna, but it still should be possible to reliably 'see' the sun with a very good receiver. Ambient noise temperature for a beam at zero elevation here in suburbia varies from 1000K to 6000K depending on the day and time... so a very low temperature receiver is wasted for terrestrial contacts. Owen Owen I would like to see what mods are made to the 221 to do that and also what kind of antenna system. I have a 221 I am using with a gasfet preamp in the shack that should be less than 1 db of noise fugure and about 20 db of gain. The antenna is a klm 22c and 75 feet of 9913 type of coax. I can just see some sun noise with the antenna aimed at the sun. It sure does not deflect the smeter several sunits. The antenna is on an azel mount. I am sure the system is working as I compaired it to an Icom 706 and another antenna that is mounted on a tower and I am getting about the differance in signal levels I would expect at the horizon. So, lets take some guesses about things here. FT221 native NF ~8dB, line loss 0.2dB, preamp 1dB NF, 20dB gain, 9913 loss 1.2dB (75', load end VSWR 1.5). On my reckoning, Teq at antenna connector is 200K. The antenna is I understand a 22 element crossed Yagi, let's assign it 15dBi for the purposes of discussion. Looking back at the earlier scenario, ambient (cold sky and earth) of 225K (ignoring the prospect of worse spillover with the smaller antenna) and sun noise of 160K, sun noise rise would be (225+200+160)/(225+200) or 1.4dB. That is not going to be very noticeable on an S meter. O&OE!!! At higher points in the solar cycle or if the sun is disturbed, the rise will be greater, but genuine quiet sun measurements should not capture disturbed sun, should they? Antennas more sensitive off the back / sides will be worse. In that scenario, moving the preamp to the antenna would improve G/T from -11.3dB to -10.1, yielding a 1.2dB improvement in S/N ratio. At zero elevation, the ambient noise is typically much higher and the improvement would be much less. Ambient noise is pretty easy to measure on 144MHz in a simple station. If you know the noise figure of your receiver and loss to the antenna, and providing ambient noise is not sufficient to cause AGC, note the audio power output change between antenna and a dummy load, you can calculate the ambient noise, see http://www.vk1od.net/sc/anc.htm for more information. In fact, with this tool, I can measure the change in audio output on my TS2000 by switching the internal 12.1dB attenuator in, and calculate Ta (try Example 3). I hear sun noise rise bragged about, but the bragger often cannot quote the solar flux prevailing at the time, in which case it is rather meaningless, especially when their method is to capture the largest rise rather than largest minimum rise whithout artificial compression. The reason I wrote the online calculator is that a number of people I have discussed sun noise rise with were using solar flux figures that were old and / or the wrong frequency. Owen |
Sun noise
"Harold E. Johnson" wrote in
news:4mkAi.59515$Xa3.5320@attbi_s22: Well of course it is Jeff, but in moonbounce, noise figure is the holy grail. Is it? Knowing G/T allows you to calculate the S/N ratio for a known incoming power flux density. Better, improvement in G/T ratio yields exactly the same improvement in S/N. You cannot do that with NF alone, so it is not a single metric that characterises receive performance of a station. Moonbouncers who focus on NF alone are as blinkered as anyone else who does that. Owen |
Sun noise
Owen Duffy wrote in
: Dave Oldridge wrote in : ... I used to have an FT-221 tricked out with a hot front end. Solar noise would run the S meter up to well over the S9 mark and you could even see the galactic plane passing through the antenna pattern. Needless to say, it heard well on terrestrial 2m SSB. That is no mean feat! I think ambient noise temperature at 144MHz for an antenna pointed at cold sky is somewhere around 200K to 250K, when you add a pretty good receiver at say 30K, you are talking 230K to 280K total system noise, and the sun is probably around 800K with a low end 4 bay EME antenna setup (Gain~22dBi), for a noise rise of 10*log((800+255)/255) or 16dB. A single yagi of gain around 15dBi is much poorer, not only is the sun noise reduced proportionately to the gain reduction, but the ambient noise increases with higher gain in the side and back area of the antenna, but it still should be possible to reliably 'see' the sun with a very good receiver. Ambient noise temperature for a beam at zero elevation here in suburbia varies from 1000K to 6000K depending on the day and time... so a very low temperature receiver is wasted for terrestrial contacts. This was a VERY quiet location. The only VHF stuff around was a bit of 156mhz and 161mhz stuff on a tower about 5 miles away. I could JUST see the big tower downtown that most of the FM broadcast stations used, but it was about 12 miles away and partially obscured by trees and a hill. But to the south and west was mostly ocean. VERY quiet VHF location. One time I worked a guy in a plane flying from St. Johns, Newfoundland to Montreal, on 146.49 simplex FM. He just had a handheld in the cockpit window and we stayed in contact (from my suburban Halifax location) from the time he reached cruising altitude (30-some thousand) until he began his descent into Montreal. The distance at the end of the QSO was approaching 800 miles. I did not notice any particular propagation that day, this was just plain simplex FM. Now I was running about 600 watts at my end of the pipe, but I could hear his handheld just fine. Near as I could measure it, the NF of the receiver after my mod was 1.2db. I had to resort to boiling and freezing water and a tiny dummy load to measure it at all. -- Dave Oldridge+ ICQ 1800667 |
Sun noise
"Harold E. Johnson" wrote in
news:ci5Ai.58746$Xa3.26736@attbi_s22: I used to have an FT-221 tricked out with a hot front end. Solar noise would run the S meter up to well over the S9 mark and you could even see the galactic plane passing through the antenna pattern. Needless to say, it heard well on terrestrial 2m SSB. That is no mean feat! You better believe no mean feat! I worked off the moon on 432 MHz back in the late 70's. Sixteen 16 element Yagis (calculated gain ~26 dB) and an STA from the FCC to use 5 KW as long as the antennas were pointed above 25 degrees elevation. (Protecting Eglin radars some 200 plus miles away.) I could consistently manage some 4, 4-1/2 dB of sun noise off a quiet sun and not one smidge more even with a mast mounted GaAs Fet preamp supposedly with some 0.8 dB NF. I DID however, at around the same time, own a 2 meter Jap all-mode transceiver that I happened to measure the "S" meter accuracy with an HP signal generator. It turned out that 2 uVolts was "S"-1. THREE uVolts was "S"-9. Yeah, and that just gets worse when you put a decent preamp in front of it. -- Dave Oldridge+ ICQ 1800667 |
Sun noise
"Owen Duffy" wrote in message ... "Ralph Mowery" wrote in : "Owen Duffy" wrote in message ... Dave Oldridge wrote in : ... I used to have an FT-221 tricked out with a hot front end. Solar noise would run the S meter up to well over the S9 mark and you could even see the galactic plane passing through the antenna pattern. Needless to say, it heard well on terrestrial 2m SSB. Firstly, this was actually Howard's statement. So, lets take some guesses about things here. FT221 native NF ~8dB, line loss 0.2dB, preamp 1dB NF, 20dB gain, 9913 loss 1.2dB (75', load end VSWR 1.5). On my reckoning, Teq at antenna connector is 200K. The antenna is I understand a 22 element crossed Yagi, let's assign it 15dBi for the purposes of discussion. Looking back at the earlier scenario, ambient (cold sky and earth) of 225K (ignoring the prospect of worse spillover with the smaller antenna) and sun noise of 160K, sun noise rise would be (225+200+160)/(225+200) or 1.4dB. That is not going to be very noticeable on an S meter. O&OE!!! At higher points in the solar cycle or if the sun is disturbed, the rise will be greater, but genuine quiet sun measurements should not capture disturbed sun, should they? Antennas more sensitive off the back / sides will be worse. In that scenario, moving the preamp to the antenna would improve G/T from -11.3dB to -10.1, yielding a 1.2dB improvement in S/N ratio. At zero elevation, the ambient noise is typically much higher and the improvement would be much less. Owen Thanks for the calculations Owen. That agrees with what I thought I read many years ago. At 2 meters unless you have an antenna the size of the ham in Texas ( I think) that has about 20 or 30 yagies in about a 1/2 half acer field, it is difficult to hear any sun noise rise on 2 meters most of the time. Certainly not something that would push an smeter to s-9. As I turn my antenna to the sun (azel mount) when it is high in the sky I just barley can see the sun noise sometimes. That is with using an audio voltmeter across the speaker. I have not tried this too many times, but I don't recall ever seeing the s-meter jump up an s unit or two due to the sun. I don't do moon bounce, but did set the system to work the Oscars. Usually no problem hearing 10 and 13 when they were way out. |
Sun noise
"Ralph Mowery" wrote in
: "Owen Duffy" wrote in message ... Dave Oldridge wrote in : ... I used to have an FT-221 tricked out with a hot front end. Solar noise would run the S meter up to well over the S9 mark and you could even see the galactic plane passing through the antenna pattern. Needless to say, it heard well on terrestrial 2m SSB. That is no mean feat! I think ambient noise temperature at 144MHz for an antenna pointed at cold sky is somewhere around 200K to 250K, when you add a pretty good receiver at say 30K, you are talking 230K to 280K total system noise, and the sun is probably around 800K with a low end 4 bay EME antenna setup (Gain~22dBi), for a noise rise of 10*log((800+255)/255) or 16dB. A single yagi of gain around 15dBi is much poorer, not only is the sun noise reduced proportionately to the gain reduction, but the ambient noise increases with higher gain in the side and back area of the antenna, but it still should be possible to reliably 'see' the sun with a very good receiver. Ambient noise temperature for a beam at zero elevation here in suburbia varies from 1000K to 6000K depending on the day and time... so a very low temperature receiver is wasted for terrestrial contacts. Owen Owen I would like to see what mods are made to the 221 to do that and also what kind of antenna system. I have a 221 I am using with a gasfet preamp in the shack that should be less than 1 db of noise fugure and about 20 db of gain. The antenna is a klm 22c and 75 feet of 9913 type of coax. I can just see some sun noise with the antenna aimed at the sun. It sure does not deflect the smeter several sunits. The antenna is on an azel mount. I am sure the system is working as I compaired it to an Icom 706 and another antenna that is mounted on a tower and I am getting about the differance in signal levels I would expect at the horizon. Well, the sun is much quieter nowadays than it was back then. When I had the station at its peak, I was running a single 19-element boomer at 85 feet, with boudle shielded Belden COAX. The preamp I was using was a mosfet, which, to the best of my measurement ability (none of the signal generators at work could come close to measuring it) gave it a 1.2db noise figure. Now there was probably also a 6db ground reflection gain from measuring the solar noise at the horizon (sunset or sunrise) and, as I say, it was a period when solar activity was high (6 meters was open to Mexico city a lot). -- Dave Oldridge+ ICQ 1800667 |
Sun noise
"Jeff" wrote in
. com: I would like to see what mods are made to the 221 to do that and also what kind of antenna system. I have a 221 I am using with a gasfet preamp in the shack that should be less than 1 db of noise fugure and about 20 db of gain. The antenna is a klm 22c and 75 feet of 9913 type of coax. I can just see some sun noise with the antenna aimed at the sun. It sure does not deflect the smeter several sunits. The antenna is on an azel mount. I am sure the system is working as I compaired it to an Icom 706 and another antenna that is mounted on a tower and I am getting about the differance in signal levels I would expect at the horizon. The 75 feet of coax is your problem!!! What is the point of a 1dB NF preamp with all that loss ahead of it? To see the benefit of the low noise figure the amp must be before all that cable loss!! Yep...when I moved, I put in hardline and lowered the antenna a bunch. That's when I started to hear other things besides the sun. And I could plot it. When the galactic plane was in front of the antenna, the noise floor would be up an S-unit from base. -- Dave Oldridge+ ICQ 1800667 |
Sun noise
"Ralph Mowery" wrote in
: "Jeff" wrote in message . com... I would like to see what mods are made to the 221 to do that and also what kind of antenna system. I have a 221 I am using with a gasfet preamp in the shack that should be less than 1 db of noise fugure and about 20 db of gain. The antenna is a klm 22c and 75 feet of 9913 type of coax. I can just see some sun noise with the antenna aimed at the sun. It sure does not deflect the smeter several sunits. The antenna is on an azel mount. I am sure the system is working as I compaired it to an Icom 706 and another antenna that is mounted on a tower and I am getting about the differance in signal levels I would expect at the horizon. The 75 feet of coax is your problem!!! What is the point of a 1dB NF preamp with all that loss ahead of it? To see the benefit of the low noise figure the amp must be before all that cable loss!! 73 Jeff G8HUL The coax only has a loss of 1 db for the length I am running it. I doubt that at 2 meters I would see any benift of putting the preamp at the antenna. It is as easy to get a 1 db or less noise figure at 2 meters as it is to make any preamp and the 221 does need some help on the receiving side. From the articals I have read the beam width of Out of the box, the 221 had a noise figure somewhat north of 12db. I've seen vacuum tube radios that were not much worse. -- Dave Oldridge+ ICQ 1800667 |
Sun noise
Dave Oldridge wrote in
9: Near as I could measure it, the NF of the receiver after my mod was 1.2db. I had to resort to boiling and freezing water and a tiny dummy load to measure it at all. I haven't tried hot/cold tests using ice and boiling water, I didn't think it was practical. You finally measured a receiver noise temperature of 50K with hot and cold loads of 270 and 370. That means a Y factor of 1.059dB. If Y were just 0.1dB greater, NF would be 0.78dB, 0.1dB lower and, NF would be 1.66dB. With this configuration the sensitivity of NF to changes in Y are extreme, 0.4dB change in NF per 0.1dB change in Y around that point. If you made the Y measurements using the audio output of a narrow band receiver, it is very hard to make high resolution measurements (eg to 0.01dB resolution) with say, a multimeter. I have done these tests with a liquid nitrogen cooled load and room temperature load, and that gives more practical Y ratios, 3.7dB for a 1.2dBNF, and the sensitivity in NF is 0.08dB per 0.1dB change in Y. This still demands high resolution measurement of noise power. Owen |
Sun noise
Dear Owen:
Others too may remember the use of a noise source comprising a gas tube crossways in a piece of waveguide (with 50 ohm probes) to estimate noise figure in the VT days. One would fire the gas tube and it was estimated to have a very large, "known" noise temp. Boiling water, while a known temp., would not have been hot enough. Ice water was critical in the use of the HILLMS receiver used to measure absolute flux. The antenna was a very long horn antenna resting on the side of a deep gully. In those days, a horn antenna was one of the few antennas with a predictable gain. The receiver switched at a low frequency between the antenna and a load kept in ice water. The gain of the receiver was stabilized with a huge amount of negative feedback. Once a day, the source would pass through the antenna's beam and a strip chart recorder would indicate the difference between ice water and the source's temp. Today, with much lower NF, and much more EM pollution, different techniques might be used. 73 Mac N8TT -- J. Mc Laughlin; Michigan U.S.A. Home: "Owen Duffy" wrote in message Dave Oldridge wrote in Near as I could measure it, the NF of the receiver after my mod was 1.2db. I had to resort to boiling and freezing water and a tiny dummy load to measure it at all. I haven't tried hot/cold tests using ice and boiling water, I didn't think it was practical. You finally measured a receiver noise temperature of 50K with hot and cold loads of 270 and 370. That means a Y factor of 1.059dB. If Y were just 0.1dB greater, NF would be 0.78dB, 0.1dB lower and, NF would be 1.66dB. With this configuration the sensitivity of NF to changes in Y are extreme, 0.4dB change in NF per 0.1dB change in Y around that point. If you made the Y measurements using the audio output of a narrow band receiver, it is very hard to make high resolution measurements (eg to 0.01dB resolution) with say, a multimeter. I have done these tests with a liquid nitrogen cooled load and room temperature load, and that gives more practical Y ratios, 3.7dB for a 1.2dBNF, and the sensitivity in NF is 0.08dB per 0.1dB change in Y. This still demands high resolution measurement of noise power. Owen |
Sun noise
Any loss ahead of your preamp adds directly to the noise figure of the
system so the best NF that you could ever have is 2.5dB plus a little form the 221. Also 20dB of gain in the preamp sounds like a sure way to produce intermods in your radio. Well of course it is Jeff, but in moonbounce, noise figure is the holy grail. There weren't as many folks on 432 and moonbounce in particular back in 1979, but I NEVER was plagued with intermod off the moon, and the directivity of the array kept me from having any from anywhere else. W4ZCB Well that's the first FT221 I heard of working on 432MHz!! If you are talking about that band rather than 144MHz then the cable loss would have been far higher and the NF of the system much higher. Having a preamp with a gain of 20dB right in front of the radio is just plain silly. Pre-amp gains should be kept as low as possible. They should have just sufficient gain so that their low noise figure defines the system noise figure. Any excess gain is wasted and just asking for large signal problems. If NF was such a Holy Grail then why throw away a significant improvement by putting the preamp after the feeder any degrading the system NF by the feeder loss? Jeff |
Sun noise
Out of the box, the 221 had a noise figure somewhat north of 12db. I've seen vacuum tube radios that were not much worse. I have never seen an FT221 with a NF anywhere near that. They normally cone in at about 5-6dB as standard and about 1.5 with the Mutek front-end board fitted. 73 Jeff |
Sun noise
Jeff wrote:
Out of the box, the 221 had a noise figure somewhat north of 12db. I've seen vacuum tube radios that were not much worse. I have never seen an FT221 with a NF anywhere near that. They normally cone in at about 5-6dB as standard and about 1.5 with the Mutek front-end board fitted. I'd agree with 5-6dB as much more typical, but more like 2-2.5dB when the muTek board is simply plugged in. Another 0.5dB can be gained by avoiding the card edge connector, and installing a direct coax link from the antenna relay onto the board. As the person who designed the original FT221 board (which Chris Bartram at muTek re-engineered for production) I wore out two dial drives chasing 2m DX with that rig! But in all that time, I never saw S9 sun noise. The only time I've seen anything approaching that level, it has been from a major solar flare. -- 73 from Ian GM3SEK http://www.ifwtech.co.uk/g3sek |
Sun noise
Owen Duffy wrote:
Dave Oldridge wrote in 9: Near as I could measure it, the NF of the receiver after my mod was 1.2db. I had to resort to boiling and freezing water and a tiny dummy load to measure it at all. I haven't tried hot/cold tests using ice and boiling water, I didn't think it was practical. You finally measured a receiver noise temperature of 50K with hot and cold loads of 270 and 370. That means a Y factor of 1.059dB. If Y were just 0.1dB greater, NF would be 0.78dB, 0.1dB lower and, NF would be 1.66dB. With this configuration the sensitivity of NF to changes in Y are extreme, 0.4dB change in NF per 0.1dB change in Y around that point. If you made the Y measurements using the audio output of a narrow band receiver, it is very hard to make high resolution measurements (eg to 0.01dB resolution) with say, a multimeter. I have done these tests with a liquid nitrogen cooled load and room temperature load, and that gives more practical Y ratios, 3.7dB for a 1.2dBNF, and the sensitivity in NF is 0.08dB per 0.1dB change in Y. This still demands high resolution measurement of noise power. Owen Indeed, this would be a very challenging measurement, because you also have to take into account the match of that load, and if it's just a resistor that you're plunging into hot and cold, its resistance will almost certainly change. At microwave frequencies, a more common technique for radiometers is to use a flat plate absorber that has been characterized for changes in absorption over temperature. One might want to take a look at how NIST does this kind of thing. Here's the slides from a talk by Jim Randa http://www.boulder.nist.gov/div818/8...t%20Crs_06.pdf he's a noise measurement guru at NIST.. check out the website: http://www.boulder.nist.gov/div818/81801/Noise/ I've had a precision noise source (used to do Y factor measurements on a precision 13.402 GHz receiver) measured in their lab over a week. The measurement uncertainty (for a system with waveguide connections) was in the few Kelvins range (out of a noise power of 7000K or so), and the connect/reconnect uncertainty dominates. I doubt one could get this kind of performance with a coaxial connector (the uncertainty in the mismatch). By the way, a good noise diode source is probably a better standard for the hot side than heating a resistor. They're very, very stable over time, once calibrated, and if properly designed, have a very stable match as the noise is turned on and off. (that's what we were using in the above system, a temperature controlled Noise/Com style source). http://www.boulder.nist.gov/div818/8...ability_IM.pdf http://www.boulder.nist.gov/div818/8...ility_CPEM.pdf describes the performance Another useful link might be: http://www.boulder.nist.gov/div818/8...97_Amps_IM.pdf D.F. Wait, J. Randa, "Amplifier Noise Measurements at NIST", IEEE Trans on Inst. and Meas., v.46, n.2, Apr 1997 They give measurement uncertainties of 0.04dB on a 0.5 dB NF for 2-4 GHz.. |
Sun noise
Jim Lux wrote in
: .... Indeed, this would be a very challenging measurement, because you also have to take into account the match of that load, and if it's just a Jim, an important point, and more generally on the mismatch issue... It seems that many measuring sun noise rise prefer to measure the rise by attenuator substitution. Though it seems a simple and sound method of measurement, the effects of mismatch need to be considered, not only on the power delivered to the receiver chain, but also the noise figure of the device with the changing input or output loads. The effects are not necessarily easy to quantify, which makes the apparently simple method a bit of a trap. Owen |
Sun noise
On Mon, 27 Aug 2007 23:02:06 GMT, Owen Duffy wrote:
Indeed, this would be a very challenging measurement, because you also have to take into account the match of that load, and if it's just a Jim, an important point, and more generally on the mismatch issue... Hi All, Given the notoriety that follows discussion about the Real component of the transmitter's source Z.... Let's see, How do I measure thee, let me count the ways. Anyone want to venture a guess on the value of the Real component of the receivers load Z? (And then we game this into the S-9 50µV across those myriad resistors.) 73's Richard Clark, KB7QHC |
Sun noise
Owen Duffy wrote in news:Xns999955EE72868nonenowhere@
61.9.191.5: Dave Oldridge wrote in 9: Near as I could measure it, the NF of the receiver after my mod was 1.2db. I had to resort to boiling and freezing water and a tiny dummy load to measure it at all. I haven't tried hot/cold tests using ice and boiling water, I didn't think it was practical. Only just and you need a good 4-digit or better AC voltmeter to do it at all. I wasn't after accuracy, just a ball-park estimate and I know I got it fairly close because the receiver did show a marked increase in noise when any decent antenna was connected. You finally measured a receiver noise temperature of 50K with hot and cold loads of 270 and 370. That means a Y factor of 1.059dB. If Y were just 0.1dB greater, NF would be 0.78dB, 0.1dB lower and, NF would be 1.66dB. Yep...the most I'd be willing to commit to with that measurement would be that it was below around 2.5 and PROBABLY fairly close to my measurement. I measured the voltages alternately 25 times and took a mean to try to smooth out the errors. With this configuration the sensitivity of NF to changes in Y are extreme, 0.4dB change in NF per 0.1dB change in Y around that point. If you made the Y measurements using the audio output of a narrow band receiver, it is very hard to make high resolution measurements (eg to 0.01dB resolution) with say, a multimeter. It is. You need a good AC voltmeter with decent digital accuracy and resolution and you have to average a bunch of readings. I have done these tests with a liquid nitrogen cooled load and room temperature load, and that gives more practical Y ratios, 3.7dB for a 1.2dBNF, and the sensitivity in NF is 0.08dB per 0.1dB change in Y. This still demands high resolution measurement of noise power. Yes, anything less than 4 digits is just about useless. -- Dave Oldridge+ ICQ 1800667 |
Sun noise
Dave Oldridge wrote in
9: Owen Duffy wrote in news:Xns999955EE72868nonenowhere@ 61.9.191.5: .... If you made the Y measurements using the audio output of a narrow band receiver, it is very hard to make high resolution measurements (eg to 0.01dB resolution) with say, a multimeter. It is. You need a good AC voltmeter with decent digital accuracy and resolution and you have to average a bunch of readings. I put some notes together on a perspective of the noise measurement (sampling) process and its statistical uncertainty, they are at http://www.vk1od.net/fsm/nmu.htm . It is my experience that a digital voltmeter probably samples for something around 100ms, and with a 2kHz wide noise bandwidth, you might expect an uncertainty of near 0.5dB at the 90% confidence level. Just watch the readings bounce around. Sure, if you average 100 of those measurements (actually, you should get the root of the sum of the squares... because it is power you average, not voltage), you might reduce that uncertainty to around 0.05dB... but it is not a very practical manual method, if recording accuracy (ie writing down the wrong number) doesn't get you, environmental drift will. Owen |
Sun noise
Dave Oldridge wrote:
Owen Duffy wrote in news:Xns999955EE72868nonenowhere@ 61.9.191.5: Dave Oldridge wrote in . 159: Near as I could measure it, the NF of the receiver after my mod was 1.2db. I had to resort to boiling and freezing water and a tiny dummy load to measure it at all. snip This still demands high resolution measurement of noise power. Yes, anything less than 4 digits is just about useless. That would be necessary but not sufficient. I suspect that other aspects of the measurement introduce greater uncertainty than the voltmeter. For instance, do you know the reflection coefficient of the load to 4 digits? (that would be knowing Z to about 0.1 ohm, at the RF frequency of interest), and is it stable with temperature to that level? For instance, a high quality load from Maury Microwave (a 2610F ) is specified to have a VSWR of 1.005 from DC-1GHz, which is a reflection coefficient of 0.0025. But that's only at 25C. An Agilent metrology grade cal kit with N connectors specifies rho0.00398 for the lowband loads, but only within 1 degree of the specified temperature. See, for example: http://cp.literature.agilent.com/lit...5054-90049.pdf A good thinfilm resistor might have a tempco of 5 ppm, with metal film being around 50 ppm, and thick film more in the 200 ppm area. For a 100 degree change, that's a 500 ppm (for the thin film) or a reflection coefficient change of 0.00025. Clearly one doesn't want to use any old resistor for the calibration load here. Measuring RF power to an accuracy of 1% is challenging. Your system is measuring a change in noise power of 100K out of 300K, roughly, so you've got a 30% change in noise power into the system. The Y-factor method essentially plots two points (one at 273K another at 373K, if you're using ice and boiling water), and then calculates the intercept at 0K. Since zeroK is about 3 times farther away than the measurement's width, errors in the measurement are roughly tripled at the intercept, and then doubled because you're using two measurements, so an error of 1% in the power measurement leads to about 5% error in the NF (if you're around 100K) (and this also applies if you have consistent errors.. say both power measurements are 1% high, the NF will come back as 105K instead of 100K). A 5% measurement uncertainty for power (0.2dB) gets you about 25-30% uncertainty in NF. The best way to improve the accuracy is to push the low temperature lower (e.g. with dry ice (195K) or LN2 (77K)), but that, of course, aggravates the change in reflection coefficient of your load with temperature. |
Sun noise
Jim Lux wrote in news:46D463CF.1080309
@jpl.nasa.gov: Dave Oldridge wrote: Owen Duffy wrote in news:Xns999955EE72868nonenowhere@ 61.9.191.5: Dave Oldridge wrote in .159: Near as I could measure it, the NF of the receiver after my mod was 1.2db. I had to resort to boiling and freezing water and a tiny dummy load to measure it at all. snip This still demands high resolution measurement of noise power. Yes, anything less than 4 digits is just about useless. That would be necessary but not sufficient. .... Just following through on the '4 digit' issue... I have done two series of 250 measurements of audio noise voltage from a SSB receiver using two different digital multimeters, the 9932 is a modern digital multimeter that is NOT true RMS responding, and the 506 is a modern digital multimeter that is true RMS responding with bandwidth adequate to cover the receiver output response. From observation with a stopwatch, I estimate that the 9932 updates 3 times per second, and the 506 updates 2 times per second. The integration times are probably .33 and .5 seconds respectively. I have measured the receiver equivalent noise bandwidth and it is 1600Hz. 95% of 250 readings were within 0.41dB for the 9932 and 0.31dB for the 506. These observations reconcile well with my Chi-squared based estimate of the uncertainty that I referred to in an earlier post. As for the number of digits, they are both 4 digit multimeters which doesn't mean a lot. They were used to measure 200mV with 1mV resolution, so the representational error is 0.04dB. The error due to the number of digits in this downscale three digit application is insignificant compared to the sampling error of 0.4dB and 0.3dB. Graphically, the distributions are shown at http://www.vk1od.net/nfm/temp.gif . Different meters with different integration times, and different receivers with different noise bandwidth will result in different outcomes, but I argue that the uncertainty is predictable. Owen |
Sun noise
On Thu, 30 Aug 2007 10:11:22 GMT, Owen Duffy wrote:
they are both 4 digit multimeters which doesn't mean a lot. They were used to measure 200mV with 1mV resolution, Hi Owen, The convention for decades has been to describe them as 3½ Digits, or 2000 count, not 4 digit unless they could represent 9999. Adding digits does not generally add precision, resolution, monotonicity, or accuracy. However, as it costs money to add a digit, the underlying circuitry could usually support "some" of these attributes. Better instruments perform rounding after the last digit. 73's Richard Clark, KB7QHC |
Sun noise
Owen Duffy wrote:
Jim Lux wrote in news:46D463CF.1080309 @jpl.nasa.gov: Dave Oldridge wrote: Owen Duffy wrote in news:Xns999955EE72868nonenowhere@ 61.9.191.5: Dave Oldridge wrote in 5.159: Near as I could measure it, the NF of the receiver after my mod was 1.2db. I had to resort to boiling and freezing water and a tiny dummy load to measure it at all. snip This still demands high resolution measurement of noise power. Yes, anything less than 4 digits is just about useless. That would be necessary but not sufficient. ... Just following through on the '4 digit' issue... I have done two series of 250 measurements of audio noise voltage from a SSB receiver using two different digital multimeters, the 9932 is a modern digital multimeter that is NOT true RMS responding, and the 506 is a modern digital multimeter that is true RMS responding with bandwidth adequate to cover the receiver output response. From observation with a stopwatch, I estimate that the 9932 updates 3 times per second, and the 506 updates 2 times per second. The integration times are probably .33 and .5 seconds respectively. I have measured the receiver equivalent noise bandwidth and it is 1600Hz. 95% of 250 readings were within 0.41dB for the 9932 and 0.31dB for the 506. These observations reconcile well with my Chi-squared based estimate of the uncertainty that I referred to in an earlier post. As for the number of digits, they are both 4 digit multimeters which doesn't mean a lot. They were used to measure 200mV with 1mV resolution, so the representational error is 0.04dB. Gotta be a bit careful there, because quantization error has a uniform distribution, so the variance is 1/12 of the span. This is different than the (presumably) normally distributed actual measurands. When giving an uncertainty (sampling error), one should also say whether it's a one sigma, two sigma, or 3 sigma number. *Standard uncertainty* is 1 sigma... *expanded uncertainty*, often given as a +/- number is usually the 95% percent confidence interval, which, for normal distributions, is 2 sigma given your statistics above, you would be giving the expanded uncertainty as 0.41dB By the way, unless your device actually directly measures dB (e.g. it has a log detector) or the errors are inherently ratios, it's probably better to give the value in a linear scale (milliwatts?) with the uncertainty in the same units. That gets you around the "ratio" problem where log(1+delta) -log(1-delta) http://physics.nist.gov/cuu/Uncertainty/index.html has the simple explanation, and the technical note (TN1297) , and references to the ISO Guide to Expression of Uncertainty in Measurment (aka the GUM) The error due to the number of digits in this downscale three digit application is insignificant compared to the sampling error of 0.4dB and 0.3dB. Graphically, the distributions are shown at http://www.vk1od.net/nfm/temp.gif . Different meters with different integration times, and different receivers with different noise bandwidth will result in different outcomes, but I argue that the uncertainty is predictable. Indeed, it is. Owen |
Sun noise
Jim Lux wrote in
: Owen Duffy wrote: ... Just following through on the '4 digit' issue... I have done two series of 250 measurements of audio noise voltage from a SSB receiver using two different digital multimeters, the 9932 is a modern digital multimeter that is NOT true RMS responding, and the 506 is a modern digital multimeter that is true RMS responding with bandwidth adequate to cover the receiver output response. From observation with a stopwatch, I estimate that the 9932 updates 3 times per second, and the 506 updates 2 times per second. The integration times are probably .33 and .5 seconds respectively. I have measured the receiver equivalent noise bandwidth and it is 1600Hz. 95% of 250 readings were within 0.41dB for the 9932 and 0.31dB for the 506. These observations reconcile well with my Chi-squared based estimate of the uncertainty that I referred to in an earlier post. As for the number of digits, they are both 4 digit multimeters which doesn't mean a lot. They were used to measure 200mV with 1mV resolution, so the representational error is 0.04dB. Gotta be a bit careful there, because quantization error has a uniform distribution, so the variance is 1/12 of the span. This is different than the (presumably) normally distributed actual measurands. Ok, point taken. I think more correctly, the maximum error would be 20 *log(1+1/200/2) or 0.0217dB. The expected error due to representation in three digits does not account for the variation in measurements. When giving an uncertainty (sampling error), one should also say whether it's a one sigma, two sigma, or 3 sigma number. *Standard uncertainty* is 1 sigma... *expanded uncertainty*, often given as a +/- number is usually the 95% percent confidence interval, which, for normal distributions, is 2 sigma Whilst it might be reasonable to assume that the combined error in measurement of a high S/N sine wave voltage might be normally distributed, and that might also be true of measurement of noise voltage in some circumstances, I propose that measurement of noise power in narrow bandwidth with short integration times is distributed as Chi- squared and the number of samples becomes relevant in determining the number of degrees of freedom for the distribution. For this reason, I have talked about a confidence level rather than sigma (which is more applicable to normally distributed data). Just for interest, in the case of the 9932 measurement set: Average=0.201, sigma=0.0046, 1sigma based uncertainty estimate=0.20dB, 2sigma based uncertainty estimate=0.41dB, 3sigma based uncertainty estimate=0.62dB. given your statistics above, you would be giving the expanded uncertainty as 0.41dB I stated it as 95% of obs within 0.41, I should have said 95% of obs within +/-0.41, I was explicit about the implied confidence, the 95% doesn't equate to either the 1sigma or 3sigma confidence, it is very close to the 2sigma confidence (95.45%), and it is at the high confidence end of the scale. By the way, unless your device actually directly measures dB (e.g. it has a log detector) or the errors are inherently ratios, it's probably better to give the value in a linear scale (milliwatts?) with the uncertainty in the same units. That gets you around the "ratio" problem where log(1+delta) -log(1-delta) I understand what you mean in your last sentence. I did record the voltage, and converted the values to dB for analysis. The interval was calculated by taking the average of the 2.5 percentile and 97.5 percentile, which is an approximation, but as such small values is pretty close. I have converted results to dB to make it easier to see the relevance of the error or uncertainty, but in so doing, another (small in this case) error is introduced. http://physics.nist.gov/cuu/Uncertainty/index.html has the simple explanation, and the technical note (TN1297) , and references to the ISO Guide to Expression of Uncertainty in Measurment (aka the GUM) In terms of the above, I am proposing that measurements of narrowband noise with short integration time is not strictly normally distributed, and an estimate of its uncertainty to a given confidence level can be obtained from the Chi-square distribution. One could not estimate the results of the test from knowledge of the instrument accuracy (inherent and representational error) alone. I think the experiment supports the proposition that digital multimeters with typically short integration times do not deliver high resolution measurement of narrow band (eg SSB telephony) noise. The error due to the number of digits in this downscale three digit application is insignificant compared to the sampling error of 0.4dB and 0.3dB. Graphically, the distributions are shown at http://www.vk1od.net/nfm/temp.gif . Different meters with different integration times, and different receivers with different noise bandwidth will result in different outcomes, but I argue that the uncertainty is predictable. Indeed, it is. Thanks, appreciate the comments. Owen |
Sun noise
Richard Clark wrote in
: On Thu, 30 Aug 2007 10:11:22 GMT, Owen Duffy wrote: they are both 4 digit multimeters which doesn't mean a lot. They were used to measure 200mV with 1mV resolution, Hi Owen, The convention for decades has been to describe them as 3½ Digits, or 2000 count, not 4 digit unless they could represent 9999. Adding digits does not generally add precision, resolution, monotonicity, or accuracy. However, as it costs money to add a digit, the underlying circuitry could usually support "some" of these attributes. Better instruments perform rounding after the last digit. Hi Richard, It is interesting in marketing hype that reference is made to 2 digit and 3 digit instruments, which implies a log based metric (10*log (MaxReading)) when you assume a 'full count', and the same hype refers to the upper digit if it can only have values of 0 or 1 as half a digit, whereas it probably has a weight of log(0.5) or 0.3... so in utility terms, a 2 1/2 digit instrument is really a 2.3 digit instrument. In my case, I was making the measurements straddling 200mV, so I needed a bit of headroom for outliers, say 1dB or 225mV fsd, so it was effectively 2.35 digit instrument if you followed that argument. Nevertheless, the error introduced by the resolution issue and instrument accuracy does not explain the experimental results... something else is happening, and one needs to look beyond the instrument itself to form a realistic view of measurement uncertainty when measuring narrowband noise. Owen |
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