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#1
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Steve,
There is another method of measuring noise figure that does not need a noise source. The reason that it is not used as often is that it is harder to measure low noise figures accurately. Since there are still a few layers of rust on my brain I will not geive the details here, since i do not want to say anything that is too far off. This other method involves measuring the gain of the device under test and then measuring the noise power output with the input terminated properly. The noise output is a combination of the DUT noise and the 'ambient' noise from the input termination, times the gain of the receiver. Working backwards you can then determine the DUT noise. As you can probably guess, the reason it doesn't work so good on small noise figures is that any errors in the measurement can easily outnumber the actual noise figure you are trying to measure! And I have actually had to deal with this problem. That is where half my gray hairs came from (the other half from being laid off). If you are measuring an entire receiver there are a few things you have to be careful with. The receiver must be a linear receiver (no FM, AM diode detector, etc.---basically just SSB). There should be a filter to pick just one sideband. Turn the AGC off. Make sure you measure the gain in the linear region, which also applies to a simple amplifier. If you have a DUT with a known noise figure, I think that this would be one way of calibrating a homebrew noise source. Jim N8EE |
#2
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"Jim" wrote in message ...
This other method involves measuring the gain of the device under test and then measuring the noise power output with the input terminated properly ....any errors in the measurement can easily outnumber the actual noise figure Thanks for reminding me of that one, Jim. But I see your point about errors. For example, in measuring the gain one needs a standard. One of the few pieces of real test gear I have is a bolometer-type RF power meter which can measure about -13 dBm accurately. If the measurement bandwidth is 1 MHz (suitable for VHF, perhaps) then thermal noise is -114 dBm. So I need about 100 dB gain for a very low noise figure DUT. To measure that I might need five 20 dB attenuators as a standard, each with perhaps +/-0.5 dB accuracy if I am lucky...so there's +/-2.5 dB error (well, I suppose I could cross my fingers and RSS the numbers). Or I have a diode-type power meter that will measure lower power, which leads to issues of how the detector responds to noise. And then there's the problem of knowing the noise bandwidth precisely... is where half my gray hairs came from (the other half from being laid off). I think about half of mine come from the latter factor too ! If you are measuring an entire receiver there are a few things you have to be careful with. The receiver must be a linear receiver (no FM, AM diode detector, etc.---basically just SSB). There should be a filter to pick just one sideband. Turn the AGC off. Make sure you measure the gain in the linear region, which also applies to a simple amplifier. Definitely. Though, with my method the input never gets more than 3 dB above the receiver noise floor and in most cases a well designed receiver will have no AGC response at that level. But with a preamp in front it usually will activate the AGC, so AGC has to be switched off when comparing preamps - which of course is impossible to do in most ham rigs ! If you have a DUT with a known noise figure, I think that this would be one way of calibrating a homebrew noise source. Hence the interest in NF repeatability of MMIC amps, since they are hard to build wrong, are well matched over a wide bandwidth and don't require tuning for best noise figure. The known DUT can also be the standard itself (in association with a receiver of only roughly known NF) to avoid issues of errors in calibrating the noise source ENR. 73, Steve VE3SMA |
#3
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On 14 Oct 2004 16:44:35 -0700, (Steve
Kavanagh) wrote: I haven't really been following this thread but if you are looking for noise diodes and associated stuff, you should take a look at: http://www.noisecom.com/ Seems to me that at one time they would calibrate a homebrew source for a modest fee. That may have been a ham that worked there paving the way; not sure. |"Jim" wrote in message ... | | This other method involves measuring the gain of the device under test and | then measuring the noise power output with the input terminated | properly | | ....any errors | in the measurement can easily outnumber the actual noise figure | |Thanks for reminding me of that one, Jim. But I see your point about |errors. For example, in measuring the gain one needs a standard. One |of the few pieces of real test gear I have is a bolometer-type RF |power meter which can measure about -13 dBm accurately. If the |measurement bandwidth is 1 MHz (suitable for VHF, perhaps) then |thermal noise is -114 dBm. So I need about 100 dB gain for a very low |noise figure DUT. To measure that I might need five 20 dB attenuators |as a standard, each with perhaps +/-0.5 dB accuracy if I am lucky...so |there's +/-2.5 dB error (well, I suppose I could cross my fingers and |RSS the numbers). Or I have a diode-type power meter that will |measure lower power, which leads to issues of how the detector |responds to noise. And then there's the problem of knowing the noise |bandwidth precisely... | | is where half my gray hairs came from (the other half from being laid off). | |I think about half of mine come from the latter factor too ! | | If you are measuring an entire receiver there are a few things you have to | be careful with. The receiver must be a linear receiver (no FM, AM diode | detector, etc.---basically just SSB). There should be a filter to pick just | one sideband. Turn the AGC off. Make sure you measure the gain in the | linear region, which also applies to a simple amplifier. | |Definitely. Though, with my method the input never gets more than 3 |dB above the receiver noise floor and in most cases a well designed |receiver will have no AGC response at that level. But with a preamp |in front it usually will activate the AGC, so AGC has to be switched |off when comparing preamps - which of course is impossible to do in |most ham rigs ! | | If you have a DUT with a known noise figure, I think that this would be one | way of calibrating a homebrew noise source. | |Hence the interest in NF repeatability of MMIC amps, since they are |hard to build wrong, are well matched over a wide bandwidth and don't |require tuning for best noise figure. The known DUT can also be the |standard itself (in association with a receiver of only roughly known |NF) to avoid issues of errors in calibrating the noise source ENR. | |73, |Steve VE3SMA |
#4
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Wes Stewart wrote:
I haven't really been following this thread but if you are looking for noise diodes and associated stuff, you should take a look at: http://www.noisecom.com/ Seems to me that at one time they would calibrate a homebrew source for a modest fee. That may have been a ham that worked there paving the way; not sure. In connection with the QST article by Bill Sabin, W0IYH, NoiseCom had a program of single-quantity sales to amateurs, and offered a low-cost calibration service. I'm not sure whether that program still operates, but some years ago NoiseCom were very helpful in supplying a free sample outside of the USA. If I were building a noise source again, I wouldn't put NoiseCom to any trouble. Instead, I would use the base-emitter junction of a small-signal UHF/microwave transistor. It seems that just about any junction that goes into avalanche breakdown at a reverse voltage of 5-7V will give an Excess Noise Ratio of about 35dB. With a modern SMD package, the device parasitics will be lower than the wire-ended diodes that NC were offering, so the ENR will be flatter with frequency into the microwave region. ENR flatness of the whole noise source depends on your construction. The other important factor is that the output impedance must not change significantly between the 'on' and 'off' states of the diode. To achieve both of these, the best technique is to build the noise source using SMD/microstrip technique including an attenuator of about 7dB made from 0805 SMD resistors; and then buy a high-class 20dB attenuator (N or SMA) which need not be expensive at a hamfest. This attenuator then becomes a permanent part of the noise source - not to be taken off and used for something else! This level of attenuation will give you an ENR of about 5-6dB, which is what you need to measure typical modern low-noise amplifiers. On the DC side, it is a very good idea to include a constant current source for long-term stability of ENR. Most devices will give a peak of ENR at a few mA, so you need to adjust the current to the top of this peak where the variability is least. (There is much more noise at currents of a few hundred uA, but you don't want to go there - the noise output there is far too sensitive to the DC current, temperature, color of carpet, phase of moon etc.) It is VERY important to design for the industry-standard power supply of +28V DC, so that your noise source is a simple plug-in replacement for any professional source... because that is how you're going to get your ENR calibration. All of these ideas come from an article by DJ9BV in DUBUS magazine which described a very high-class noise source, good up to 10GHz. Mine uses simpler and much less precise construction, so the ENR begins to wobble above a few GHz due to resonances. The DJ9BV articles (in both English and German) are on the DUBUS website, at: http://www.marsport.demon.co.uk/archive.htm Look around 1990, and there are a few follow-ups in later years. (Unfortunately both archive sites are down right now, but do keep trying - these articles are *exactly* what you've been looking for!) As for calibration, the best way is to take your noise head to a microwave meet where there is professional NF measuring equipment, and take a cal from the HP346A there. Alternatively, get to know someone - anywhere - who has access to these facilities, and can do it for you one lunchtime. Other useful background information is in an Agilent App Note, at: http://literature.agilent.com/litweb/pdf/5952-3706E.pdf If you can still find a copy of 'The VHF/UHF/DX Book' (out of print) there's a lot of information in there too. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
#5
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"Ian White, G3SEK" wrote in message ...
Instead, I would use the base-emitter junction of a small-signal UHF/microwave transistor. It seems that just about any junction that goes into avalanche breakdown at a reverse voltage of 5-7V will give an Excess Noise Ratio of about 35dB. With a modern SMD package, the device parasitics will be lower than the wire-ended diodes that NC were offering, so the ENR will be flatter with frequency into the microwave region. That's an interesting suggestion...but presumably the 35 dB ENR can't be counted upon to the level needed for measurements without calibration. The DJ9BV articles (in both English and German) are on the DUBUS website, at: http://www.marsport.demon.co.uk/archive.htm Look around 1990, and there are a few follow-ups in later years. (Unfortunately both archive sites are down right now, but do keep trying - these articles are *exactly* what you've been looking for!) Thanks, Ian. I found the DUBUS articles...though I haven't read them through yet. They look very interesting. Though perhaps not *exactly* what I was after, as they use a real microwave noise diode, whereas my thrust is more in the direction of what can be done with more common parts and without any need for calibration of individual noise standards. I recognize that this probably won't be possible in the microwave range but it seems likely (to me, anyway) that reasonable accuracy (not good enough for EME preamps !) can probably be attained up to VHF, even with these constraints. 73, Steve, VE3SMA |
#6
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Steve Kavanagh wrote:
"Ian White, G3SEK" wrote in message ... Instead, I would use the base-emitter junction of a small-signal UHF/microwave transistor. It seems that just about any junction that goes into avalanche breakdown at a reverse voltage of 5-7V will give an Excess Noise Ratio of about 35dB. With a modern SMD package, the device parasitics will be lower than the wire-ended diodes that NC were offering, so the ENR will be flatter with frequency into the microwave region. That's an interesting suggestion...but presumably the 35 dB ENR can't be counted upon to the level needed for measurements without calibration. That's correct; it is only a design guide to the amount of attenuation that will be needed to give an ENR that's in the right ballpark. But then you need to know what the actual value *is* - and for that, you still need a calibration. The DJ9BV articles (in both English and German) are on the DUBUS website, at: http://www.marsport.demon.co.uk/archive.htm Look around 1990, and there are a few follow-ups in later years. (Unfortunately both archive sites are down right now, but do keep trying - these articles are *exactly* what you've been looking for!) Thanks, Ian. I found the DUBUS articles...though I haven't read them through yet. They look very interesting. Though perhaps not *exactly* what I was after, as they use a real microwave noise diode, whereas my thrust is more in the direction of what can be done with more common parts and without any need for calibration of individual noise standards. My point was that you don't need a real microwave noise diode - any small, cheap UHF/microwave bipolar transistor will give almost the same performance up to several GHz. I recognize that this probably won't be possible in the microwave range but it seems likely (to me, anyway) that reasonable accuracy (not good enough for EME preamps !) can probably be attained up to VHF, even with these constraints. You can optimize NF using an uncalibrated source, but with this kind of source you cannot can make worthwhile quantitative measurements without an external calibration. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
#7
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You also have to know the noise bandwidth of the system to use this
method. If the response is dominated by a single, fairly steep-sided filter, this is easy. But otherwise (such as if both IF filtering and AF amplifier response shape the overall response), some calculation and/or measurement is required. I've gotten what I believe are reasonable results on HF and AF amplifiers using this method. But I haven't tried it on a really low NF system, so would heed Jim's caution. Roy Lewallen, W7EL Jim wrote: Steve, There is another method of measuring noise figure that does not need a noise source. The reason that it is not used as often is that it is harder to measure low noise figures accurately. Since there are still a few layers of rust on my brain I will not geive the details here, since i do not want to say anything that is too far off. This other method involves measuring the gain of the device under test and then measuring the noise power output with the input terminated properly. The noise output is a combination of the DUT noise and the 'ambient' noise from the input termination, times the gain of the receiver. Working backwards you can then determine the DUT noise. As you can probably guess, the reason it doesn't work so good on small noise figures is that any errors in the measurement can easily outnumber the actual noise figure you are trying to measure! And I have actually had to deal with this problem. That is where half my gray hairs came from (the other half from being laid off). If you are measuring an entire receiver there are a few things you have to be careful with. The receiver must be a linear receiver (no FM, AM diode detector, etc.---basically just SSB). There should be a filter to pick just one sideband. Turn the AGC off. Make sure you measure the gain in the linear region, which also applies to a simple amplifier. If you have a DUT with a known noise figure, I think that this would be one way of calibrating a homebrew noise source. Jim N8EE |
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