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Any FM band RX projects?
Hi. Anybody developed a nbfm RX project covering the FM band (appx
87.5Mhz-108Mhx). With digital readout, though I suppose I could just hook up a simple a frequency counter to read frequency. TIA. Rich. |
In article ,
says... Hi. Anybody developed a nbfm RX project covering the FM band (appx 87.5Mhz-108Mhx). NBFM? As in narrow band? What would be the point? Here in the U.S. at least, that entire band is assigned to FM broadcasting, and it is anything but narrow-band. Typical deviation from a broadcast station is 75+ kHz. -- Dr. Anton Squeegee, Director, Dutch Surrealist Plumbing Institute (Known to some as Bruce Lane, KC7GR) kyrrin a/t bluefeathertech d-o=t c&o&m Motorola Radio Programming & Service Available - http://www.bluefeathertech.com/rf.html "Quando Omni Flunkus Moritati" (Red Green) |
Dr. A.T. Squeegee wrote:
In article , says... Hi. Anybody developed a nbfm RX project covering the FM band (appx 87.5Mhz-108Mhx). NBFM? As in narrow band? What would be the point? Here in the U.S. at least, that entire band is assigned to FM broadcasting, and it is anything but narrow-band. Typical deviation from a broadcast station is 75+ kHz. Maybe I used the wrong term. I think lots of HiFi tuners have very wide filters much greater than 75 Khz. For DXing it seems then you need no more than say 75Khz. A tuner with that bandwidth would, in a sense, (Ithink) compared to a regular HiFi tuneer be a narrow bandwidth tuner. BTW, what would be the result if you used say a 20Khz filter on a FM signal with 75 Khz deviation? Would you get distortion or a perfectly copyable signal. I mean is it the analagous to using a 2Khz filter for an AM signal transmitted at 6Khz wide? |
Dr. A.T. Squeegee wrote:
In article , says... Hi. Anybody developed a nbfm RX project covering the FM band (appx 87.5Mhz-108Mhx). NBFM? As in narrow band? What would be the point? Here in the U.S. at least, that entire band is assigned to FM broadcasting, and it is anything but narrow-band. Typical deviation from a broadcast station is 75+ kHz. Maybe I used the wrong term. I think lots of HiFi tuners have very wide filters much greater than 75 Khz. For DXing it seems then you need no more than say 75Khz. A tuner with that bandwidth would, in a sense, (Ithink) compared to a regular HiFi tuneer be a narrow bandwidth tuner. BTW, what would be the result if you used say a 20Khz filter on a FM signal with 75 Khz deviation? Would you get distortion or a perfectly copyable signal. I mean is it the analagous to using a 2Khz filter for an AM signal transmitted at 6Khz wide? |
It wouldn't be quite the same. You would be clipping the sidebands, and
experience quite a bit of distortion. A 110kHz filter is about as narrow as you can go. I've been meaning to come up with a tuner that would be in the class of a McIntosh MR78 for the past couple of years, but something has always come up. Maybe after my current project, I will do this, if there is enough interest. Pete Richard wrote in message ... Dr. A.T. Squeegee wrote: In article , says... Hi. Anybody developed a nbfm RX project covering the FM band (appx 87.5Mhz-108Mhx). NBFM? As in narrow band? What would be the point? Here in the U.S. at least, that entire band is assigned to FM broadcasting, and it is anything but narrow-band. Typical deviation from a broadcast station is 75+ kHz. Maybe I used the wrong term. I think lots of HiFi tuners have very wide filters much greater than 75 Khz. For DXing it seems then you need no more than say 75Khz. A tuner with that bandwidth would, in a sense, (Ithink) compared to a regular HiFi tuneer be a narrow bandwidth tuner. BTW, what would be the result if you used say a 20Khz filter on a FM signal with 75 Khz deviation? Would you get distortion or a perfectly copyable signal. I mean is it the analagous to using a 2Khz filter for an AM signal transmitted at 6Khz wide? |
It wouldn't be quite the same. You would be clipping the sidebands, and
experience quite a bit of distortion. A 110kHz filter is about as narrow as you can go. I've been meaning to come up with a tuner that would be in the class of a McIntosh MR78 for the past couple of years, but something has always come up. Maybe after my current project, I will do this, if there is enough interest. Pete Richard wrote in message ... Dr. A.T. Squeegee wrote: In article , says... Hi. Anybody developed a nbfm RX project covering the FM band (appx 87.5Mhz-108Mhx). NBFM? As in narrow band? What would be the point? Here in the U.S. at least, that entire band is assigned to FM broadcasting, and it is anything but narrow-band. Typical deviation from a broadcast station is 75+ kHz. Maybe I used the wrong term. I think lots of HiFi tuners have very wide filters much greater than 75 Khz. For DXing it seems then you need no more than say 75Khz. A tuner with that bandwidth would, in a sense, (Ithink) compared to a regular HiFi tuneer be a narrow bandwidth tuner. BTW, what would be the result if you used say a 20Khz filter on a FM signal with 75 Khz deviation? Would you get distortion or a perfectly copyable signal. I mean is it the analagous to using a 2Khz filter for an AM signal transmitted at 6Khz wide? |
"Pete KE9OA" wrote in message ... It wouldn't be quite the same. You would be clipping the sidebands, and experience quite a bit of distortion. A 110kHz filter is about as narrow as you can go. I've been meaning to come up with a tuner that would be in the class of a McIntosh MR78 for the past couple of years, but something has always come up. Maybe after my current project, I will do this, if there is enough interest. Pete I hear that FM RX's are pretty complicated affairs. Most FM DXers it seems just modify commercial sets. Reduce bandwidth from say 230Khz to 110 Khz. I suppose that going this way has quite a lot of merit. Cheaper probably. |
"Pete KE9OA" wrote in message ... It wouldn't be quite the same. You would be clipping the sidebands, and experience quite a bit of distortion. A 110kHz filter is about as narrow as you can go. I've been meaning to come up with a tuner that would be in the class of a McIntosh MR78 for the past couple of years, but something has always come up. Maybe after my current project, I will do this, if there is enough interest. Pete I hear that FM RX's are pretty complicated affairs. Most FM DXers it seems just modify commercial sets. Reduce bandwidth from say 230Khz to 110 Khz. I suppose that going this way has quite a lot of merit. Cheaper probably. |
In article , "Richard"
writes: Dr. A.T. Squeegee wrote: In article , says... Hi. Anybody developed a nbfm RX project covering the FM band (appx 87.5Mhz-108Mhx). NBFM? As in narrow band? What would be the point? Here in the U.S. at least, that entire band is assigned to FM broadcasting, and it is anything but narrow-band. Typical deviation from a broadcast station is 75+ kHz. Maybe I used the wrong term. I think lots of HiFi tuners have very wide filters much greater than 75 Khz. For DXing it seems then you need no more than say 75Khz. A tuner with that bandwidth would, in a sense, (Ithink) compared to a regular HiFi tuneer be a narrow bandwidth tuner. BTW, what would be the result if you used say a 20Khz filter on a FM signal with 75 Khz deviation? Would you get distortion or a perfectly copyable signal. I mean is it the analagous to using a 2Khz filter for an AM signal transmitted at 6Khz wide? You need to refresh your personal databanks on basic modulation. In FM the modulation amplitude "swings the frequency up and down in frequency." [close and simplistic, there's a bit more to it...] Limiting the bandwidth of the receiver is the same as clipping the peaks of an amplitude modulation. You WILL get a LOT of distortion on high-amplitude modulation input at a station. In a limited-bandwidth AM receiver there is no limit on the amplitude of an AM signal, just the frequency range of the modulation signal. In a limited-bandwidth FM receiver there is both a limit on the amplitude and frequency range of the modulation signal. Len Anderson retired (from regular hours) electronic engineer person |
In article , "Richard"
writes: Dr. A.T. Squeegee wrote: In article , says... Hi. Anybody developed a nbfm RX project covering the FM band (appx 87.5Mhz-108Mhx). NBFM? As in narrow band? What would be the point? Here in the U.S. at least, that entire band is assigned to FM broadcasting, and it is anything but narrow-band. Typical deviation from a broadcast station is 75+ kHz. Maybe I used the wrong term. I think lots of HiFi tuners have very wide filters much greater than 75 Khz. For DXing it seems then you need no more than say 75Khz. A tuner with that bandwidth would, in a sense, (Ithink) compared to a regular HiFi tuneer be a narrow bandwidth tuner. BTW, what would be the result if you used say a 20Khz filter on a FM signal with 75 Khz deviation? Would you get distortion or a perfectly copyable signal. I mean is it the analagous to using a 2Khz filter for an AM signal transmitted at 6Khz wide? You need to refresh your personal databanks on basic modulation. In FM the modulation amplitude "swings the frequency up and down in frequency." [close and simplistic, there's a bit more to it...] Limiting the bandwidth of the receiver is the same as clipping the peaks of an amplitude modulation. You WILL get a LOT of distortion on high-amplitude modulation input at a station. In a limited-bandwidth AM receiver there is no limit on the amplitude of an AM signal, just the frequency range of the modulation signal. In a limited-bandwidth FM receiver there is both a limit on the amplitude and frequency range of the modulation signal. Len Anderson retired (from regular hours) electronic engineer person |
"Pete KE9OA" ) writes:
It wouldn't be quite the same. You would be clipping the sidebands, and experience quite a bit of distortion. A 110kHz filter is about as narrow as you can go. I've been meaning to come up with a tuner that would be in the class of a McIntosh MR78 for the past couple of years, but something has always come up. Maybe after my current project, I will do this, if there is enough interest. Pete For most people, it makes more sense to simply change the filters in an existing FM BCB receiver than start from scratch. Indeed, it seems to be a relatively common practice among people who DX that band. Not that building something from scratch wouldn't be interesing, only that if "narrow bandwidth" is all that's wanted, then there's no sense in building it all. And there isn't much sense in putting narrow filters in a mediocre homebuilt FM receiver, which is the sort of thing you see in construction articles. I use Delco digitally tuned car radios as my "table radios", running them off power supplies. For the price, a few dollars at garage sales, they are pretty good receivers on the FM band. I know it would benefit from a narrow filter for a few stations I like to listen to. But of course, a lot of FM receivers aren't that great for distant reception, being too sensitive, without good overload protection. In some cases, it might be intriguing to build a single channel FM BCB receiver. Build it like a ham band converter, with plenty of tuned circuits at the fronte end, little or no RF amplification, and a good mixer. Being fixed tuned, one could optimize it for that frequency, and not worry about tracking, or the problems of ganging a number of tuned circuits. For the local oscillator, one could go with a crystal oscillator chain. Michael VE2BVW Richard wrote in message ... Dr. A.T. Squeegee wrote: In article , says... Hi. Anybody developed a nbfm RX project covering the FM band (appx 87.5Mhz-108Mhx). NBFM? As in narrow band? What would be the point? Here in the U.S. at least, that entire band is assigned to FM broadcasting, and it is anything but narrow-band. Typical deviation from a broadcast station is 75+ kHz. Maybe I used the wrong term. I think lots of HiFi tuners have very wide filters much greater than 75 Khz. For DXing it seems then you need no more than say 75Khz. A tuner with that bandwidth would, in a sense, (Ithink) compared to a regular HiFi tuneer be a narrow bandwidth tuner. BTW, what would be the result if you used say a 20Khz filter on a FM signal with 75 Khz deviation? Would you get distortion or a perfectly copyable signal. I mean is it the analagous to using a 2Khz filter for an AM signal transmitted at 6Khz wide? |
"Pete KE9OA" ) writes:
It wouldn't be quite the same. You would be clipping the sidebands, and experience quite a bit of distortion. A 110kHz filter is about as narrow as you can go. I've been meaning to come up with a tuner that would be in the class of a McIntosh MR78 for the past couple of years, but something has always come up. Maybe after my current project, I will do this, if there is enough interest. Pete For most people, it makes more sense to simply change the filters in an existing FM BCB receiver than start from scratch. Indeed, it seems to be a relatively common practice among people who DX that band. Not that building something from scratch wouldn't be interesing, only that if "narrow bandwidth" is all that's wanted, then there's no sense in building it all. And there isn't much sense in putting narrow filters in a mediocre homebuilt FM receiver, which is the sort of thing you see in construction articles. I use Delco digitally tuned car radios as my "table radios", running them off power supplies. For the price, a few dollars at garage sales, they are pretty good receivers on the FM band. I know it would benefit from a narrow filter for a few stations I like to listen to. But of course, a lot of FM receivers aren't that great for distant reception, being too sensitive, without good overload protection. In some cases, it might be intriguing to build a single channel FM BCB receiver. Build it like a ham band converter, with plenty of tuned circuits at the fronte end, little or no RF amplification, and a good mixer. Being fixed tuned, one could optimize it for that frequency, and not worry about tracking, or the problems of ganging a number of tuned circuits. For the local oscillator, one could go with a crystal oscillator chain. Michael VE2BVW Richard wrote in message ... Dr. A.T. Squeegee wrote: In article , says... Hi. Anybody developed a nbfm RX project covering the FM band (appx 87.5Mhz-108Mhx). NBFM? As in narrow band? What would be the point? Here in the U.S. at least, that entire band is assigned to FM broadcasting, and it is anything but narrow-band. Typical deviation from a broadcast station is 75+ kHz. Maybe I used the wrong term. I think lots of HiFi tuners have very wide filters much greater than 75 Khz. For DXing it seems then you need no more than say 75Khz. A tuner with that bandwidth would, in a sense, (Ithink) compared to a regular HiFi tuneer be a narrow bandwidth tuner. BTW, what would be the result if you used say a 20Khz filter on a FM signal with 75 Khz deviation? Would you get distortion or a perfectly copyable signal. I mean is it the analagous to using a 2Khz filter for an AM signal transmitted at 6Khz wide? |
Richard wrote:
I hear that FM RX's are pretty complicated affairs. Most FM DXers it seems just modify commercial sets. Reduce bandwidth from say 230Khz to 110 Khz. I suppose that going this way has quite a lot of merit. Cheaper probably. It's certainly the most bang for the buck. Really, a good FM receiver isn't much more complicated than a good AM receiver -- but a bare-bones-just-barely-receives-the-strongest-stations FM receiver is quite a bit more complicated than a bare-bones-..... AM set. Usually you can reduce the bandwidth of a FM receiver by simply removing the monolithic ceramic filters and replacing them. I've done that on my Technics ST-G50; at my location 30 miles outside Nashville, I have received at least one DX station on every frequency that doesn't have a local. (yes, that includes the frequencies adjacent to 100,000-watt locals) Forget what I paid for the filters - it was definitely less than $5 apiece. My tuner needed two. ============================= Regarding going to 20KHz bandwidth... In DX situations with heavy interference, a very narrow bandwidth might be helpful for identifying DX stations. The programming will be mostly unintelligible in a 20KHz bandwidth, but maybe it'll be more intelligible than it would be against the interference from adjacent channels in a more reasonable bandwidth. I occasionally use the narrow filters in my TH-F6 HT to DX television audio. Nobody would dream of listening to that audio for entertainment but one can identify things they'd never ID on a TV set. Don't know anyone who's tried putting a 10.7MHz IF 20KHz or similar bandwidth filter in a FM broadcast tuner. You'd want it to be one of multiple bandwidths, so you could select something more reasonable for stronger signals. -- Doug Smith W9WI Pleasant View (Nashville), TN EM66 http://www.w9wi.com |
Richard wrote:
I hear that FM RX's are pretty complicated affairs. Most FM DXers it seems just modify commercial sets. Reduce bandwidth from say 230Khz to 110 Khz. I suppose that going this way has quite a lot of merit. Cheaper probably. It's certainly the most bang for the buck. Really, a good FM receiver isn't much more complicated than a good AM receiver -- but a bare-bones-just-barely-receives-the-strongest-stations FM receiver is quite a bit more complicated than a bare-bones-..... AM set. Usually you can reduce the bandwidth of a FM receiver by simply removing the monolithic ceramic filters and replacing them. I've done that on my Technics ST-G50; at my location 30 miles outside Nashville, I have received at least one DX station on every frequency that doesn't have a local. (yes, that includes the frequencies adjacent to 100,000-watt locals) Forget what I paid for the filters - it was definitely less than $5 apiece. My tuner needed two. ============================= Regarding going to 20KHz bandwidth... In DX situations with heavy interference, a very narrow bandwidth might be helpful for identifying DX stations. The programming will be mostly unintelligible in a 20KHz bandwidth, but maybe it'll be more intelligible than it would be against the interference from adjacent channels in a more reasonable bandwidth. I occasionally use the narrow filters in my TH-F6 HT to DX television audio. Nobody would dream of listening to that audio for entertainment but one can identify things they'd never ID on a TV set. Don't know anyone who's tried putting a 10.7MHz IF 20KHz or similar bandwidth filter in a FM broadcast tuner. You'd want it to be one of multiple bandwidths, so you could select something more reasonable for stronger signals. -- Doug Smith W9WI Pleasant View (Nashville), TN EM66 http://www.w9wi.com |
Richard wrote:
Maybe I used the wrong term. I think lots of HiFi tuners have very wide filters much greater than 75 Khz. For DXing it seems then you need no more than say 75Khz. A tuner with that bandwidth would, in a sense, (Ithink) compared to a regular HiFi tuneer be a narrow bandwidth tuner. Yes, you used the wrong termgrin. "NBFM" has a specific technical meaning, IIRC a system where the modulation index (ratio of peak deviation to maximum modulating frequency) is less than 1. For FM broadcast, the peak deviation is 75KHz and the maximum modulating frequency 15KHz. (OK, I'm ignoring stereo...) So the modulation index is 5. For police radio, the peak deviation is roughly 3KHz and so is the maximum modulating frequency. So the modulation index is 1. BTW, what would be the result if you used say a 20Khz filter on a FM signal with 75 Khz deviation? Would you get distortion or a perfectly copyable signal. I mean is it the analagous to using a 2Khz filter for an AM signal transmitted at 6Khz wide? Lots of distortion. In AM, distance from the center of the channel correlates to modulating frequency. Restricting the filter bandwidth in the receiver restricts the frequency response - the high audio frequencies ("treble") are rolled off. But it has no effect on the range of *amplitudes* that can be received; a loud sound within the bandpass of the filter will still be reproduced accurately. In FM, distance from the center of the channel correlates to modulating *amplitude*. A loud sound will push the transmitted signal to the outer edges of the channel. If the receiver's filter is too narrow to pass that, the signal peaks will be chopped off, resulting in severe interference. Remember that the 75KHz peak deviation for FM broadcast is 75KHz *either side* of center. To get the actual bandwidth required you have to add the peak modulating frequency to that. 165KHz, not counting stereo. You can chop some of that off at the expense of some distortion. I've found in practice, 110KHz filters work fine for DXing though I wouldn't want to listen to a symphony through them. 75KHz would probably be reasonably intelligible. -- Doug Smith W9WI Pleasant View (Nashville), TN EM66 http://www.w9wi.com |
Richard wrote:
Maybe I used the wrong term. I think lots of HiFi tuners have very wide filters much greater than 75 Khz. For DXing it seems then you need no more than say 75Khz. A tuner with that bandwidth would, in a sense, (Ithink) compared to a regular HiFi tuneer be a narrow bandwidth tuner. Yes, you used the wrong termgrin. "NBFM" has a specific technical meaning, IIRC a system where the modulation index (ratio of peak deviation to maximum modulating frequency) is less than 1. For FM broadcast, the peak deviation is 75KHz and the maximum modulating frequency 15KHz. (OK, I'm ignoring stereo...) So the modulation index is 5. For police radio, the peak deviation is roughly 3KHz and so is the maximum modulating frequency. So the modulation index is 1. BTW, what would be the result if you used say a 20Khz filter on a FM signal with 75 Khz deviation? Would you get distortion or a perfectly copyable signal. I mean is it the analagous to using a 2Khz filter for an AM signal transmitted at 6Khz wide? Lots of distortion. In AM, distance from the center of the channel correlates to modulating frequency. Restricting the filter bandwidth in the receiver restricts the frequency response - the high audio frequencies ("treble") are rolled off. But it has no effect on the range of *amplitudes* that can be received; a loud sound within the bandpass of the filter will still be reproduced accurately. In FM, distance from the center of the channel correlates to modulating *amplitude*. A loud sound will push the transmitted signal to the outer edges of the channel. If the receiver's filter is too narrow to pass that, the signal peaks will be chopped off, resulting in severe interference. Remember that the 75KHz peak deviation for FM broadcast is 75KHz *either side* of center. To get the actual bandwidth required you have to add the peak modulating frequency to that. 165KHz, not counting stereo. You can chop some of that off at the expense of some distortion. I've found in practice, 110KHz filters work fine for DXing though I wouldn't want to listen to a symphony through them. 75KHz would probably be reasonably intelligible. -- Doug Smith W9WI Pleasant View (Nashville), TN EM66 http://www.w9wi.com |
On Fri, 19 Dec 2003 22:11:23 -0000, "Richard"
wrote: Hi. Anybody developed a nbfm RX project covering the FM band (appx 87.5Mhz-108Mhx). With digital readout, though I suppose I could just hook up a simple a frequency counter to read frequency. TIA. Rich. If you want to do DX'ing get an existing receiver and put in a narrower filter. Many of the older tuners are ideal for this. I saw writeups on this in magazines over 20 years ago and don't remember what bandwidth filter was recommended. An interesting project would be mods to the GE SuperRadio II or III. They are cheap, available and have a tuned RF stage. 73, Jim |
On Fri, 19 Dec 2003 22:11:23 -0000, "Richard"
wrote: Hi. Anybody developed a nbfm RX project covering the FM band (appx 87.5Mhz-108Mhx). With digital readout, though I suppose I could just hook up a simple a frequency counter to read frequency. TIA. Rich. If you want to do DX'ing get an existing receiver and put in a narrower filter. Many of the older tuners are ideal for this. I saw writeups on this in magazines over 20 years ago and don't remember what bandwidth filter was recommended. An interesting project would be mods to the GE SuperRadio II or III. They are cheap, available and have a tuned RF stage. 73, Jim |
"Jim, N2VX" ) writes:
On Fri, 19 Dec 2003 22:11:23 -0000, "Richard" wrote: Hi. Anybody developed a nbfm RX project covering the FM band (appx 87.5Mhz-108Mhx). With digital readout, though I suppose I could just hook up a simple a frequency counter to read frequency. TIA. Rich. If you want to do DX'ing get an existing receiver and put in a narrower filter. Many of the older tuners are ideal for this. I saw writeups on this in magazines over 20 years ago and don't remember what bandwidth filter was recommended. An interesting project would be mods to the GE SuperRadio II or III. They are cheap, available and have a tuned RF stage. 73, Jim That's a bit misleading. Most FM BCB receivers, now and in the past, have an RF stage ahead of the mixer, and it is tuned. I once had a nice Sony stereo receiver that worked well on FM, and it's lack of an amplifer ahead of the mixer was pretty uncommon (and likely accounted for it's good overload resistance). The "TRF" stage in a Superradio is in reference to the AM band, where in non-car radios, an RF stage is an exception. Also, while it does seem a bit of work has been done in the design for better AM reception, that RF stage and the better than average loopstick, it does not seem like the FM section is anything to write home about. I gather it's a fairly generic design. This is part of the mythology of the Superradio. It's not all that great, just maybe a little bit better than the average portable or table radio, but people point to it when the concept of "a better radio" comes up. Once you start spending the money, better to spend it on a better design. Or better, buy a car radio that will have good reception, or some used stereo that did have a bit of extra care in it's design, so it actually won't overload in an urban environment. Michael VE2BVW |
"Jim, N2VX" ) writes:
On Fri, 19 Dec 2003 22:11:23 -0000, "Richard" wrote: Hi. Anybody developed a nbfm RX project covering the FM band (appx 87.5Mhz-108Mhx). With digital readout, though I suppose I could just hook up a simple a frequency counter to read frequency. TIA. Rich. If you want to do DX'ing get an existing receiver and put in a narrower filter. Many of the older tuners are ideal for this. I saw writeups on this in magazines over 20 years ago and don't remember what bandwidth filter was recommended. An interesting project would be mods to the GE SuperRadio II or III. They are cheap, available and have a tuned RF stage. 73, Jim That's a bit misleading. Most FM BCB receivers, now and in the past, have an RF stage ahead of the mixer, and it is tuned. I once had a nice Sony stereo receiver that worked well on FM, and it's lack of an amplifer ahead of the mixer was pretty uncommon (and likely accounted for it's good overload resistance). The "TRF" stage in a Superradio is in reference to the AM band, where in non-car radios, an RF stage is an exception. Also, while it does seem a bit of work has been done in the design for better AM reception, that RF stage and the better than average loopstick, it does not seem like the FM section is anything to write home about. I gather it's a fairly generic design. This is part of the mythology of the Superradio. It's not all that great, just maybe a little bit better than the average portable or table radio, but people point to it when the concept of "a better radio" comes up. Once you start spending the money, better to spend it on a better design. Or better, buy a car radio that will have good reception, or some used stereo that did have a bit of extra care in it's design, so it actually won't overload in an urban environment. Michael VE2BVW |
Doug Smith W9WI wrote:
edges of the channel. If the receiver's filter is too narrow to pass that, the signal peaks will be chopped off, resulting in severe interference. Oops. That should have read "...severe *distortion*." -- Doug Smith W9WI Pleasant View (Nashville), TN EM66 http://www.w9wi.com |
Doug Smith W9WI wrote:
edges of the channel. If the receiver's filter is too narrow to pass that, the signal peaks will be chopped off, resulting in severe interference. Oops. That should have read "...severe *distortion*." -- Doug Smith W9WI Pleasant View (Nashville), TN EM66 http://www.w9wi.com |
"Jim, N2VX" wrote in message ... An interesting project would be mods to the GE SuperRadio II or III. They are cheap, available and have a tuned RF stage. And an essentially uncalibrated frequency dial. Pretty bad for DXing IMO. Almost any digitally-tuned car radio would be better for DXing. Remember that cars drive through all the worst reception areas and their radios have to at least try to work there. |
"Jim, N2VX" wrote in message ... An interesting project would be mods to the GE SuperRadio II or III. They are cheap, available and have a tuned RF stage. And an essentially uncalibrated frequency dial. Pretty bad for DXing IMO. Almost any digitally-tuned car radio would be better for DXing. Remember that cars drive through all the worst reception areas and their radios have to at least try to work there. |
In article ,
(Michael Black) writes: In some cases, it might be intriguing to build a single channel FM BCB receiver. Build it like a ham band converter, with plenty of tuned circuits at the fronte end, little or no RF amplification, and a good mixer. Being fixed tuned, one could optimize it for that frequency, and not worry about tracking, or the problems of ganging a number of tuned circuits. For the local oscillator, one could go with a crystal oscillator chain. Going to www.KitsAndParts.com by Dieter Gentzow, W8DIZ, you can find the old Motorola MC3362P (2 for $4) which is a complete FM receiver IC. There's a full Motorola datasheet there with application data. A one-IC FM receiver for headphones. For some audio power output, he has National LM380s at 4 for $5. For an RF preamp, he has Fairchild J310 J-FETs at 12 for $4. There's also a respectable number of Micrometals toroid cores for IF and RF filtering. Datasheets for all except a full info range on the Micrometals cores (which some think are made by Amidon). www.micrometals.com for full core information. If the MC3362 is too much or too little, Dieter has 612 Gilbert cell ICs for mixer-oscillator applications and MC1349 gain blocks (its a slightly higher gain version of MC1350) for the IF. A fairly good supplier of oldies but goodies in semiconductors and toroid cores. LHA retired (from regular hours) electronic engineer person |
In article ,
(Michael Black) writes: In some cases, it might be intriguing to build a single channel FM BCB receiver. Build it like a ham band converter, with plenty of tuned circuits at the fronte end, little or no RF amplification, and a good mixer. Being fixed tuned, one could optimize it for that frequency, and not worry about tracking, or the problems of ganging a number of tuned circuits. For the local oscillator, one could go with a crystal oscillator chain. Going to www.KitsAndParts.com by Dieter Gentzow, W8DIZ, you can find the old Motorola MC3362P (2 for $4) which is a complete FM receiver IC. There's a full Motorola datasheet there with application data. A one-IC FM receiver for headphones. For some audio power output, he has National LM380s at 4 for $5. For an RF preamp, he has Fairchild J310 J-FETs at 12 for $4. There's also a respectable number of Micrometals toroid cores for IF and RF filtering. Datasheets for all except a full info range on the Micrometals cores (which some think are made by Amidon). www.micrometals.com for full core information. If the MC3362 is too much or too little, Dieter has 612 Gilbert cell ICs for mixer-oscillator applications and MC1349 gain blocks (its a slightly higher gain version of MC1350) for the IF. A fairly good supplier of oldies but goodies in semiconductors and toroid cores. LHA retired (from regular hours) electronic engineer person |
Actually, they aren't much different than an AM design, except that
generally, you use quadrature detection for demodulation, and that you have a deemphasis filter. The I.F. bandwidth is different, but pretty much everything else is the same. Pete Richard wrote in message ... "Pete KE9OA" wrote in message ... It wouldn't be quite the same. You would be clipping the sidebands, and experience quite a bit of distortion. A 110kHz filter is about as narrow as you can go. I've been meaning to come up with a tuner that would be in the class of a McIntosh MR78 for the past couple of years, but something has always come up. Maybe after my current project, I will do this, if there is enough interest. Pete I hear that FM RX's are pretty complicated affairs. Most FM DXers it seems just modify commercial sets. Reduce bandwidth from say 230Khz to 110 Khz. I suppose that going this way has quite a lot of merit. Cheaper probably. |
Actually, they aren't much different than an AM design, except that
generally, you use quadrature detection for demodulation, and that you have a deemphasis filter. The I.F. bandwidth is different, but pretty much everything else is the same. Pete Richard wrote in message ... "Pete KE9OA" wrote in message ... It wouldn't be quite the same. You would be clipping the sidebands, and experience quite a bit of distortion. A 110kHz filter is about as narrow as you can go. I've been meaning to come up with a tuner that would be in the class of a McIntosh MR78 for the past couple of years, but something has always come up. Maybe after my current project, I will do this, if there is enough interest. Pete I hear that FM RX's are pretty complicated affairs. Most FM DXers it seems just modify commercial sets. Reduce bandwidth from say 230Khz to 110 Khz. I suppose that going this way has quite a lot of merit. Cheaper probably. |
I was thinking about going with a high level mixer, something with an IP3 in
the +30dBm range. I would probably make it a double conversion unit. This way, I could use a bandpass filter at the front end. You are right, though.................there are quite a few good performers already on the market, and those auto radios are no exception. The newer ones that use the Philips dual conversion chipset are very good on the AM broadcast band, too. Pete Michael Black wrote in message ... "Pete KE9OA" ) writes: It wouldn't be quite the same. You would be clipping the sidebands, and experience quite a bit of distortion. A 110kHz filter is about as narrow as you can go. I've been meaning to come up with a tuner that would be in the class of a McIntosh MR78 for the past couple of years, but something has always come up. Maybe after my current project, I will do this, if there is enough interest. Pete For most people, it makes more sense to simply change the filters in an existing FM BCB receiver than start from scratch. Indeed, it seems to be a relatively common practice among people who DX that band. Not that building something from scratch wouldn't be interesing, only that if "narrow bandwidth" is all that's wanted, then there's no sense in building it all. And there isn't much sense in putting narrow filters in a mediocre homebuilt FM receiver, which is the sort of thing you see in construction articles. I use Delco digitally tuned car radios as my "table radios", running them off power supplies. For the price, a few dollars at garage sales, they are pretty good receivers on the FM band. I know it would benefit from a narrow filter for a few stations I like to listen to. But of course, a lot of FM receivers aren't that great for distant reception, being too sensitive, without good overload protection. In some cases, it might be intriguing to build a single channel FM BCB receiver. Build it like a ham band converter, with plenty of tuned circuits at the fronte end, little or no RF amplification, and a good mixer. Being fixed tuned, one could optimize it for that frequency, and not worry about tracking, or the problems of ganging a number of tuned circuits. For the local oscillator, one could go with a crystal oscillator chain. Michael VE2BVW Richard wrote in message ... Dr. A.T. Squeegee wrote: In article , says... Hi. Anybody developed a nbfm RX project covering the FM band (appx 87.5Mhz-108Mhx). NBFM? As in narrow band? What would be the point? Here in the U.S. at least, that entire band is assigned to FM broadcasting, and it is anything but narrow-band. Typical deviation from a broadcast station is 75+ kHz. Maybe I used the wrong term. I think lots of HiFi tuners have very wide filters much greater than 75 Khz. For DXing it seems then you need no more than say 75Khz. A tuner with that bandwidth would, in a sense, (Ithink) compared to a regular HiFi tuneer be a narrow bandwidth tuner. BTW, what would be the result if you used say a 20Khz filter on a FM signal with 75 Khz deviation? Would you get distortion or a perfectly copyable signal. I mean is it the analagous to using a 2Khz filter for an AM signal transmitted at 6Khz wide? |
I was thinking about going with a high level mixer, something with an IP3 in
the +30dBm range. I would probably make it a double conversion unit. This way, I could use a bandpass filter at the front end. You are right, though.................there are quite a few good performers already on the market, and those auto radios are no exception. The newer ones that use the Philips dual conversion chipset are very good on the AM broadcast band, too. Pete Michael Black wrote in message ... "Pete KE9OA" ) writes: It wouldn't be quite the same. You would be clipping the sidebands, and experience quite a bit of distortion. A 110kHz filter is about as narrow as you can go. I've been meaning to come up with a tuner that would be in the class of a McIntosh MR78 for the past couple of years, but something has always come up. Maybe after my current project, I will do this, if there is enough interest. Pete For most people, it makes more sense to simply change the filters in an existing FM BCB receiver than start from scratch. Indeed, it seems to be a relatively common practice among people who DX that band. Not that building something from scratch wouldn't be interesing, only that if "narrow bandwidth" is all that's wanted, then there's no sense in building it all. And there isn't much sense in putting narrow filters in a mediocre homebuilt FM receiver, which is the sort of thing you see in construction articles. I use Delco digitally tuned car radios as my "table radios", running them off power supplies. For the price, a few dollars at garage sales, they are pretty good receivers on the FM band. I know it would benefit from a narrow filter for a few stations I like to listen to. But of course, a lot of FM receivers aren't that great for distant reception, being too sensitive, without good overload protection. In some cases, it might be intriguing to build a single channel FM BCB receiver. Build it like a ham band converter, with plenty of tuned circuits at the fronte end, little or no RF amplification, and a good mixer. Being fixed tuned, one could optimize it for that frequency, and not worry about tracking, or the problems of ganging a number of tuned circuits. For the local oscillator, one could go with a crystal oscillator chain. Michael VE2BVW Richard wrote in message ... Dr. A.T. Squeegee wrote: In article , says... Hi. Anybody developed a nbfm RX project covering the FM band (appx 87.5Mhz-108Mhx). NBFM? As in narrow band? What would be the point? Here in the U.S. at least, that entire band is assigned to FM broadcasting, and it is anything but narrow-band. Typical deviation from a broadcast station is 75+ kHz. Maybe I used the wrong term. I think lots of HiFi tuners have very wide filters much greater than 75 Khz. For DXing it seems then you need no more than say 75Khz. A tuner with that bandwidth would, in a sense, (Ithink) compared to a regular HiFi tuneer be a narrow bandwidth tuner. BTW, what would be the result if you used say a 20Khz filter on a FM signal with 75 Khz deviation? Would you get distortion or a perfectly copyable signal. I mean is it the analagous to using a 2Khz filter for an AM signal transmitted at 6Khz wide? |
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