FM radio reception at ~24MHz?
Hi all, I picked up an SDR radio dongle, and have been playing around
with receiving. As I am playing around with it today, I am noticing something odd (to me). We have a college station locally that broadcasts at 88.1MHz FM. As with many college radio stations, it has a very weak signal, but I can just pull it in with rabbit ear antennas. As I was poking around at ~24MHz, and I was able to pull this same station in at 23.645MHz. Even more odd, I was getting a much better signal at 23.645MHz than at the 88.1MHz "official" frequency. Additionally, as I scanned around, I also found all of the other stations doing this as well; 91.9MHz, which is too weak to tune in, I can hear perfectly at 28.035MHz. I can also tune in to 96.1MHz at 30.240MHz, 105.5MHz at 30.325, and so forth. So here's my question, is this something that is "normal" in radio, or does this instead point to some peculiarity with my radio/software setup. Thanks for any suggestions, Jon |
FM radio reception at ~24MHz?
On Mon, 17 Mar 2014, Jon Danniken wrote:
Hi all, I picked up an SDR radio dongle, and have been playing around with receiving. As I am playing around with it today, I am noticing something odd (to me). We have a college station locally that broadcasts at 88.1MHz FM. As with many college radio stations, it has a very weak signal, but I can just pull it in with rabbit ear antennas. As I was poking around at ~24MHz, and I was able to pull this same station in at 23.645MHz. Even more odd, I was getting a much better signal at 23.645MHz than at the 88.1MHz "official" frequency. Additionally, as I scanned around, I also found all of the other stations doing this as well; 91.9MHz, which is too weak to tune in, I can hear perfectly at 28.035MHz. I can also tune in to 96.1MHz at 30.240MHz, 105.5MHz at 30.325, and so forth. So here's my question, is this something that is "normal" in radio, or does this instead point to some peculiarity with my radio/software setup. Thanks for any suggestions, Jon FM stations apparently can use something around 26 MHz for remote feeds, but I don't know the details other than it has come up in one of the newsgroups before, someonehearing FM broadcasting in the wrong place. Do those things do any conversion, or is it straight to baseband? If it converts to some other frequency first, then maybe you are seeing an image. And since those things aren't so well tuned, so image rejection might not be so great. That doesn't explain why the wrong frequency would have a stronger signal. Find out if it converts to an IF first, then do some math. MIchael |
FM radio reception at ~24MHz?
On Mon, 17 Mar 2014 13:15:16 -0700, Jon Danniken
wrote: Hi all, I picked up an SDR radio dongle, and have been playing around with receiving. As I am playing around with it today, I am noticing something odd (to me). We have a college station locally that broadcasts at 88.1MHz FM. As with many college radio stations, it has a very weak signal, but I can just pull it in with rabbit ear antennas. As I was poking around at ~24MHz, and I was able to pull this same station in at 23.645MHz. Even more odd, I was getting a much better signal at 23.645MHz than at the 88.1MHz "official" frequency. Additionally, as I scanned around, I also found all of the other stations doing this as well; 91.9MHz, which is too weak to tune in, I can hear perfectly at 28.035MHz. I can also tune in to 96.1MHz at 30.240MHz, 105.5MHz at 30.325, and so forth. So here's my question, is this something that is "normal" in radio, or does this instead point to some peculiarity with my radio/software setup. Thanks for any suggestions, Jon The 'front' end of many relatively low cost, wide range receivers has very poor selectivity. The output of the Mixer stage is the SUM and the DIFFERENCE between the local oscillator frequency and the incoming signal. The result is when you tune a frequency, you are really tuning to two different frequencies. The difference between one signal and the IF and the Sum of another signal and the IF, Usually there is some sort of pre-selector, bandpass filter or semi-tuned RF amplier that effectively blocks the unwanted frequency, so there is no energy in one of the two possibly tuned frequencies For example if you have a conventional FM receiver, traditionally the Intermediate frequency is 10.7 Mhz. So if you tuned the receiver Another strategy is to 'up convert' instead of down convert. This puts much more spectrum between the sum and difference frequency. All that your experience says is that SDR probably has poor front end selectivity. |
FM radio reception at ~24MHz?
On 2014-03-17 21:29:35 +0000, Michael Black said:
On Mon, 17 Mar 2014, Jon Danniken wrote: Hi all, I picked up an SDR radio dongle, and have been playing around with receiving. As I am playing around with it today, I am noticing something odd (to me). We have a college station locally that broadcasts at 88.1MHz FM. As with many college radio stations, it has a very weak signal, but I can just pull it in with rabbit ear antennas. As I was poking around at ~24MHz, and I was able to pull this same station in at 23.645MHz. Even more odd, I was getting a much better signal at 23.645MHz than at the 88.1MHz "official" frequency. Additionally, as I scanned around, I also found all of the other stations doing this as well; 91.9MHz, which is too weak to tune in, I can hear perfectly at 28.035MHz. I can also tune in to 96.1MHz at 30.240MHz, 105.5MHz at 30.325, and so forth. So here's my question, is this something that is "normal" in radio, or does this instead point to some peculiarity with my radio/software setup. Thanks for any suggestions, Jon FM stations apparently can use something around 26 MHz for remote feeds, but I don't know the details other than it has come up in one of the newsgroups before, someonehearing FM broadcasting in the wrong place. Do those things do any conversion, or is it straight to baseband? If it converts to some other frequency first, then maybe you are seeing an image. And since those things aren't so well tuned, so image rejection might not be so great. That doesn't explain why the wrong frequency would have a stronger signal. Find out if it converts to an IF first, then do some math. MIchael It is also common to find FM broadcast feeds up around 460 MHz. |
FM radio reception at ~24MHz?
On 03/17/2014 02:29 PM, Michael Black wrote:
FM stations apparently can use something around 26 MHz for remote feeds, but I don't know the details other than it has come up in one of the newsgroups before, someonehearing FM broadcasting in the wrong place. Do those things do any conversion, or is it straight to baseband? From my understanding, the tuner chip sends out an 8MHz (I think) "window" of information at zero-IF to the modulator/interface chip (RTL2832U). I did see a 28.800 crystal on mine when I opened it up to put in some shielding, though. Unfortunately I am rather new at all of this, so I'm not really up to speed on radio stuff yet. If it converts to some other frequency first, then maybe you are seeing an image. And since those things aren't so well tuned, so image rejection might not be so great. That doesn't explain why the wrong frequency would have a stronger signal. Find out if it converts to an IF first, then do some math. Thanks, Jon |
FM radio reception at ~24MHz?
On 03/17/2014 03:10 PM, matt weber wrote:
The 'front' end of many relatively low cost, wide range receivers has very poor selectivity. The output of the Mixer stage is the SUM and the DIFFERENCE between the local oscillator frequency and the incoming signal. The result is when you tune a frequency, you are really tuning to two different frequencies. The difference between one signal and the IF and the Sum of another signal and the IF, Usually there is some sort of pre-selector, bandpass filter or semi-tuned RF amplier that effectively blocks the unwanted frequency, so there is no energy in one of the two possibly tuned frequencies For example if you have a conventional FM receiver, traditionally the Intermediate frequency is 10.7 Mhz. So if you tuned the receiver Another strategy is to 'up convert' instead of down convert. This puts much more spectrum between the sum and difference frequency. All that your experience says is that SDR probably has poor front end selectivity. Thanks Matt. Jon |
FM radio reception at ~24MHz?
matt weber wrote:
All that your experience says is that SDR probably has poor front end selectivity. Almost none. The dongle was designed to pick up megawatt digital TV stations with short antennas (about 4-6 inches) antennas. Using it as an SDR is a happy accident. The ones sold as DVB-T dongles come with software for decoding digital TV broadcasts, some come with digital audio broadcast decoding software, most don't. The most popular E4000 receiver dongle did not come with FM broadcast reception software. Geoff. -- Geoffrey S. Mendelson, N3OWJ/4X1GM/KBUH7245/KBUW5379 |
FM radio reception at ~24MHz?
On Tue, 18 Mar 2014, Geoffrey S. Mendelson wrote:
matt weber wrote: All that your experience says is that SDR probably has poor front end selectivity. Almost none. The dongle was designed to pick up megawatt digital TV stations with short antennas (about 4-6 inches) antennas. Using it as an SDR is a happy accident. To be fair, it's not related to an SDR, but to a specific and cheap implemtnation of the idea. One can argue that a lot of low end wide band receivers don't have good front end selectivity, either. If there's a conversion to a high IF, image rejection can be done with a low pass filter. That's true with any receiver that converts to a high enough first IF. I seem to recall from a description that the Racal receiver that used the Wadley Loop had both a good low pass filter and a traditional LC front end, and one could switch between the two. The low pass was good enough for the image rejection, but of course meant all the signals from DC to 30MHz or wherever the LPF cutoff was seen by the first tube, so it had to handle all of that. Not necessarily a good thing. Michael The ones sold as DVB-T dongles come with software for decoding digital TV broadcasts, some come with digital audio broadcast decoding software, most don't. The most popular E4000 receiver dongle did not come with FM broadcast reception software. Geoff. -- Geoffrey S. Mendelson, N3OWJ/4X1GM/KBUH7245/KBUW5379 |
FM radio reception at ~24MHz?
matt weber wrote: All that your experience says is that SDR probably has poor front end selectivity. On 3/18/2014 12:59 AM, Geoffrey S. Mendelson wrote: Almost none. The dongle was designed to pick up megawatt digital TV stations with short antennas (about 4-6 inches) antennas. Using it as an SDR is a happy accident. A great tutorial / construction article on how to use the dongle as a DC to daylight software defined radio (SDR): QST magazine, January 2013, pp 30-35. If you know a member of the ARRL, they can print a copy of this article. |
FM radio reception at ~24MHz?
On 03/17/2014 01:15 PM, Jon Danniken wrote:
Hi all, I picked up an SDR radio dongle, and have been playing around with receiving. As I am playing around with it today, I am noticing something odd (to me). We have a college station locally that broadcasts at 88.1MHz FM. As with many college radio stations, it has a very weak signal, but I can just pull it in with rabbit ear antennas. As I was poking around at ~24MHz, and I was able to pull this same station in at 23.645MHz. Even more odd, I was getting a much better signal at 23.645MHz than at the 88.1MHz "official" frequency. Additionally, as I scanned around, I also found all of the other stations doing this as well; 91.9MHz, which is too weak to tune in, I can hear perfectly at 28.035MHz. I can also tune in to 96.1MHz at 30.240MHz, 105.5MHz at 30.325, and so forth. So here's my question, is this something that is "normal" in radio, or does this instead point to some peculiarity with my radio/software setup. Thanks for any suggestions, Jon Now you get to experience the joy of building a crude preselector. A 20 dB pad might work, try that first. |
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