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#1
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#2
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Paul Keinanen wrote:
... Even with a proper S&H (ten nanosecond sampling and several microsecond hold times) for decimation, the 280 Ksamples/s sounds a bit low for FM broadcast detection. The higher order (Bessel function) sidebands are still quite strong with the modulation indexes used, so truncating the bandwidth to the 100-140 kHz range might not be a good idea. ... The IF passband of an FM receiver needs to be at least 200 KHz for good quality and -- counterintuitive to me -- I'm told that the capture ratio improves as the bandwidth increases. Back in the 50s, one premium receiver -- the first I knew of to use semiconductor diodes in the detector -- had a half-MHz IF. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ |
#3
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On Jun 23, 11:42 am, Jerry Avins wrote:
Paul Keinanen wrote: ... Even with a proper S&H (ten nanosecond sampling and several microsecond hold times) for decimation, the 280 Ksamples/s sounds a bit low for FM broadcast detection. The higher order (Bessel function) sidebands are still quite strong with the modulation indexes used, so truncating the bandwidth to the 100-140 kHz range might not be a good idea. ... The IF passband of an FM receiver needs to be at least 200 KHz for good quality and -- counterintuitive to me -- I'm told that the capture ratio improves as the bandwidth increases. Back in the 50s, one premium receiver -- the first I knew of to use semiconductor diodes in the detector -- had a half-MHz IF. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ Assuming cochannel signals with small power difference, as the BW increases the weaker one will hit threshold (knee in the SNRout vs SNRin curve) first. Does that explain it? John |
#4
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On Jun 23, 6:17 am, Paul Keinanen wrote:
Even with a proper S&H (ten nanosecond sampling and several microsecond hold times) for decimation, the 280 Ksamples/s sounds a bit low for FM broadcast detection. The IF is 1.7 MHz which is sampled using a 13.6 MHz clock by the AD9874 and decimated by a factor of 48 to produce the 280 Ksamples/s output. Looking at: http://en.wikipedia.org/wiki/Fm_broadcasting I see that FM audio goes from 0 to 53 KHz and RDS is at 57 KHz. The AD9874 decimation filter should prevent any signal from aliasing into the bandwidth of interest and the resulting sample rate is above Nyquist. I'm missing something ... assuming no aliasing problems why is a higher sampling rate necessary for recovering the audio and RDS? -- John |
#6
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On Sat, 23 Jun 2007 11:56:21 -0700, wrote:
The IF is 1.7 MHz which is sampled using a 13.6 MHz clock by the AD9874 and decimated by a factor of 48 to produce the 280 Ksamples/s output. With 1.7 MHz IF you are going to have to construct your own LC band pass filters in front of the AD9874, if 1.7 MHz is the first IF. If this is the second IF and you have proper filtering (ceramic or crystal) at the first IF, say 10.7 MHz, a very simple filter at 1.7 MHz should be sufficient. Due to not so spectacular performance of that chip, I would suggest using a selectable IF filter at 10,7 MHz (or whatever the first IF is), to even roughly match the required bandwidth of the transmission. If the IF filter would be 110-280 kHz wide to allow broadcast FM reception, using such receiver in Europe to receive the 7.0-7.1 MHz amateur band when there are very high power international broadcasters starting at 7.1 MHz, might give quite disappointing results, since those strong broadcast signals would control the AGC reducing sensitivity and you still would get a lot of intermodulation products. It also appears that the quoted dynamic range applies only to certain AGC settings. The use of 1.7 MHz IF with a very wide input filter may be problematic at least in countries that are still using the AM broadcast band actively, with very strong signals up to 1.6 MHz. On the other hand, local 1.8 MHz amateur radio signals may be quite strong. In any superheterodyne receiver, there is always a risk for signal break-through from the environment to the IF stage at that frequency range. For this reason, common IF frequencies, such as 10.7 MHz and 455 kHz, are usually excluded from transmitter frequency assignments. Looking at: http://en.wikipedia.org/wiki/Fm_broadcasting I see that FM audio goes from 0 to 53 KHz and RDS is at 57 KHz. While the modulation index for the RDS signal is quite low, thus producing only the first order Bessel sidebands at +/- 57 kHz from the carrier, the stereo difference signal S can have a quite high modulation index, thus producing the first pair of Bessel sidebands below +/-53 kHz from the carrier but also producing some significant second order Bessel sidebands below +/-106 kHz and possibly even the third order at +/-159 kHz. Of course, this requires that there is a strong high pitch tone only in one audio channel to get a huge difference signal. Paul OH3LWR |
#7
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![]() wrote in message ups.com... I'm working on a SDR design using the AD9874 to digitize the IF producing 280 Ksamples/sec and am trying to size the DSP. I'm leaning towards the ADSP-BF532 which is a fixed point DSP rated at 400 MIPS / 800 MMACS and is available in a LQFP package. I'd like to be able to handle everything from decoding standard shortwave SSB signals to broadcast FM stereo signals (including RDS). 1) Any pointers to information which can be used to size the DSP in terms of MIPS, program RAM, and data RAM? Something like a minimum of X MIPS are needed to do a reasonable job handling broadcast FM stereo signals (which I'm assuming require more MIPS than SSB) would be useful to know. 2) Anyone have a specific DSP they favored for this type of application? Keep in mind I need the DSP to be in a package I can handle using home equipment. It would be nice if the DSP was flexible enough so that it can be used for general purpose things such as controlling the keyboard and LCD instead of adding a microcontroller. I'm not looking to use a FPGA at this time. -- John Can you use one of the blackfin stamp boards ? http://blackfin.uclinux.org/gf/project/stamp Digikey sells the BF537 stamp board for US$226 http://www.digikey.com/scripts/DkSea...440505&Site=US http://www.analog.com/en/prod/0,2877...DSTAMP,00.html Could always try and make a network interface for controlling the radio. I'm just starting to look at what I need for getting a similar set up but A.M and ssb only for students to use in a lab to introduce them to the bascs of sdr. The lecturers want the students to program different modules , different types of modulation , encoding and decoding, set different sampling rates etc They want the students to be able use both c and matlab. Looks like we'll be using a c6713 dsk as the base (thanks to TI providing free software) I just get to assemble/build/integrate what gets specified. Someone in the faculty has a couple of lyrtech signalwave boards which they aren't using(been sitting in their filling cabinet for a few years), have to see if we can prise them off them. Then see what it costs to get support (need the up to date software). http://www.lyrtech.com/DSP-developme...signalwave.php Alex |
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