On Fri, Sep 30, 2005 at 10:47:47AM -0500, Jorge Chávez wrote:
> I'm trying to FM TX with the fm_tx4.py but it doesn work for comercial FM
> Band (88 to 108MHz) but I need to TX in this frecuencys.
> Whe I submit my needs in the gnuradio discuss I get the next tip:
>
> Another trick to get higher frequencies is to use an image that is
> produced the DAC. If you program the DAC to 29.3 MHz, you'll also get RF
> at the SampleRate +/- DAC Freq. So you'll get a signal at 128 - 29.3 and
> 128 + 29.3. So in this case 98.7 and 157.3. The signal levels will be
> lower, but with the proper bandpass filter and amplifier chain, you can
> generate a complete signal.
It really works. No sample rate change required.
If you have access to a spectrum analyzer, this is easily verified.
Richard Lyons' book "Understanding Digital Signal Processing" has a
very clear explanation of the phenomenon.
There are images produced each 64 MHz (sampling rate / 2). Every
other one of them has the spectrum inverted, but that won't matter for
FM. You might try googling for "Nyquist zone" or "bandpass sampling"
http://www.national.com/appinfo/adc/files/Undersampling.pdf
Note that broadcast FM is much wider-band than narrow-band FM.
Very similar, but putting together a wfm tx block would make sense.
If you just connect a wire to the SMA connector and the FM receiver is
close by you can probably hear it. For a real solution, you'll want
a bandpass filter and amp as described above.