[Discuss-gnuradio] Re: Need some answers about fm_tx

[Eric Blossom]

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.

>  Does it realy works? How and where can I change the sample-rate to make
> this image?

Yes.  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.

> Sorry for not submit this to the Discuss.
> and sorry for my english

No problem with your English. It's completely understandable.
But please post questions to the list.

> I hope your answer, and your advise.
> Thanks
> 
> Jorge Chavez

Eric