Re: [Discuss-gnuradio] Need some answers about FM TX

[Robert McGwier]

Jorge:

The paper you included and the mathematical and electrical phenomenon 
you are talking about applies to the analog to digital converter, the 
receiver only.  This is undersampling as you call it.  Most engineers 
call it bandpass sampling.  It applies to the use of aliasing to bring a 
higher frequencies down low on receive.  This cannot be done with all 
analog to digital converters.  It can be done with those analog to 
digital converters that can "see" the high frequency.  In order for them 
to see the high frequency, their conversion time must be small compared 
to the high frequency, and they must respond to the higher frequency. 

You cannot do the reverse on transmit.  It is a "one to many" problem.  
The only way you can get a higher frequency is for some (BAD) 
nonlinearity to be producing the higher frequency content. 

My apologies, I wish my Spanish was good.  I have included a Google 
translation and I hope it helps rather than hurts:

"El papel usted incluido y el fenómeno matemático y eléctrico que usted 
está hablando se aplica a convertidor a digital análogo, el receptor 
solamente.  Esto undersampling como usted lo llama. La mayoría de la 
llamada de los ingenieros él muestreo bandpass.  Se aplica al uso del 
aliasing de traer frecuencias más altas abajo bajo en recibe.  Esto no 
se puede hacer con los convertidores a digitales todo análogos.  Puede 
ser hecha con esos los convertidores a digitales análogos que pueden 
"ver" el de alta frecuencia.  En la orden para que consideren el de alta 
frecuencia, su tiempo de la conversión debe ser pequeño comparado al de 
alta frecuencia, y deben responder a la frecuencia más alta.  Usted no 
puede hacer el revés encendido transmite.  Es "el que esta' a muchos" 
problema.  La única manera que usted puede conseguir una frecuencia más 
alta está para que un cierto nonlinearity (del MALO) produzca el 
contenido de una frecuencia más alta.
"

Bob

Jorge Chávez wrote:

> If I run the fm_tx4.py, I TX in 29.3MHz and?? Which other 
> frecuencies??, Which one is the frecuency of the Sample Rate??
> Please Help me, and sorry for my english
>
>      
>     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.
>
>      
>
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