Re: [Discuss-gnuradio] WBFM modulation/demodulation high frequencies highly attenuated

[Johannes Demel]

Hi Fernando,

If you want to use FM modulation, WBFM might still be the block to use.
The const values for the low_pass filter can be changed. You just need 
to take care that your sample rates etc. do not cause aliasing.

The two values that need to be reassigned are passband cutoff and 
stopband cutoff. For FM Radio transmissions the values 16kHz and 18kHz 
make a lot of sense. (15kHz is the maximum audio frequency and there's a 
pilot sine at 19kHz).

There are multiple ways to fix your problem.
1. Just hard code new values in. It'll fix your particular problem but 
will break other flowgraphs you might use.
2. Derive the cutoff frequencies from audio and quad sample rate.
3. Add new options to the block where you can specify these two values. 
(Make 16 and 18kHz the respective defaults).
4. Add another optional parameter, where you can provide the filter taps 
for the interpolator. It'll low you to specify all your properties 
(including cutoff frequencies)

Of course there's at least one more option. WBFM is a hier block. Just 
manually put interpolator, preemph and modulator together. Compare line 
81. Then you'll be able to control all your parameters.

Have fun coding!

Cheers
Johannes

On 27.04.2017 00:35, Fernando wrote:
> It seems I using the block I should not use:  WBFM TRansmit
> Documentation for this block may be found at
> https://github.com/gnuradio/gnuradio/blob/master/gr-analog/python/analog/wfm_tx.py#L67
> It includes a preenfasis and a low pass filter with passband ending at
> 16Khz  so I can't use it for a FM stereo transmission whose bandwidth is
> 53Khz.
> I think i should use the block Frequency Mod, although I can find many
> information about this block.
>
> regards
>
>
>
> El 26/04/17 a las 20:02, Fernando escribió:
> > I'm modulating/demodulating a wide band FM signal and I observe that
> > high frequencies are highly  attenuated.
> >
> > This is the diagram
> >
> >
> > And this is the spectrum
> >
> > High frequencies are attenuated more than 40dB over low frequencies.
> >
> > It seems to happen for frequencies above 20Khz.
> >
> > Changing max deviation in WBFM Transmit has no effect.
> >
> > What am I doing wrong?
> >
> >
> > regards
> >
> >
> >
> >
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>
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