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

[Fernando]

It worked using  "Frequency Mod"    and "FM Demod" this way I attach.



El 27/04/17 a las 09:33, Johannes Demel escribió:
> 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)

You mean modifying WBFM Transmit block?   I can see where I can change
it in the properties of the block.  It will be necessary to create a new
block?


regards







>
> 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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StereoFM-RX.grc.png
Description: PNG image

StereoFM-TX.grc.png
Description: PNG image