Re: [Discuss-gnuradio] Source of annoying problem found

[Rob Judd]

Ian,

I can only assume that you are using a frequency that is too high for
the card. If the samplerate selected was, say 48kHz then your IF cannot
be above 12kHz. Go do some reading on subjects like Nyquist frequency
and aliasing for a better understanding.

Rob VK3XVK


Ian Wraith wrote:
> 
> Hello
>       For the last week I have been chasing an annoying bug in my
> experimental DQPSK demodulator which was causing the I & Q channel
> waveforms from my quadrature demodulator to be distorted. To try and
> pin down this bug I borrowed a Agilent signal generator and set it
> to produce a 18 KHz waveform. I fed this into the sound cards line in
> and and set my program to write the data captured (from GrAudioSource
> <VrComplex>) to a file. When I looked at the initial data which had
> been through the root raised cosine filter then sample rate converted
> the sine waves had wildly varying amplitudes and the square waves
> looked very similar. Even when I removed the root raised filters
> and the sample rate converters the waveforms were still distorted.
> 
>   Just to confirm this wasn't due to a problem with GrAudioSource I
> used Audacity to record from the signal generator and to my suprise
> the waveforms looked the same. However when I lowered the output of the
> generator to 10 KHz the waveforms were much larger and not in the
> slightest distorted. I can only conclude therefore that the soundcard
> built onto my motherboard (an Asus A7V8X-X) has a hardware lowpass
> filter on its line inputs which is affecting my 18 KHz signals. Are such
> filters normal on a soundcard ??
> 
>   I'm afraid looks like my idea to demodulate PI/4 DQPSK with a soundcard
> isn't possible with this motherboard. So its roll on the USRP (I have a
> credit card burning a hole in my pocket !) then I can get back to this mode.
> Until then I might look at writing something to decode one of the simpler
> data modes in use over here.
> 
> Regards
> 
> Ian
> 
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