Re: Signal Package pwelch

[Thomas D. Dean]

On 8/13/19 2:12 PM, Thomas D. Dean wrote:
> On 8/13/19 1:32 PM, Thomas D. Dean wrote:
> > I have a application using the signal package pwelch function that
> > seems to produce an incorrect plot.
> >
> > [s, f] = pwelch(iq,[], overlap=0, NFFT=2048, Fs=sample_rate,
> > detrend='none', range='shift', plot_type = 'dB')
> >
> > iq is read from an rtl-sdr
> >
> > knowing the location of some signals, like the Hydrogen line, and,
> > comparing this to python, pwelch puts the "hydrogen bump" about .3MHz
> > too high.
> >
> > pwelch(iq,[], overlap=0, NFFT=2048, Fs=sample_rate, detrend='none',
> > range='shift', plot_type = 'dB');
> >
> > How do I upload an image to show this?
>
> More information,
>
> The "hydrogen bump" should be between 1420400000 and 1420500000 Hz.
> But, pwelch places it between 1420700000 and 1420850000
>
> octave:100> [s, f] = pwelch(iq,[], overlap=0, NFFT=2048, ...
>          Fs=sample_rate,  detrend='none', range='shift', ...
>          plot_type = 'dB');
> octave:101> freq
> freq =  1420400000
> octave:102> x = freq + f;
> octave:103> idx=find( ( x >= 1420700000 ) & ( x <= 1420850000 ) );
> octave:104> plot( x(idx), 10*log10(s(idx)));
>
> I can post the .cc and the .m code, if anyone is interested.
>
> Tom Dean

Fixed.  Strange character in the .m file.

Sorry for the noise.

Tom Dean