[Lf] [Fwd: LF: RE DDS Sources in receivers]

[Andre' Kesteloot]

Klaus von der Heide wrote:

> Hello all, hello Andy,
>
> Andy wrote:
> > As a test last night, I mixed the output of my DDS LF generator set to
> > generate 59.8kHz, (DDS clocked at 5MHz) with an off air MSF signal at
> > 60kHz.   The resulting 200 Hz output was low pass filtered and fed to
> > the soundcard and monitored with Gram and also on similar software for
> > the 56002EVm - EVMSPEC..  The only sidebands visible were those caused
> > by the switching of the MSF carrier, no sign of the jitter mentioned by
> > Klaus, DJ5HG.
>
> Fine! Obviously this chip is designed to that accuracy. Sorry, at the
> moment I have no time to measure the AD9850 down to 120 dB with my
> DSP56002EVM.
> (At that resolution you can see funny things. Some months ago I found
> spurs +-50 Hz and +-100 Hz from any analog carrier fed into the EVM.
> They are due to an amplitude modulation of the ADC by the supply!)
>
> > I still maintain that a DDS is one of the cleanest in terms of phase
> > noise, and certainly the most flexible frequency source around these
> > days.
>
> Yes, and the spurs are not an intrinsic DDS-problem, but only a
> problem introduced by simplification.
>
> > What are the implications of 60dBc sideband / phase noise anyway ?  They
> > are only going to matter if an INBAND signal is 60dB up on the wanted
> > one, and isn't that quite likely to give problems elsewhere in the
> > processing chain ?
>
> If your input filter stops all OUTBAND signals in the entire spectrum
> sufficiently, then only the INBAND problem will be present. May be,
> that's not a real problem.
>
> If this processing chain is digital at say 24 bits, the processing
> chain does not generate further problems.
>
> 73 de Klaus, DJ5HG