Re: [Discuss-gnuradio] Quadrature demodulator update

[Dave Emery]

On Tue, Feb 11, 2003 at 07:16:47PM -0000, Philip Mackenzie wrote:
> Yes we did. I keep 2 channels at 50. I've tried it at both impedances and I
> experience the same interference. I have a feeling the answer lies in the
> realm of smith charts...
> 
> Philip.

        Not so clear to me.  If I understand you correctly the IF sampling
clock is getting into the front end of the receiver ?

        This would seem like a classic E and M sheilding, grounding and
isolation problem, not a matter of impedance matching per see.   And
indeed these can be bears to cure, though the usual ferrite beads,
filters, shielded tin plate boxes, good solid ground planes, and
isolation of the antenna are the standard cures.

        A spectrum analyzer might and might not help locate the 
problem, but look up material on shielding, grounding, interference
control and EMC testing and remediation (VDE and FCC A and B).

        Obviously a A/D card contains a good deal of energy at the
sampling clock rate and its harmonics as all sorts of signal lines
wiggle up and down at that rate.   What you need to do is ensure
that this energy stays with the card and does not get radiated into
your antenna or coupled into the radio via the IF tap.  Ground loop
issues can count here...



-- 
        Dave Emery N1PRE,  address@hidden  DIE Consulting, Weston, Mass. 
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