I've been following the thread about Erik's DIY PN analyzer, and
wondering if it might be easy enough to use a frequency discrimination
method. I'm opening this in a different thread to avoid muddying the
water on the original (and long) one.
What I'm picturing is putting the DUT's output into a quadrature power
splitter that optionally has a voltage-tuned slight phase shift feature.
The I and Q outputs would go into the DBM and produce the nearly-zero DC
plus baseband signal for analysis as in the original story.
If the quadrature is precise and stable enough, the DC out should be
close to zero, and since the baseband is ultimately AC coupled to the
analyzer, small offset should be OK, within reason.
If this is not sufficient, then having a phase tuning feature could be
used to form a PLL to hold the DC at zero. The big difference here is
that instead of locking a separate reference source to the DUT, the
relative phase at the mixer just has to be fine tuned to maintain the
output DC. The same sorts of PLL requirements are encountered to get the
results, but no external reference (and its noise and lock range etc
issues) is needed.
The downside is that a different quadrature splitter would probably be
needed for each DUT frequency to be applied - I'm picturing ones for 5
and 10 MHz initially. Those 90 degree broadband splitters that Mike
mentioned seem very interesting too.
There is still the necessity of calibration, either way.
Ed
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