Ed,
On 7/9/22 22:26, ed breya via time-nuts wrote:
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.
There is no need for it. Using the PI loop, it will drive the phase
detector into quadrature and as it does this, the DC component of the
detector is cancelled as it is integrated into the integrator path I.
There is however use for quadrature splitter as you do a Costas loop,
which is needed for some modulation schemes. Then again, you really do
not need to use a quadrature splitter to achieve the needed quadrature
pair, but there is other tricks to achieve the same thing. The Tayloe
detector comes to mind, which uses a frequency 4 times higher, divides
it down and then drive the detector for a S/H style of mixer. See for
instance Elecraft KX3.
Cheers,
Magnus
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