Re: Basics of Frequency Shifting


The problem is unusual in that I don't have physical access to either the
sources or the circuit inputs. They are remote. I am stuck with having
two independent sources at slightly different frequencies arriving via
independent circuits with different delays. What I do know is the
relative signal phases at the sources at a specific time.

Interesting! How?

There are two separate circuits with equal delays between the sources and my
remote measurement point independent of the two other circuits whose delays
I am trying to measure. I didn't want to complicate the question by going
into this, but I do accurately know the relative source phases. If I could,
I'd use conventional delay measurement techniques, but these two
uncorrelated signals are the only measurement means I have and I'm trying to
work with them.


So one way to determine differential circuit delay is to measure relative
phase of the arriving signals at the same time when the relative phases
are known at the sources. However this is subject to errors in
measurement time.

Indeed. And to other errors as well.

Yes, but the other errors are manageable. Phase measurement error due to
imprecise measurement time is the one that is difficult.


I'm trying to reduce or eliminate sensitivity to time of measurement by
shifting both received signals to a common frequency. The shifting
process may alter the signal phases, but it should do the same to both
circuits thereby not affecting the relative received phases that I want
to measure. Conceptually is there a flaw in this approach?

Yes. The phase shift will depend on original and final frequencies,
thereby differing from one source to another. The phase shift through the
network will also depend on the actual frequency.

Understood. Picture it this way: Assume the uncorrelated signals are at
exactly the same frequency. We can measure their relative phase at the
circuit output and compare that to their relative phase at the circuit input
to determine the relative circuit delay. We are looking for time delay
rather than phase itself.

Now assume slightly different signal frequencies. Now relative phase in and
out have to be measured at the same time to determine circuit delay time.
That's hard. Can I elminate the time variable by shifting both signals to
the same frequency while preserving the relative circuit delay so I can
measure it?

Phase shift also depends on signal frequency as you stated, but in this case
circuit phase shift vs. signal frequency is negligible. The frequency
difference is very small, but still bothersome for my purposes and I'd like
to eliminate it.


There is a fundamental conceptual error here. relative phase has a well
defined meaning only with signals that are exactly the same frequency. It
is possible to assign meanings in other cases, but the assignments must be
made explicitly for each case. How do _you_ propose to compare the phases
of the signals _in this case_? If you cannot be explicit, you are blowing
smoke.

Understood. My goal is to measure relative circuit time delay. I can
compare the input and output phases to/from the two circuits. This
information is available at the measurement location via independent paths.
But both relative phase measurements have to be done at the same time
because the frequencies are different. Making simultaneous measurements is
hard. To reduce the dependency on time of measurement I am proposing to
shift the signals to exactly the same frequency, and my question is will
doing this destroy their relative phase relationship that I am trying to
measure in order to determine relative circuit delay time?

Mason



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