Basics of Frequency Shifting

Sorry if I was not clear.

I have no control over the sources.  The two sines are correlated in that I 
know their relative phase at a specific time at the sources.  They are at 
slightly different frequencies and arriving through two circuits with 
different delays.  I'm measuring the difference in time between selected 
events in the two sines - ie, waveform maxima - to determine the difference 
in the circuit delays, and I'm making that measurement at about the same 
time when I know their relative phase at the sources.

The problem is, errors in the time of measurement cause inaccurate 
measurement results because the waveforms are moving relative to each other 
vs. time as a result of their different frequencies.

So I believe a solution is to shift the frequency of each signal to a 
nominal frequency.  This would eliminate the requirement for precise 
measurement time because the sines will no longer have time-dependent phase 
difference.  The frequency shifting process will affect the signal phases 
but I believe it will affect both signals the same and therefore not alter 
the phase difference due to different circuit delays that I am trying to 
measure.

Am I going in the right direction?

Mason


=============
Jerry is right. You can't compare the phase between the two sines from
two oscillators unless they are somehow correlated. Maybe we did not
understand your question correctly?

James
www.go-ci.com 

Mason wrote:

> The input signal is essentially a pure tone as it is received, at approx but 
> not exactly 5 kHz.  I don't follow what you mean by mixing it with a pure 
> sine.  My goal is to shift (mix) it to precisely 5 kHz, always 5 kHz, like 
> an AFC circuit.

Do you mean that you can change the source?  Then you want a PLL which
will supply the signal to lock your source to a reference.

> The application is to determine the difference in phase shift through two 
> circuits driven by separate oscillators.  Since the oscillators have 
> slightly different frequencies I first need to normalize the frequencies 
> then measure the phase difference between the signals.

If you just want to know the difference between your source and a
reference 5kHz source then a mixer will supply the difference signal.

-- glen

On 6 Nov, 06:20, "Mason" <plsnos...@plsnospam.net> wrote:
> Sorry if I was not clear.
>
> I have no control over the sources. &#4294967295;The two sines are correlated in that I
> know their relative phase at a specific time at the sources. &#4294967295;They are at
> slightly different frequencies and arriving through two circuits with
> different delays.

This is a very elaborate way of saying that you have two
independent sources. The fact that you can compare them
at one time does not change the fact that they are independent.

>&#4294967295;I'm measuring the difference in time between selected
> events in the two sines - ie, waveform maxima

Beginner's mistake. It's way easier to use zero crossings
if you want accurate timings.

> - to determine the difference
> in the circuit delays, and I'm making that measurement at about the same
> time when I know their relative phase at the sources.

If you want to measure or characterize each of the cirquits,
then investigate each cirquit on its own.

> The problem is, errors in the time of measurement cause inaccurate
> measurement results because the waveforms are moving relative to each other
> vs. time as a result of their different frequencies.

That's a problem you can't solve. The reason you have that
problem is that you have chosen a very bad strategy to solve
a seemingly standard problem.

> So I believe a solution is

Don't believe. Investigate.

> to shift the frequency of each signal to a
> nominal frequency. &#4294967295;This would eliminate the requirement for precise
> measurement time because the sines will no longer have time-dependent phase
> difference.

Of course. "Eliminate the requirement for precise measurements."
Contemplate that sentence. Why not re-iterate the argument of
yours and get rid of the "frequency normalization" and eliminate
the need for measurements alltogether?

>&#4294967295;The frequency shifting process will affect the signal phases
> but I believe it will affect both signals the same and therefore not alter
> the phase difference due to different circuit delays that I am trying to
> measure.

If you want to investigate cirquit delays, then investigate
cirquit delays. Quit messing about with BS.

> Am I going in the right direction?

No.

Rune

Frequency is change phase in time. If you want measure only
dynamic difference of phases it is available. You should  use
two frequency detectors on both channels. Then submit
frequencies and weight result with integrating filter. It is your
phase difference. Minimal measurement frequency of changes
this phase depends of stability components. You may take 1/minute.
 But measurements on zero frequency naturally can�t be done.
 
Sergey Volodin

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.

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.

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?



"Rune Allnor" <allnor@tele.ntnu.no> wrote in message 
news:85e39370-5113-48a7-b347-b3d048ee8ccb@40g2000prx.googlegroups.com...
On 6 Nov, 06:20, "Mason" <plsnos...@plsnospam.net> wrote:
> Sorry if I was not clear.
>
> I have no control over the sources. The two sines are correlated in that I
> know their relative phase at a specific time at the sources. They are at
> slightly different frequencies and arriving through two circuits with
> different delays.

This is a very elaborate way of saying that you have two
independent sources. The fact that you can compare them
at one time does not change the fact that they are independent.

> I'm measuring the difference in time between selected
> events in the two sines - ie, waveform maxima

Beginner's mistake. It's way easier to use zero crossings
if you want accurate timings.

> - to determine the difference
> in the circuit delays, and I'm making that measurement at about the same
> time when I know their relative phase at the sources.

If you want to measure or characterize each of the cirquits,
then investigate each cirquit on its own.

> The problem is, errors in the time of measurement cause inaccurate
> measurement results because the waveforms are moving relative to each 
> other
> vs. time as a result of their different frequencies.

That's a problem you can't solve. The reason you have that
problem is that you have chosen a very bad strategy to solve
a seemingly standard problem.

> So I believe a solution is

Don't believe. Investigate.

> to shift the frequency of each signal to a
> nominal frequency. This would eliminate the requirement for precise
> measurement time because the sines will no longer have time-dependent 
> phase
> difference.

Of course. "Eliminate the requirement for precise measurements."
Contemplate that sentence. Why not re-iterate the argument of
yours and get rid of the "frequency normalization" and eliminate
the need for measurements alltogether?

> The frequency shifting process will affect the signal phases
> but I believe it will affect both signals the same and therefore not alter
> the phase difference due to different circuit delays that I am trying to
> measure.

If you want to investigate cirquit delays, then investigate
cirquit delays. Quit messing about with BS.

> Am I going in the right direction?

No.

Rune 

> Here is what I would do:
>
> take the original signal, x(t), multiply it by: cos(2 PI dF t), sin(2
> PI dF t)
>
> R(t) = x(t) cos(2 PI dF t)
> I(t) = x(t) sin(2 PI dF t)
>
> where dF is the delta frequency to be shifted.
>
> Now you have two time series signals: R(t) and I(t).
>
> Conduct complex Fourier transform using R(t) as real and I(t) as
> imaginary parts. The results will be frequency shifted spectrum.
>
> James
> www.go-ci.com

This is interesting, is it just a simple foreward then inverse FT on this, 
or is there another step?
Is this like single-side-band pitch shifting?

Thanks,
Dave

Mason wrote:
> 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?

> 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.

> 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.

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.

   ...

jerry
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

>> 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

Mason wrote:
>>> 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.

Unless the frequencies are precisely the same, the term "relative 
phases" is meaningless. I can't guess what you think you know, but it 
isn't relative phase.

>                                                     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?

The only way to know the relative phase between the input and output of 
any one circuit is to know that circuit's delay. So if you knew the 
delays of both paths, you could calculate the pseudo phases that would 
in turn allow you to determine the (already known) delays.

> 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.

The relative phase of the original signal and its frequency-shifted 
version is not definable. Don't tell me that you can look at graphs and 
tell what it is. When you can define criteria and write them into a 
program to do it according to a rule, then and only then will you 
convert me to the belief that you stand a chance of doing what you want.

>> 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. 

Not unless there frequencies are the same, or you rigorously define what 
you mean by phase _in this case_.

>                                                                  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. 

The same time at the input, or out the output? It can't be both.

>                                   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?

Unless the signals are at the same frequency, there is no relative phase.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

Mason wrote:
> 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.

> 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.

> 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?

If you shift the two differently then the relative phase will be
different.  If you want more answer, you need to supply more details
about what you are trying to measure and why.

If you shift them both the same (using mixers with the same input)
then only one can be tracked.

Is it that the sources are synchronous, but the path in between
is variable?  That is different.

-- glen