Seeing phase of an RF signal after down conversion to audio

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neazoi

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Hi I want to see the phase on an HF RF signal but my scope has gone bad.
Having an HF receiver available though.
So I am thinking to down convert this signal to audio, using this receiver, and then fed the audio to the PC, using an audio oscilloscope software.

As I am changing the phase of the RF signal from zero to 360 degrees, will I see an equal change in the phase of the audio signal from zero to 360 degrees?

I am only interested in relative phase change, not absolute. I.e I want to see the 0-360 degrees phase change. I do not care if some phase shift is introduced at the mixer stages of the receiver, as long as the phase can be varied at full 360 degrees.
 

How are you going to measure phase? Phase relative to what reference?
 

    neazoi

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barry said:
How are you going to measure phase? Phase relative to what reference?
Click to expand...
I am going to just measure it with the scope display squares. It does not need to be accurate, there may be some overlap, I do not care, as long as I can see a full cycle "moving" on (360 degrees approximately).


If this causes triggering problems, I could alternatively feed an audio signal of the same frequency as the downconverted, at the other channel of the PC scope, and compare these.

But will the phase change will be the same on the AF signal compared to the RF? I.e. if I change from zero to 10 degrees the phase of the RF signal, will the downconverted audio signal change also 0-10 degrees?
 
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FvM said:
Yes, this can be derived from general AM relations, see e.g. https://en.wikipedia.org/wiki/Amplitude_modulation

You need a SSB converter to perform the intended down conversion. The reference signal problem mentioned by barry remains nevertheless.
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Yes it is a ham radio transceiver, so the receiver works on SSB. It also has a TCXO reference driving the DDS and it is very stable.
I am not sure if the audio scope will trigger on every 0 degrees and ignore the phase change in the display of the waveform. If that happens I won't be able to see any visual phase change, as the sine waveform will always start at 0 degrees at the start of the screen.

If that is the case then I could use the internal audio generator function of the software scope, set at exactly the same frequency as the RF downconverted signal and maybe I can compare the phases then.
 

betwixt said:
Possibly use X-Y mode?

Brian.
Click to expand...
Feeding the audio signal from the generator to X and the downconverted AF signal of the same frequency to the Y and then notice the pattern on the screen?

What shape should I see on 0 degrees and what on 180 degrees?
 

Hi:

(both Sine waveform)
0°: diagonal line, bottom left to top right
90°, 270°: circle (may be elongated in X or Y)
180°: diagonal line, top left to bottom right

Klaus
 

    neazoi

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Not so sure XY mode is usable over this range/ratio of frequencies -

X-Y Mode - T&M Atlantic

Learn more about X-Y Mode in our on-line encyclopedia.
www.tmatlantic.com


Regards, Dana.
 

Frequency is irrelevant in the XY mode. The whole principle is that you have two signals of the SAME frequency and the shape of the X-Y output indicates the phase, as explained by Klaus.

You COULD use a different frequency reference, but that just makes the measurement more difficult.
 


I may be wrong but I thought the object was to compare the phase of the RF signal against the
phase of the Audio signal, which means the folding of the Lissajous pattern is unreadable ....?

But if he does use down converted against a external known audio freq it works, but is no longer
comparing the original system RF phase with the down converted signal....

Regards, Dana.
 

Your phase reference is the BFO signal phase.

Here you can find how to use and understand Lissajous curves:

I like what the guy said at the end, that in 42 years working with oscilloscopes he never used Lissajous curves...
 

I've been using scopes for at least 54 years and I only recall using them once. That was to track down a curious intermittent fault on a digital desktop telephone. Of thousands produced, a few would sometimes mis-dial numbers and the problem wasn't repeatable. Very difficult to track down! A scope was used to monitor the clock on the MCU with a stable external oscillator used as reference, they were not phase locked so a slowly 'rotating' loop showed on the screen. What was happening was when a number was keyed on the keypad, the PCB (components on one side, carbon pad switches on the other) would flex slightly and the microphony on ceramic capacitors loading the quartz crystal caused a sudden phase shift. It could clearly be seen as a wobble in the Lissajous curve but would be impossible to find any other way.

Brian.
 

    M

    mtwieg

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