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Phase and level monitor of two signals

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neazoi

neazoi

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Hi, I want to monitor the phase and level of two signals. Actually there is one VFO which is split in two signals using a splitter, so the frequency is the same. Then one of the two signals is phase shifted using a PI LC network.

To be able to set accurate 90 degrees phase and same level to the signals, I am thinking of doing this:

Connect a combiner at the two signals ports and a diode detector at the combined port. If the two signals are 180deg out of phase, minimum voltage will output the detector. If they are in phase, there will be double voltage and if they are 90 degrees out of phase (the desired thing) there will be a certain voltage.
For the power monitor it is easy to be done by a diode detector at each of the two ports prior to the combiner.

Will this work?
 

Yes, but there is a much easier way to detect the phase difference. Use an XOR logic gate and feed the signals to its two inputs. The logic of XOR says the output will be high only if one or the other, but not both are high. So the output is actually the phase difference between the inputs represented as a pulse width. Follow it with a simple RC filter and you get a voltage between 0V and 5V (or whatever logic supply you use) which is proportional to the phase difference. Half supply means 90 degrees.

The other method if you have an oscilloscope is to connect one signal to the X input and the other to the Y input. Put the scope in XY display mode and you get a loop on the display. The phase difference is demonstrated as the difference in height and width of the loop, when you have a circle they are the same so you have exactly 90 degrees shift. You essentially are showing sine and cosine plotted against each other and mathematically they are 90 degrees apart.

Brian.
 

betwixt said:
Yes, but there is a much easier way to detect the phase difference. Use an XOR logic gate and feed the signals to its two inputs. The logic of XOR says the output will be high only if one or the other, but not both are high. So the output is actually the phase difference between the inputs represented as a pulse width. Follow it with a simple RC filter and you get a voltage between 0V and 5V (or whatever logic supply you use) which is proportional to the phase difference. Half supply means 90 degrees.

The other method if you have an oscilloscope is to connect one signal to the X input and the other to the Y input. Put the scope in XY display mode and you get a loop on the display. The phase difference is demonstrated as the difference in height and width of the loop, when you have a circle they are the same so you have exactly 90 degrees shift. You essentially are showing sine and cosine plotted against each other and mathematically they are 90 degrees apart.

Brian.
Click to expand...

I just found this one https://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/Simple_Phase_Detector.gif the reference and the signal in, will be the two signals. Can it be used?
 

It can be, it looks like part of a PLL circuit. One signal 'gates' the other so that more of less of the waveform reaches the RC filter, how well they coincide determines the output voltage. Note that that circuit is critical of signal voltages, if they are too low it will fail to work.

Brian.
 

betwixt said:
It can be, it looks like part of a PLL circuit. One signal 'gates' the other so that more of less of the waveform reaches the RC filter, how well they coincide determines the output voltage. Note that that circuit is critical of signal voltages, if they are too low it will fail to work.

Brian.
Click to expand...
Is there any other way you can think of, so I could save non worthy effort in deciding what to build?
 

The XY scope method is probably most accurate and it will tell you levels as well as phase shift without needing to build anything at all. Do you have a scope with an X axis input available?

Brian.
 

Maybe you can use a 90 degree hybrid coupler for I/Q signal seperation.
 

betwixt said:
The XY scope method is probably most accurate and it will tell you levels as well as phase shift without needing to build anything at all. Do you have a scope with an X axis input available?

Brian.
Click to expand...

I am sorry I haven't specified if previously but it has to be kind of standalone, not requiring a bulky scope. Ok so there are o other methods?
 

In fact you have specified yet about nothing. In my view, it's useless to discuss practical solutions without mentioning a frequency and level range and giving an idea of the intended accuracy.
 

Hi,

What about a LIA = lock in amplifier?

Klaus
 
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    neazoi

    neazoi

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Use an XOR logic gate and feed the signals to its two inputs. The logic of XOR says the output will be high only if one or the other, but not both are high.
Click to expand...

This is basically (in principle) the same as the product detector. The results can be very accurate if the inputs have the same level (amplitude).

But using the scope X-Y input can be tricky: without any phase shift you will get a straight line (45 deg), with a phase shift of 90 deg you will get a circle and anything in between you will get an ellipse.

But I guess measuring from the osc screen will be less accurate.
 
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    neazoi

    neazoi

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This is for HF 1-30MHz. I did not thought the solution would be frequency depended. Wouldn't the solution in post #1 work with reasonable accuracy?
Reasonable meaning to be able to keep the required 90 degrees phase difference with equal amplitudes, so that the opposite sideband can be cancelled in an SSB phasing receiver.
I want to build a simple monitor circuit because the phasing usially depends on the frequency of operation and when frequency is changed, the controls need to be readjusted.

- - - Updated - - -

KlausST said:
Hi,

What about a LIA = lock in amplifier?

Klaus
Click to expand...

As far as I read the lock in amplifier is no different than a direct conversion receiver, in a more advanced form. This is a bit out of topic but how is it able to detect very low S/N signals?
 

I understand that you are looking for a simple tool to adjust quadrature phase shifters.

I would prefer a phase detector that is 1. symmetrical (gives unique, e.g. zero volts output with 90° phase) and 2. can process analog input signals directly. The suggested XOR detector or the detector linked in #3 don't, a double balanced diode mixer is an appropriate solution.
 

Hi,
As far as I read the lock in amplifier is no different than a direct conversion receiver, in a more advanced form. This is a bit out of topic but how is it able to detect very low S/N signals?
Click to expand...
Did you write about signal levels, S/N ratio, expected output precision, frequency, waveform...yet?

I just did a bit of guessing.


****
You wrote about "level monitoring". This can´t be made with a XOR, because it needs logic level input .... but a LIA can detect level and phase.

Klaus
 

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