Phase difference measurement of two RF signals using Network Analyzer

[per_lube]

Hi,

I want to measure the phase difference between two RF signals of a receiver circuit.

Can I use Network Analyzer (VNA) for this measurement? or is there any other technique that can be used to measure the phase difference of two RF signals in the laboratory?

regards,

 

[vfone]

VNA's can measure the phase (in +/- 180 degrees range), introduced by a transmission medium. Phase values could be negative at some frequencies and positive at others, depending on the initial phase value and the nature of the transmission line.

To measure the phase difference between two RF signals you can use an oscilloscope.
oscilloscope Tutorials.

 

[biff44]

Yes, but usually the network analyzer has to be the thing generating the signal, and then you would need two other channels to receive the signal and determine the phase.

There are older instruments that measure phase and amplitude of two signals, like the HP 3575A. But they are usually broken when you buy them used (I have a couple lying around in the scrap heap somewhere).

As said, an oscilloscope can do it. You use a high speed ADC, and sample both signals using the same sample clock, and doing an fft to figure out the phase. I recently did a project where I did just that using a tektronix MSO4104 to sample two singals for a 10K record length, and dumped the data files into mathcad to figure out the phases. The MSO4104 seemed to handle the phase measurement well, but with a few tens of ps of random jitter.

Cruder methods are to use a mixer with an I/Q output and just look at the two voltages. One signal is the LO, and the other is the RF on the mixer. But then you need to worry about dc offsets, and the shape in the I/Q space moving with varying input powers. Never the less, it works well for low cost approaches.

At lower frequencies, there are many ways. You can trigger a ramp generator with one signal's zero crossing, and sample that ramp with the other's zero crossing, and read the voltage. I got one of those to work up to 27 Mhz on a project once. I have seen some papers suggesting you could go much higher in frequency, even up to microwaves, but by using monolithic circuits. I think they use them in nuclear decay measurements.

You could also try to use a PLL phase detector, and use filtered Vout as being proportional to Phase (some can give you +/- 2 Π degrees of info)

Rich

 

[BigBoss]

Modern VNAs have time domain option but they are expensive..
But quite accurate

 

[per_lube]

Thanks a lot vfone,biff44 and BigBoss for your replies...

If I be more specific about my requirement, the frequency of two RF signals which I need to measure is 1 GHz. There is no oscilloscope available which can be used to record the signals at this frequency in my uni. laboratory.

The VNA available is Agilent E8361A model and it has a mode called "Receiver mode" .

The user manual (Agilent PNA Series Help) says there are four receivers available, and there are six jumpers coming out from the VNA.

I'm wondering whether I can feed the two RF signals into two of these available receivers and measure the phase difference. Do you think my idea is possible or is there any other way using this VNA (Agilent E8361A) ?

Could you please comment on this...

regards,

 

[biff44]

where are the two 1 ghz signals coming from, and are they phase coherent to each other?

 

[per_lube]

Hi biff44,

Thanks for the reply,

I've attached a block diagram explaining where are they coming from.



Here I'm driving the two VCOs with two voltage ramps to keep the down-converted RF signals (RF1 & RF2) in a constant value (1 GHz).

What I want to measure is the exact phase difference introduced by the band stop filter in this circuit (there can be multiple band stop filters tuned to different frequencies).

At the moment I don't have access to a high speed oscilloscope, I'm wondering whether I can use the Agilent E8361A network analyzer to take this measurement. Could you please comment on my idea on this measurement...

regards,

 

[per_lube]

I resolved my problem with the help of Dr. Joel from Agilent technologies...

I've posted a new thread explaining how I did the measurement (https://www.edaboard.com/threads/209151/#post883290)

Please have a look at it and correct/comment if there is anything wrong or should be improved...

regards,
per_lube