Anyone (else) using a vector voltmeter?

[shabaz]

I have a few analog meters if I ever get around to experimenting a little. It would be interesting to recreate a simple experiment using a sig-gen.

Just for a bit of fun I gave it a quick attempt. I have not investigated what circuit topology was actually used in the past, my really ugly circuit just uses what I had, in terms of existing scraps of experiments all joined up. All passive, no power supply!
I manually slowly adjusted the phase on one sig-gen output, from -180 degrees to +180 degrees, and observed the output on a 'scope with a very slow timebase setting.  I guess the output could be low-pass-filtered and drive a meter needle, although as-is, there's no indication of positive or negative phase.

[pdenisowski]

I had a go at logging my newer HP 8405A  VVM (made in 1986) to see how stable the phase is over time. This is done using a DVM connected to the phase recorder at the back of the meter.

Very cool.  Now I'm wondering if that methodology could be used to measure Allan variance of sinusoidal signals ....

[G0HZU]

I had a go at logging my newer HP 8405A  VVM (made in 1986) to see how stable the phase is over time. This is done using a DVM connected to the phase recorder at the back of the meter.

Very cool.  Now I'm wondering if that methodology could be used to measure Allan variance of sinusoidal signals ....

I have read that the NPL used the HP 8405A to compare frequency standards in the long and short term but I don’t know what they use now.

My HP 8405A had to be on for about 2 or 3 hours to get the stability in my previous plot. I think it would be even better if left running 24/7 in a controlled lab.

[pdenisowski]

I have read that the NPL used the HP 8405A to compare frequency standards in the long and short term but I don’t know what they use now.

My HP 8405A had to be on for about 2 or 3 hours to get the stability in my previous plot. I think it would be even better if left running 24/7 in a controlled lab.

The two main ways I see Allan variance measured are either using (good) counters or using phase noise analyzers, but I assume that a VVM could be used to do this as well. 

Even today, most modern T&M instruments (even ones with OCXOs, external references, etc.) get their optimum performance after being given time (minutes to hours) to warm up.

If you're measuring Allan variance, you're probably going to be leaving your instrument (whatever you use) on for a long time anyway  

[G0HZU]

I found a link to the classic HP app note AN-77-3. This shows how to measure impedance using a vector voltmeter and a decent directional coupler.

https://www.hpmemoryproject.org/an/pdf/an_77-3.pdf

The HP 778D coupler is the usual choice here as it offers >40dB directivity (typical) across 100MHz to 1GHz and it can be used a bit below 100MHz because the directivity is still good here.

I've got a 778D coupler here and use it fairly regularly for various tasks. There is a nice image of an HP engineer using the classic combo of the HP 8405A and the HP 778D coupler in figure 2 of the link above. I've also added the figure 2 image below for convenience. The engineer can be seen using a smith chart to convert the reflection coefficient (indicated by the dials of the HP 8405A) to impedance.

The AN-77-3 app note provides all the theory and equations for this.

How lucky we are today to have modern VNAs to compute all this stuff across hundreds of frequency points in the blink of an eye 

[pdenisowski]

I found a link to the classic HP app note AN-77-3. This shows how to measure impedance using a vector voltmeter and a decent directional coupler.

Wow - I somehow missed that app note.  Thanks!!!

(And the pictures in the app note are absolutely priceless)

[shabaz]

Experimenting a bit further with the previous setup (and realized that the ~10 Hz was simply the sum from the mixer, aliased due to sample rate, and not a frequency discrepancy between the two input signals!).

The addition from the previous experiment, is that I've added a 500 kHz low-pass filter on the output, so that I can remove the sum and retain just the DC value. As before, I then adjusted the phase from -180 to +180 manually turning the dial, and captured that to a 'scope. This time I'm using 1 MHz signals because I forgot to set them to 10 MHz when I captured the 'scope trace. Everything is quite wideband in the circuit anyway, so that doesn't change anything material.

The output is pretty rock-solid, easy to measure with a multimeter or view with 'scope.
I *think* this could be useful to observe phase noise. This is for no real reason, I'm just kicking tyres with this experiment.
Currently I'm just using two outputs from a single 'sig-gen, but might try to split that across two tomorrow just to see what happens. Or I could try to use a scrap AliExpress OCXO for one of the inputs, I have a few and been meaning to do something with them.

[quince]

How lucky we are today to have modern VNAs to compute all this stuff across hundreds of frequency points in the blink of an eye 

Won't someone think of the slide rule and paper Smith chart manufacturers?