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Anyone (else) using a vector voltmeter?

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pdenisowski:

--- Quote from: G0HZU on June 24, 2024, 10:07:08 pm ---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.

--- End quote ---

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

G0HZU:

--- Quote from: pdenisowski on June 26, 2024, 12:15:16 am ---
--- Quote from: G0HZU on June 24, 2024, 10:07:08 pm ---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.

--- End quote ---

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

--- End quote ---

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:

--- Quote from: G0HZU on June 26, 2024, 08:36:02 am ---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.

--- End quote ---

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:

--- Quote from: G0HZU on June 26, 2024, 06:31:54 pm ---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.

--- End quote ---

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

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

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