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Copper Mountain Technologies VNA

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joeqsmith:
I attempted to repeat Brian's test using the latest hardware revision of the LiteVNA.   I am using home made cables and Midwest Microwave ATT-0290-10-HEX-02, 10dB 18GHz attenuator.  The VNA was left on for the entire 24 hour duration.   Keep in mind, this $120 VNA is only spec'ed for 6GHz.  The firmware I am using can use harmonics to match Brian's 9GHz.  As you can see though, it's performance is about what you would expect. 

The two 3D graphs show the mag and phase for the first half hour of warmup.   The lab's temperature started at 26.7C and rose to 27.3C during this time.   

METAS showing after 24 hours of operation normalized to the initial cold startup.   The office temperature was 26.7C at the start and ended at 27.0C.

To use is above 8GHz, I assembled an poor man's frequency extender that can be used from roughly 8-12GHz. 
https://www.eevblog.com/forum/rf-microwave/experimenting-with-waveguides-using-the-litevna/msg4903016/#msg4903016

virtualparticles:

--- Quote from: joeqsmith on August 04, 2023, 03:17:03 pm ---
--- Quote from: virtualparticles on August 04, 2023, 03:11:01 pm ---I ran an SC5090 compact VNA, measuring a 10 dB attenuator over the full span.  ..

--- End quote ---

Any idea how much the rooms air temp varied between those two data points?   

***
Looking at the temperature test I ran with the LiteVNA, I was concerned about the cables adding error and this is why I did not use them.   Still looks like the majority of the error was the phase.   I only ran a narrow range but can try to repeat your test.  For the $120, don't expect much out of it. 
***
Attached looking at your normalized mag&phase data.   I wonder how much of this is due to a mechanical change.   Interesting.  And thanks for running it.

--- End quote ---

I don't have a temp gauge in my office but I suspect that it stayed within 5 degrees F. Cables do add error. BTW, attaching the drift after almost 3 days. Looks about the same.

joeqsmith:
Overlaying both your 1 & 3day drift normalized to the start.  Note how much tighter it is now.  Guessing temperature is closer to the initial condition.   Sounds like I need to send you a meter, or send me a loaner and I'll toss it into the chamber where we can run it over a +/-2.5C change and see how stable it is. 

I thought what I would do is take my setup apart and wait a couple of weeks.  Then reinsert the attenuator and load the same cal and measure it again. 

G0HZU:
I think it would be useful to measure the drift from +30 minutes from cold and then every 30 minutes or so for the equivalent of a working day. That's kind of what I tried to do. Measuring two spots measurements over three days might not reveal how it drifts during the first hour or so.

I usually leave my VNA running for a couple of hours before doing a measurement of (say) a BJT because I'll be measuring it at 3Vce, 5Vce, 7Vce, 10Vce, 12Vce and 15Vce and at every one of these points I'll also be measuring at 500uA, 1mA, 2mA, 3mA, 5mA, 7mA, 10mA and maybe through to 20mA for typical small signal BJTs. So that's lots of measurements inside about 30 minutes to an hour. That's when I really need the stability from the VNA. I usually repeat the first couple of measurements at the end of the session to gain some confidence that any drift was negligible.

There are some very challenging vector measurements to be made here, especially for the s11 and s22 measurements of the BJT and (minimising) drift over time is really important here.

joeqsmith:
This is basically what the 3D graphs show for the first half hour.   Still, the temp change isn't constant from lab to lab and it's best to use a chamber, or at least log the ambient temperatures with it.

A problem I am running into with the LiteVNA is the frequency drift.  Even though my homemade frequency extender is phase locked to the GPS reference clock, the LiteVNA doesn't support it.  Even if it did, the waveguides are made from copper, brass, silver which have a fairly high thermal expansion coefficient. 

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