Author Topic: Copper Mountain Technologies VNA  (Read 4370 times)

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Offline G0HZU

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Re: Copper Mountain Technologies VNA
« Reply #25 on: August 04, 2023, 06:47:24 pm »
As for the VNAs described here, I'd rather see sample measurements than opinions. With a description of what calibrators, test leads, etc. were used.

One interesting test would be to measure a DUT (from a cold start) with an old stored calibration using the same test fixture.

Looking back through my recent files, I measured an HP 8120-4781 cable back on the 26th July with my old E5071 VNA. This would have been calibrated using an unknown through cal after the analyser was allowed plenty of time to warm up.

Today is the 4th August and I fired up the VNA and loaded up the 8 day old cal corrections and measured the cable again today from a cold start. As soon as the VNA would let me, I took a plot of the cable. This can be compared with the original measurement out to 3GHz.

There's bound to be some phase shift even though I used the same decent HP cal cable (the same way around) as last time but I would expect the amplitude agreement to be really close even though the cal was done 8 days ago and I'm also comparing cold to the original cal 8 days ago (note: the analyser was fully warmed up 8 days ago when the calibration was done).

There's about an hour between the cold and the warm test today. The Cal plot is the 8 day old data for the same cable.

I used a decent HP N cable as part of the initial calibration. This cable is in very good condition and gives repeatable results up to 3GHz although some minor differences are inevitable across 8 days and through removing and reattaching the cable. The main aim is to show the change in drift from cold to warmed up (today) to show how stable the VNA is from a cold start onwards.

Sadly, I did knock/move the cable between cold and warm so I'd really like to repeat this again and make sure I don't touch anything.

To clarify, the warm test was taken about 1 hour after today's initial cold test. The Cal plot is the original from 8 days ago.

The results below are quite good I think. There is hardly any drift at all between all three cases. The E5071 is a very old VNA so I'd expect any modern alternative to be able to compete well against this old workhorse.

« Last Edit: August 04, 2023, 06:52:03 pm by G0HZU »
 

Online tautech

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Re: Copper Mountain Technologies VNA
« Reply #26 on: August 04, 2023, 08:24:37 pm »
Also, my main desktop PC is in the opposite corner to the VNA in my workroom now. So any cable routing would be a pain.
Have you considered a WiFi solution, something you can easily open and close at will ?

Away from the bench/lab I often need a LAN connection but cabling is 10m away and sneakernet was the normal solution until another member suggested using one of these in Client mode:
https://www.tp-link.com/us/home-networking/wifi-router/tl-wr802n/

Found I can power it from an instruments USB port too and it's invisible to the network.
So armed with the instruments IP and especially when you have a well featured webrowser remotely accessing instrument files is now a breeze which also saves on sneaker rubber.  :)
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Some stuff seen @ Siglent HQ cannot be shared.
 

Offline G0HZU

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Re: Copper Mountain Technologies VNA
« Reply #27 on: August 05, 2023, 08:44:55 pm »
Thanks, yes I've thought of other ways to do this. All of them are more efficient but I don't want to network the Win 2k VNA. I can't avoid the trip across the room anyway.

I did an insertable device calibration last night and measured a nice 10dB attenuator up to 6GHz. I left the cables and attenuator in place overnight. At around lunchtime today I powered up the E5071 VNA and loaded the previous cal into it and immediately measured the 10dB attenuator.

The first plot shows the difference between last night and the initial cold measurement today. This is a really harsh test but I think the VNA did OK here.

The second plot shows the response after a short warmup and then at +3 hours and +5 hours. This looks a lot better. Hardly any drift at all.

This is only a scalar measurement of s21 though. Usually, I measure two port models of semiconductors (eg BJT or FET) and this involves measurements near the edge of the smith chart with a reflection coefficient very close to 1 at some frequencies. This is where I really need the stability of the E5071 VNA. It does seem to do a good job when measuring semiconductors up to many GHz.

« Last Edit: August 05, 2023, 09:08:55 pm by G0HZU »
 
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Offline G0HZU

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Re: Copper Mountain Technologies VNA
« Reply #28 on: August 05, 2023, 09:06:32 pm »
Note that this is an N type attenuator. The fine trace ripple seen above about 4GHz is probably due to the limits of my homebrew N type cal kit. I have worked out the Cx and Lx coefficients for this cal kit out to 6GHz and it does use extremely nice Suhner 18GHz N connectors with 8 slits in the female centre contact for example. I used very expensive N connectors in the cal kit. They are similar quality to decent VNA N connectors. Now that it is corrected, this cal kit does work extremely well. But not as well as a decent Keysight N cal kit.

I do have an Anritsu N kit but it doesn't offer male and female calibration for an insertable device. I've got various cal kits for SMA and 3.5mm including an N4431B Ecal and an 85033E mechanical kit.

« Last Edit: August 05, 2023, 09:10:08 pm by G0HZU »
 

Offline G0HZU

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Re: Copper Mountain Technologies VNA
« Reply #29 on: August 05, 2023, 11:48:13 pm »
All I meant to do in my last post was to add some background info about the calibration setup I used as requested below. I also used an HP 8120-4781 N cable as part of the test setup.
This cable is old but still quite good. I don't have any $$$ metrology grade N cables here but this one should be quite stable over time if not moved.

Quote
With a description of what calibrators, test leads, etc. were used.


 

Offline joeqsmith

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Re: Copper Mountain Technologies VNA
« Reply #30 on: August 06, 2023, 01:12:06 am »
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

Offline virtualparticles

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Re: Copper Mountain Technologies VNA
« Reply #31 on: August 07, 2023, 11:44:59 am »
I ran an SC5090 compact VNA, measuring a 10 dB attenuator over the full span.  ..

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.

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.
 

Offline joeqsmith

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Re: Copper Mountain Technologies VNA
« Reply #32 on: August 07, 2023, 01:56:38 pm »
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. 

Offline G0HZU

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Re: Copper Mountain Technologies VNA
« Reply #33 on: August 07, 2023, 02:33:25 pm »
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.
 

Offline joeqsmith

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Re: Copper Mountain Technologies VNA
« Reply #34 on: August 07, 2023, 03:48:53 pm »
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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