Electronics > RF, Microwave, Ham Radio

Clone 85033E VNA cal kit measured against PICO cal kit using LibreVNA

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TopQuark:
A while back I purchased a LibreVNA after deciding I've outgrown what my nanoVNA can do. I have been pretty happy with it and I might review it proper later.

However, like most low cost VNAs, the calibration kit that comes with the LibreVNA is basically a set of glorified dust covers along with a cheap terminator. There are people in the community that have characterized the LibreVNA cal kit with more professional equipment against proper standards, but the low cost nature of the kit means there's no guarantee for the consistency of the cal kit manufacturing process (the only thing that makes characterization data transferrable between kits). Furthermore, it is near impossible to screw in the cal kit pieces without rotating the center pin, as the piece that holds the short pin barely protrudes out from the rotating nut. For a VNA, garbage in = garbage out is especially true at high frequencies, and bad (or lack of) cal kit offset delay data makes impedance matching at high frequencies an exercise in vain. Long story short, I wanted something better to calibrate my VNA.

I decided to get two sets of 3.5mm female kits. One is the premium 3.5mm female cal kit from Pico technologies (859 USD, ouch) , the other one a Chinese clone of the 85033E female cal kit purchased from Taobao (165 USD, good match for my LibreVNA). I avoided SMA kits as IMO 3.5mm connectors are manufactured with tighter tolerances and more stable. The big difference between the two kits is the Pico kit comes with individually characterized data (calibration artefact + traceable touchstone data), whereas the clone 85033E kit just says "85033E" on the item listing (calibration artefact, no traceable data). The idea is to use the Pico kit as the lab reference standard, then characterize the clone 85033E and use it as the lab working standard.

My hope is the clone 85033E kit actually matches the real 85033E kit in terms of offset delay and calibration coefficients, failing that, I would just save the touchstone measurement files of the clone 85033E kit and use those as the cal kit data.

Note the clone 85033E kit only comes with short, open and load. The 3.5mm through adapter (25 USD) and the 2 SMA terminators (15 USD each) used for isolation calibration is purchased separately, also from Taobao.

To make the best measurements I can, I am using 3.5mm phase stable test leads, being careful not to bend them much between measurements, 0.9Nm torque wrench for every single connection, and have warmed up my LibreVNA for about an hour to let its temperature stabilise before any measurements.

TopQuark:
First step is to calibrate my LibreVNA using the Pico cal kit with the cal data provided by Pico.

The data comes in a USB stick containing a single .kit file for the SOLT artefact standards. The .kit file is basically all four touchstone files cramped into a single file with some extra formatting stuff for the picoVNA software. It is easy to determine which chunk of the file belongs to which of the SOLT standards by just looking at the value of the data.

I extracted the data and separated them into individual touchstone files for the LibreVNA software to read. I have attached the original .kit file and the separated touchstone files for anyone interested.

TopQuark:
After calibrating the LibreVNA, I immediately moved on to measure the clone 85033E kit. I have attached the measurement files for the SOLT pieces as measured.

After a brief play with the data in Simsmith, I am pleasantly surprised by the quality of the clone 85033E kit.

As I am only measuring the kit to 6GHz, I decided to compare the measured data to the published cal coefficients for the 85033D kit, the 6GHz rated version of the kit.

I modelled the open as an air transmission line with a parallel capacitor with capacitance 49.43 fF (C0 ×10−15 F), and the short as an air transmission line with a series inductor with inductance of 2.0765 pH (L0 × 10−12 H). I then tuned the length of the transmission line until the modelled short and open matches the measured data trace.

With this crude exercise, I eye-balled the open offset delay to be 29.43ps (vs 29.243ps of original 85033D), and the short offset delay to be 31.86ps (vs 31.808ps of original 85033D). This is an incredibly close match from a first pass estimation.

The load standard of the kit is also quite good compared to the corrected Pico load, measuring below 1.030 VSWR at 6G. This mismatch is low enough that I would trust it as a calibration load considering connector to connector mismatch can easily exceed that anyways.

I have attached the raw measurement data of the clone 85033E kit, along with Simsmith screenshot of my open and short models. Feel free to play with the data and see what you can make out of it.

All in all, I am really happy with the Chinese clone 85033E kit. Had I known it's close match with the real deal, I would have skipped purchasing the pico kit and just used the 85033D coefficients. But at the end of the day, measuring is knowing, and without this new found knowledge, the clone 85033E kit would not have been useful as a cal kit.

joeqsmith:

--- Quote from: TopQuark on August 10, 2022, 09:13:01 am ---A while back I purchased a LibreVNA after deciding I've outgrown what my nanoVNA can do. I have been pretty happy with it and I might review it proper later.

--- End quote ---

I stopped following the LibreVNA development a few years ago and would be very interesting in seeing your review on it.   The first thing I noticed was the large chunk of aluminum.  Until I read what you posted, I had no idea what that box was.   :-DD  Obviously your not making any use of the sinks surface area the way you are using it but maybe this suggests this is more for show than actually an oversight by the designers?

TopQuark:
You are not wrong about the peculiar "thermal solution", I have been experimenting with different ways to manage the temperature induced drift of the LibreVNA. I have been toying with different ideas and including sticking a heatsink with fan on the aluminium lid, a more effective use of sink area for sure. The fan is hooked up to the 5v USB supply.

I am still debating on whether I should use a big block of aluminium like in the original photo and have the temperature stabilized against a big thermal mass, but having to wait a while for the temperature to settle, or use a heatsink + fan to bring the device temperature to room temperature. My room temperature can fluctuate quite a bit due to the poor bang bang temperature control of the AC unit.

The goal is not to cool the VNA to ambient, but rather hold the temperature stable enough to minimize the error between calibration and measurement of the DUT. Hell, I've even thought about heating and controlling the temperature of the lid to 60C, just don't like the idea of needing an extra power source.

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