Electronics > RF, Microwave, Ham Radio

Building a 22 GHz network analyzer for under $1000

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EggertEnjoyer123:
I am thinking about trying to build a DIY network analyzer based on the LiteVNA architecture.

There are several issues (more like tradeoffs to make the device cheaper) with the LiteVNA design. They are as follows:
1. Bad directivity with the bridge. The bridge starts to lose directivity at around 6 GHz, and the noise floor for S11 quickly increases to about -15dB or so at 8-9 GHz. To be fair, it was never designed to operate that high.
2. Bad mixer. For cost reasons, the mixer used is the AD8342, which is only supposed to work up to 3.8GHz. This is why the noise floor increases quickly for S21 measurements after 4 GHz.
3. Bad isolation with the RF switches. An RF switch is used to switch between the reference, reflected, and transmitted signals. The RF switch (MXD8641) is rated only to 3 GHz. That's probably another reason why the noise floor increases past 3 GHz, at least with S21.

My idea is to use easily obtainable used RF modules which can be found at ham swap meets. I have a couple 7-18 GHz high directivity couplers from Narda, which have >25dB directivity. I also have several wideband mixers which cover the entire frequency range. My plan is to use two LMX2820 chips, which can be had for about $100, and a few RF amplifier ICs (like the MAAM-011100, which costs $12) to power the mixers and generate the signal for Port 1. The two chips will be set to have an offset of 10kHz, allowing a cheap audio ADC to be used to digitize the signals. I could also use mechanical RF relays to solve the isolation issue. Any issues with my idea?

joeqsmith:
How low of a frequency do you plan to support?  Are you planning to support more complex calibration methods?   Did you try your 7-18GHz couplers are 22GHz and found good enough performance?   

Keep us posted.

Gerhard_dk4xp:
Have you seen this here: 
<     https://www.analog.com/en/products/adl5960.html     >     ?

regards, Gerhard

(who is just writing a driver for LMX2594/95 )

In the end, directivity  performance is the result of a lot of math
invested in 12-term-error correction etc.

szoftveres:
If I could change only one thing with the existing LiteVNA, I would probably modify it for lower output power (e.g. attenuate the source by 20dB, and add 20dB LNAs before the mixers). It would enable measurement of active devices in their linear region without giving up of dynamic range of the VNA.
Beyond that, an option to be able to switch between "normal" or "-20dB" output would be the ultimate usability improvement for me.

EggertEnjoyer123:
I bought a LMX2820 test board from Aliexpress for around $120 (Link: https://www.aliexpress.us/item/3256806758049656.html, it was also on sale when I bought it).

The chip seems to work well, but amplitude falls off quickly after 20 GHz. I'm not sure if this is due to some random resonance in my setup or not (probably need to tighten down all the connectors), but the amplitude does drop sharply after around 20 GHz. I was able to get 0dBm out at 20 GHz but only around -5 to -6dBm out at 22 GHz. At lower frequencies the output amplitude is close to 7dBm.

I think a lot of it is due to FR4 losses. In any case -6dBm + 12 dB from an amplifier is enough to drive a mixer (but there probably needs to be some kind of leveling to avoid damaging the mixer at lower frequencies).

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