Building a 22 GHz network analyzer for under $1000

[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).

[ftg]

Asking FR-4 and SMA's to do 20GHz might be a bit much.
I'd blame the lossy FR-4 and then the PCB->SMA transition might not be the best available.
That said, the manufacturer does claim that the SMA's used are infact 3.5mm connectors made for 26.5GHz.

The claimed output power is 0dBm on 20GHz, -0.8dBm on 21GHz, -3.8dBm on 22GHz and -3.5dBm on 22.5GHz.

I also grabbed the images of the from that Aliexpress listing so that they can be seen more easily and will be available even when that listing is gone.

[EggertEnjoyer123]

Here's my annotated version of the board.

There are some dubious design choices (as with all cheap Chinese boards). The main ones are:

1) Using the same loop filter components as the datasheet while using a different PFD frequency (though 140 MHz vs 200 MHz might be close enough that this doesn't make a difference, assuming they are multiplying the input frequency by 2 and then 7)

2) Loading the output of a (presumably CMOS) oscillator with 25 ohms (2 x 50 ohms). I'll probably try removing them to see if that does anything.

3) Not using C0G capacitors for the loop filter

They're also using LC filters for the power rails, and I doubt they've considered resonance.

Anyways, it looks like I might be able to get the chips for $50 each off LCSC, which is much better than the $107 that Arrow charges (or $123 for Mouser). I've had good luck buying parts off of there, and they should be legit parts.

[jjoonathan]

"What is the bandwidth of your loop filter?"

"About 10 degrees Celsius"

[EggertEnjoyer123]

Removing both 50 ohm resistors seems to have decreased phase noise by a few dB (which makes sense, since loading a CMOS oscillator with 25 ohms is not a good idea).

Other changes I tried (such as more decoupling and adding resistors to get rid of potential resonances) didn't do much at all.

The power supply filtering is also not perfect, since some switching noise gets in. When I power it off a crappy USB power bank, you can see 500kHz noise from the switching.

Also, it appears that the PFD frequency is only 10 MHz, which is different from the datasheet's 200 MHz. This might mean that the loop filter needs to be changed. I determined the PFD frequency by looking for reference spurs, and there seems to be one at 10 MHz no matter what frequency I choose.

Edit: Dropping the PFD frequency by 20 times means the loop filter's bandwidth is now 20 times less - which is not very good.

Also, here is the expected phase noise (with 200MHz PFD frequency) and here is the actual phase noise (with 10 MHz PFD). You can see that the bump around 30kHz lines up with my spectrum analyzer measurements

[EggertEnjoyer123]

I'm currently working on the PCB design for the new VCO.

I found two couplers from (presumably) some HP VNAs in my rusty parts bin. The first one had 12dB directivity (and a bent APC-7 connector which I "fixed" using pliers and quite a bit of force), while the other had a bit over than 20dB (I measured from 0 - 9.3GHz, which is the best my LiteVNA could do). Before someone gets mad at me for "fixing" RF connectors, I connected a piece of coax to act as a "port saver", and then I marked all the bad connectors with Sharpie just to make sure I never use those connectors again with good parts

Here's what's inside the 12dB directivity one. It is much simpler than I expected, and a lot wider in bandwidth (2GHz to >9.3GHz) than ones made with microstrip. There is a triangular piece of metal which is clamped with pressure. Moving the triangle with a screwdriver didn't do much in terms of return loss or directivity, as measured by my cheap LiteVNA. The directivity was always around 12dB at lower frequencies.