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?