hi EggertEnjoyer123,
1st, thanks for sharing about such a high-profile project !
far from what I could...
2nd,
do you have an idea about / can you rank : what matters in this "upgrade from a nanoVNA" so to speak...?
To jump to 22Ghz, of course you can reply : all matter ! and you're right.
but,
what's the influence of the oscillator
- vs the perf of a such device ? (nano-VNA & your project) ?
- vs the other parts to improve : PCB design / SMA in/out / perf of the IC.. etc...
my e-tweak level is currently about oscilllators
The VNA works by sending out a signal of a certain frequency and measuring the reflected/transmitted signal. The LiteVNA has a PLL IC (MAX2870) that generates the 6.3 GHz signal (I blelieve they are cheating and using the chip out of the rated frequency limit, but it works anyways). You can go past 6.3GHz, but then you're using the harmonics of the PLL IC to generate the signal (so at 7 GHz, the oscillator IC will actually send out 3.5GHz, and we measure the 7GHz harmonic). The downside of this approach is that with amplifiers, it could be the case that the lower harmonic saturates the amplifier, which would throw off your measurements). Of course, my design isn't much better (since the LMX2820 outputs a square wave at lower frequencies), and commercial VNAs have filter banks to deal with this issue (but that would cost too much for me). For a VNA, you actually need two oscillators (one to drive the mixer and one to drive the output of Port 1). The second oscillator is set to a slightly lower frequency than the first oscillator (for me, probably somewhere around 10-20kHz lower, since the output would then be in the audio range, and I can use a cheap audio ADC to digitize it). That way, after mixing you get a very low frequency which is much easier to analyze and get the magnitude/phase information from.
The architecture of my design is going to be pretty much the exact same as the LiteVNA, but with higher quality parts. You'd actually be surprised by how much the LiteVNA creators got away with to save money. The mixer that is used in the LiteVNA is only rated to 3.8 GHz, which is why the S21 noise floor degrades after that frequency. The switches in the LiteVNA are actually only rated for 3 GHz, and leakage becomes a big issue at 6 GHz (another reason why the noise floor increases). The directional bridge in the LiteVNA is built using ferrite baluns, and those also lose directivity at high frequencies (causing your S11 noise floor to increase). In my design, I'll use mixer modules that are rated to 26.5 GHz, some 18GHz directional coupler modules (hopefully they still work at 22 GHz), and mechanical RF switches to solve all of those issues. The LiteVNA also is unable to switch the ports (you can get S11 and S21, but not S12 and S22), but my design will have a transfer switch.
PCB design is actually a lot easier than you'd think. Since all the chips are matched, routing the PCB is just making sure all the traces are 50 ohms impedance. The SMA connector just has to have a center pin with the same thickness as the trace. The return loss of the entire system is dominated by the amplifier MMIC I'm using, which could have S22 as bad as -5dB. The MMIC return loss will definitely dominate over everything else in the system. To fix the return loss issue I'll probably just put an attenuator after everything. Also, I'm still using FR-4 because the Aliexpress board uses FR-4 and the output amplitude is still high enough to get to the 10dBm needed to drive the mixers (if I use two amplifiers in the middle). Switching to a good substrate like Rogers would cost way more than the $5 I spent on the extra amplifier. The only other real gotcha is making sure all the SMAs are torqued correctly. Unfortunately, the only torque wrench I have is from Aliexpress.
I also got all the boards back, and I'm working on assembling everything. So far I found a mistake with the TL431 on one of my boards
. Going to have to bodge that one...