I gleaned the following information from one of the other nanoVNA user groups regarding nanoVNA version 2.
1. The nanoVNA will eventually reach 3GHz (and at a similar price to version 1).
2. It's going to be based on the adf4350 + si5351.
3. The 3 mixers are replaced with one higher spec mixer (ad8342) that is switched between the 3 channels.
4. A variable gain amplifier is added at baseband using one opamp and switched feedback resistors for improved dynamic range.
5. The Audio codec is removed and the stm32 built in ADC is used instead.
6. The performance should be comparable or better to V1.
7. Info about the baseband VGA design: A RFIC switch is used to switch the shunt resistor in the feedback path. The switch is basically "transparent" because the off state capacitance is in the femtofarad range (it is an RF switch) which is negligible at the IF frequency. The on state resistance is small compared to the resistors being switched in. Since the amplifier gain is mainly dictated by the feedback network, and the switch is "transparent", there is nothing other than the tempco of the physical resistors that can cause a temperature dependence. The RFIC used is the same as for the receiver RF switch, and it turns out all the maxscend switches do not have the shunt diode problem (most RF switch ICs have parasitic diodes from RF input to ground which will start to conduct at lower frequencies), so it has no theoretical lower frequency limit and can be applied at the IF frequency. This is a big improvement over using normal analog switch ICs which have capacitance in the pF range.
8. Info about linearity: The code will perform a calibration of each VGA step on boot up. Since there is no temperature dependence the calibration only needs to happen once.
A preliminary block diagram is attached. You might want to treat this as hearsay until the design appears on GitHub. The design of the original nanoVNA was released in 2016 and it took almost three years for Hugen to take the ball and run with it.
