This change will reduce ADC dynamic range from 100 dB to 70 dB. So, it seems that the new NanoVNA will be worse...
No, the VGA will extend dynamic range to 100dB, plus ADC dynamic range is nowhere near being the bottleneck anyway.
Where exactly you can get (genuine & new) ADF4350 for 0.4$? AD web: $6.05 @1000+
Not my decision. I just heard this from the internal chat group.
Right, MiniVNA Tiny. Existence of such design does not mean it is good. Problem with such approach - leakage through switches. Search this forum to see how bad this VNA actually is.
Please do not jump to conclusions before you've seen the design. There are 3 stages of switches between the reflection path and the receiver, for a total switch isolation of 90dB.
The bottleneck of the dynamic range is actually the common mode inductance of the two edge mount SMA connectors, and this can't be improved much other than separating the two ports as far as possible. All low cost USB VNAs on the market currently suffers from this problem and is why they are all limited to a dynamic range of around 70dB < 1GHz and 50dB at 3GHz.
Again - do not agree to such design decision. ADC of stm32 have barely 11 bit ENOB and 69dB SNR. Such VGA-augmented ADC will have worse linearity and temperature stability comparing to literally any generic audio ADC. Not to mention that VGA acting as part of ADC may slow sampling speed down because some/many points needs to be sampled at least 2 times while correct VGA gain is found.
See above - ADC dynamic range is far from being the bottleneck. If the ADC is sampling at 1Msps and the VBW is 1kHz, a 60dB ADC dynamic range leads to a 90dB measurement dynamic range, which is more than sufficient. What makes you think autoranging is slow? A typical measured transfer function is fairly smooth and in practice you likely won't ever see more than 5 to 10 autoranging events per sweep, which is equivalent to adding 10 points to a 100 point sweep.