On 3/21/23 8:50 AM, Eitan Tsur wrote:
Does anyone know of any currently-on-the-market NanoVNA hardware revisions with MAX2870/MAX2871 PLL's? Seems like they have better performance at higher frequencies than the ADF4350/4351, etc... Considering single quantities for a MAX chip are ~$14 USD from Mouser, I'd be willing to pay the money for a better performing model if software is available to support the part.
Changing PLL is a major redesign of both hardware and firmware (that is, it's not a pin for pin drop in). The MAX2870 only goes down to 23.5 MHz which is fairly important for a lot of users. For that matter the ADF4350 only goes down to 137.5 MHz
The register map is quite different between the parts.
The NanoVNA uses a Si5351 part which is a clock generator that goes 2.5 kHz to 200 MHz (it will actually go higher), and costs $2.44 in single quantity.
The design of a VNA using either the MAX or ADF parts would be quite different.
So it's not the parts cost that's the driver - it's the design cost - And I'd expect that the parts cost on something covering microwave frequencies to be substantially higher than the parts going into the NanoVNA.
For example, you're going to need a better reference oscillator to go to 4 GHz, since the phase noise starts to be a bigger part of the noise budget, and that 20log(N) bites pretty hard. You'll also need to deal with layout and substrate issues - at 300 MHz, regular old FR4 works pretty well, and the small variations don't make much difference (or will calibrate out). Start getting up to 3-4 GHz and FR4 works, but it's lossy, and the piece to piece variability gets bigger. Some calibrates out, some doesn't.
All this can be solved in one way or another, but it's a fair amount of work.
