Jim Lux 2020/12/14 12:42
On 12/13/20 3:33 PM, kalashnikov.alexander.b@gmail.com wrote:
> Hello.
>
> I have checked v2_2 schematics[1] and found several places that I am
> unable to understand.
> For example: R3 and R1, or R4 and R2. Seems like U1 and U2 have
> different current limits on VDD because of that. And RFSW−REF signal
> comes through two resistors to U1 and through only one to U2.
> Is that intended?
I would assume it's for filtering - so that the leakage out through the
select port and Vdd of U1 don't get into U2.
You'll note the two switches are essentially cascaded, so the idea is to
get more isolation when switched away from the input.
The switch only has 25-30 dB of isolation, so cascading two will give
you 50-60 dB. You need that if you're getting a strong reflection on
the "sending" port, you're seeing the entire Tx power, and that would
combine with the desired signal on the "receiving" port on a S21
measurement.
The data sheet doesn't give a "control to signal" isolation or a "Vdd to
signal" isolation spec - that's typically something you'd measure in a
breadboard, and then deal with it accordingly.
They may be slow, big, and consume power, but relays are pretty good at
low loss, good match, and good isolation.
>
> Most of the inductances have only "fb" or "rffb" instead of their
> nominal. For example: L603 and L604.
>
> R607-609 + C620-622 are RC filters for SPI signals for U601. But there
> are no any for similar adf4350(U401).
The Tx synth also has more filtering on it's Vdd, as well. Maybe it was
a leakage path into the receiver.
>
> And more I am looking into schematics, more I see weird things.
>
> Is that schematics is up to date at all, or maybe I am just missing
> something?
> Could, please, someone help me to understand at least some of that things?
>
> Thank you!
>
> [1] https://raw.githubusercontent.com/nanovna/NanoVNA-V2/master/v2_2.pdf
>