On Tue, Mar 23, 2021 at 07:24 PM, jmr wrote:
I've not actually looked inside my nanovna but a quick glance at the online
schematic shows a classic resistive bridge. When you connect a test circuit to
the CH0 input the bridge in the nanoVNA should output a signal that measures
to have the magnitude of the reflection coefficient. It will also have the
phase of the reflection coefficient. The LO + NE612 mixer converts this down
to 5kHz where it can be sampled with an ADC and measured in relation to the
reference signal in terms of amplitude and phase. The reference is also mixed
down to 5kHz.
If the ADC after the NE612 mixer says the 5kHz signal is shrunk to 99.9% of
the size of the 5kHz reference input then the reflection coefficient is 0.999.
If the 5kHz signal has a phase of -112degrees with respect to the 5kHz
reference then the angle of the reflection coefficient will be -112degrees.
That's the way I assume the system works. I don't see why you and Jim think
the bridge generates I and Q signals. What am I missing here?
JMR,
Please read carefully what I wrote. I did not say the "bridge generates I and Q signals". But it is not as simple as you say. Here is edy555 commenting on the original design. How is it done now? I don't know...
/g/nanovna-users/files/Miscellaneous/NanoVNA%20design%20notes%20by%20edy555/Calculate%20reflection%20coefficient%20from%20NanoVNA%20signal%20and%20plot%20frequency%20characteristics.pdfRather than you or I speculating I hope someone here that knows the firmware details can comment.
Roger
Roger Need