On Wed, Mar 24, 2021 at 12:39 AM, Roger Need wrote:
I did some research today on how the NanoVNA calculates the reflection
coefficient, Gamma, from the digitized bridge signals. There is very little
information on this available and the firmware source code does not have a
flowchart or text description.
Roger
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?
