As discussed in prior posts the NanoVNA uses a resistive bridge.
They are wide band and easier to construct then the alternative
bridge utilizing a directional coupler.
There are excellent references on the resistive bridge. The RF
bridge being a variation of the well know Wheatstone Bridge.
An excellent treatment and discussion is presented in
"The Wheatstone Bridge:How Does It Impact VNA Measurements"
Author: Brian Walker. Presented in RF and MIcrowave Magazine.
Further work in this area is presented by Joel Dunsmore and appears
in RF design Magazine Nov 91, "Simple SMT Bridge Circuit Mimics
Ultra Boradband Coupler".
I decided to calculate the port impedance of the NanoVNA R bridge.
It is 51.27 ohms and within the specified SWR source match.
Using this bridge as a template and based on a OPAMP version
of a similar bridge, I constructed a simple Spice Version. This schematic
is attached. The simulation is run in LTSpice. A free down load of this
simulator is available from Analog Devices web. A nice treatment of this
bridge and running it in simulation is instructive! You can plot Gamma as well return loss.
See a paper by Dunbar published in Electronic Design May 29, 2000, "Build A Vector Network Analyzer".
There the discussion presents VNA's which can span audio through RF and
includes functions like metal detectors, microphone analysis, etc... This
circuit operated in Spice simulation is handy to check the understanding of what the
NanoVNA returns in measurements.
I'll see if I can place the LT Spice circuit file in a folder for those interested.
