This project is a continuation of the TAPR VNA 3.1.
It combines SW and HW designs from various VNA projects (in particular NanoVNA for the HW design and the DSP IF and the DG8SAQ VNWA for the resistive bridge design) to come with a cheap and simple design capable to measure from 1 MHz (or below) to at least 700MHz. It also can do TDR to check cables on short and open

Main changes are (see VNA current pridge.pdf in Doc folder):
- Using Arduino and SI5351 (on 5$ ebay breakout module) to generate test signal and 5kHz offset mixing signal. Both can be square wave as the mixing removes all unwanted harmonics (like Nano VNA)
- 3 SA612 to do the downmixing to IF at 5khz (like Nano VNA), LM324 to convert balanced outputs to single ended and buffer the switches,
- Added IF DSP on processing on PC (see audioinput.c) to extract the phase/magnitude from the IF. Using 16 bit audio processing delivers over 100dB dynamic range and full 360 degrees phase detection
- Removal of all complex calibration of the phase/magnitude detectors as the IF DSP is completely lineair
- Adding a mockup test device simulating the audio signals generated during a sweep of SOLT to enable testing the SW without connected HW module

An example implementation uses (see Schematic in doc folder):
 - PC with this SW
 - Auduino module controlled by USB steering the SI5351 to sweep and the transmission/reflection switch
 - Resistor based reflection bridge and 50 ohm transmission sink, both according to DG8SAQ VNWA
 - 3 SA612 + ompamps to create the reference (left audio channel), reflection and transmission (right audio channel) audio IF signals
 - two FET's and two npn transistors to switch between reflection and transmission for right audio channel.
 
 The PC software builds on  Microsoft Visual C++ 2010 Express (free) and probably also on the newer free versions
 The Arduino SW is normal arduino C++ and does run on any Arduino UNO or NANO or similar
 
 VNAR4.zip contains a ready to run executable (no installation needed). Due to the build in simulator (the mockup dialog) you can run and test the program without HW