pa0nhc SWRsweeper v2.
Detection circuit :
In this version, two AD8361 Vrms-to-Vdc converters measure two RF-voltages in a simple resistor divider measuring circuit.
In the laptop installed DC2WK Windows software generates sweep commands for the sweeper. Every frequency change, a frequency hop command is transmitted via the USB connection to the .INO software in the Arduino.
The Arduino INO software then generates a frequency change command for the DDS.
The DDS generates the RF current with the wanted frequency. The RF current runs through the DDS filter loading resistors, the input of IC1, and the remaining current through a the 50 Ohms resistor in the antenna circuit. As the output is a DC current source, it also contains DC voltage. Not all components in the circuit load for RF and DC. IC1 and IC2 only load the circuit for RF. The DC level on the DDS output depends further on the DC loading at the antenna connection.
IC1 and IC2 convert the in the resistor measuring circuit generated RF voltages into DC voltages A0 and A1.
From these voltages A0 and A1, the antenna feedpoint resistance is calculated in the Arduino .INO software. Thereby taking in account the loading effect from the relative low input resistance of IC3.
This software contains a few correction possibilities (OFFSET and VSWRCORR) for DC offset in IC1 and IC2, and possible small conversion gain differences. They can be changed to optimize the VSWR graphs of 10-270 Ohms test resistors at a test frequency of 3.5MHz. This calculation routine is therefore a bit more complicated than for a bridge circuit.
The output voltage of the DDS lowers with rising output frequency.
When IC1 and IC2 do not have EXACT equal gain characteristics, it influences the flatness of the test resistor SWR graph.
The small graph differnces above 10MHz do not influence an optimal 1:1.0 VSWR plot at least to 30MHz.
Ideally, IC1 and IC2 should be paired. But how?
From the antenna resistance, the VSWR is calculated.
From the VSWR, the reflection value is calculated.
At the end of each frequency hop, the Arduino transmits the for this one frequency calculated results via USB to the laptop.
The laptop than sends a new frequency hop command to the sweeper.
At the end of the completed sweep, the DC2WK Windows application software plots a complete SWR graph on the screen.