KD8CEC ‘s SWR senser

Joel Trenalone

Greetings All,
I have been working on my UBiTx V5 for a few months now and have added KD8CEC update with nextion screen and the I2C update. I had all the features working (CW decode etc) except the SWR meter. 

I built a kitsandparts SWR bridge and hooked it up to the second Arduino, but the meter never operated correctly. I traced the problem to the bridge after hooking an oscilloscope up to the forward and reverse outputs and noticed that the reverse voltage would bounce up to 4 volts or so before settling down in a few seconds (this was while hooked to a 50 ohm dummy load. While the probe was hooked up, the meter on the Nextion display would go high and then settle down to a good swr reading. But when I removed the oscope probe, the meter would just stay pegged at very high SWR. 

I ran across a blog here (http://www.kk5jy.net/swr-meter-v1/)     about building a digital SWR meter and noted that he stated  that “In order to prevent stray RF from getting into the A/D circuits, I placed a 0.05uF ceramic disc capacitor between each line and the GND pin.  Since the A/D inputs are very high-impedance, and the SWR sensor outputs were meant to drive current devices (e.g., analog meters), I also placed a 75k resistor across each of the 0.05uF caps, to drain the accumulated charge when no RF was applied”. 

After I did this it the SWR meter works great. I was just wondering if anyone has hooked up KD8CEC’s meter and had similar issues. In an effort to learn more, I  also wondered if anyone can better explain why the resistor and capacitor fixed the situation. 


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