Addressing issues with an SWR meter and the Nextion display

Joel KD6AGW has been working on his UBiTx V5 for a few months now and added the KD8CEC update with Nextion screen and the I2C update.  He had all the features working (CW decode etc) except the SWR meter.

Joel also built a Kits and Parts SWR bridge and hooked it up to the second Arduino.  However, the meter never operated correctly.  The home brew Stockton bridge that Ian (CEC) used apparently did not have this problem.

After hooking up an oscilloscope to the forward and reverse outputs, Joel noticed that the reverse voltage would bounce up to 4 volts or so before settling down after a few seconds.  This was while the uBITx RX input was hooked to a 50 ohm dummy load.  When the probe was hooked up, the meter on the Nextion display would go high, and then settle down to a good swr reading.   However, as soon as he removed the probe to the oscilloscope, the meter would just stay pegged at very high SWR.

He ran across a blog item here (http://www.kk5jy.net/swr-meter-v1/)  about building a digital SWR meter and noted that:

“In order to prevent stray RF from getting into the A/D circuits,  a 0.05uF ceramic disc capacitor was placed 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), a 75k resistor was placed across each of the 0.05uF caps, to drain the accumulated charge when no RF was applied”.

After doing these modifications, Joel has found the SWR meter to work great.

For information, here is a sketch of the circuit modification:

Following a question about power wastage in the Stockton Bridge SWR circuit Joel checked the output with the bridge in and out.   There was no perceptible difference in power out on the oscilloscope.

Reference

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