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

KD8CEC v1.097 Beta will be released shortly

Ian KD8CEC has been busy again in conjuring up new features in his CEC firmware.   He is tempting us with descriptions of two new features:

  • Nextion display update that makes it easy to adjust keying speed and to select the keyer type (Iambic A, Iambic B or Straight Key) even while in TX
  • Adding an additional arduino, connected to the Raduino via i2c, to provide an S-meter function.

The first feature is well explained by the photo above. The second feature is a bit more difficult to describe.   By adding a US$3 additional arduino the µBITx now has double the number of analogue and digital ports and another 32K of program space.   This could be a fun ride!  The first function is a basic one, but an important one.   A much more sensitive S-meter.   But wait there’s more, as yet to be fully described.  If you already have a Nextion screen, it looks like you may be in line for a half decent signal analyser function.

Connecting up the arduino

The circuit is pretty straight forward, and involves just a couple of resistors and a capacitor, along with wiring to +5v and ground, and the two i2c lines.

Simulated S-Meter and SWR meter on small OLED display

 

 

 

 

Duwayne KV4QB has been playing with an Arduino Pro Mini and a small OLED display to use as a stand alone replacement for a analog meter.  The existing Raduino in the µBITx does not have sufficinet spare analog input pins to provide everything that Duwayne wanted to measure.

More information can be found on his blog at:
https://kv4qb.blogspot.com/2018/04/stand-alone-simulated-analog-meter.html

He is also working on a small board to provide forward and reflected power readings for SWR metering and power measurement.

This has created a bit of interest on the list, and represents an interesting idea that is potentially adaptable to other projects and use cases.

Reference