Glad it's all working, John. It is a fun and useful tool
in the shack.
As Will said - the trimmer is to set the reference voltage
for the Arduino's A/D converter.
It normally is set as close to BUT NOT GREATER THAN 5.00
The Arduino is not happy seeing a voltage on any pin
greater than its own 5 V supply.Other than that there is
no real need for precision in setting that trimmer, it'll
all come out in the calibration. I usually set mine at
5.95V just in case the Arduino's regulator is running a
Speaking of calibration - yep, early on I didn't really
try to calibrate my power meter to an absolute level
because I didn't have a reliable reference source. I did
calibrate it with a step attenuator so the slope would be
correct. And as Will mentioned, many applications are just
looking at relative changes over a narrow range of
frequency. The slope part of the calibration is actually
more important than the intercept, since that allows you
to tell where 3 dB points are, etc.
Now that I have a decent sig gen and a better scope, and
an M^3 RF calibration source, I have gone back and done a
careful calibration on all three PHSNA that I've built.
Even built a decent 20 dB pad so I can check the output
of some 500 mW VHF beacon transmitters.
I do have a couple of questions, if one or both of
you would be so kind as to answer them.
First,there is an R17 variable resistor on the main
board that appears to set a voltage for Aref, pin 30
on the Arduino. What voltage should this resistor be
set for and what is its function?
Second, I think I remember that Jim said in one of
his posts that he had never gotten around to
calibrating his power meter because it was not
necessary for the way he used the PHSNA. Is that the
case, and what uses did not require calibration? I am
thinking they were probably relative power output
readings over a fairly narrow frequency range,
resulting for example from measuring a crystal filter
response. If so, I may also delay the calibration