Re: DDS Module is not functioning

William Kimber


Jim I think you mean 4.95v


On 12/12/15 13:39, n5ib@... [PHSNA] wrote:

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 V
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 little low.

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.

Jim, N5IB

---In PHSNA@..., wrote :

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 procedure.

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