Re: SWR


Arv Evans
 

Hello

Discussions about reading FWD and REV power and SWR bring to mind a
possibility that I have not yet seen in BITX software.  Would it be advantageous
to periodically check RF measurements and compare with some pre-set
threshold to drive an alarm function?  This way you would know if something
changed while you are using the rig.

Arv  K7HKL
_._


On Fri, May 11, 2018 at 4:10 PM, Jerry Gaffke via Groups.Io <jgaffke@...> wrote:
Problem with the two Nano readings goes away if you tune up the antenna in CW mode.
Or whistle into the mike, assuming you can whistle a pure single frequency note.

It might sort of work well enough on regular SSB phone transmisssions with large enough caps on the detector outputs.
I'd have my doubts about the SWR figure in that case, but the forward and reverse power readings might be useful.

Here's a couple old posts:
    https://groups.io/g/BITX20/message/48408
    https://groups.io/g/BITX20/message/48422

So the bias offset could be read once immediately before we start transmitting 
at the outputs of the two diode detectors, the same points we read during transmit.
Alternately, we could read the offset once during calibration and store it to eeprom.
Or read it once during development and hard code that offset into the firmware.
I think that last one would work well enough, better than most such detectors which have no offset adjustment.

Then whenever we read the diode detectors for power and swr, we subtract that offset.

I'm shooting for a max reading of 25 Watts into 50 ohms. 
So best case resolution of the detectors using the 10 bit ADC on the Nano is 25000/1024 or about 25mW
Could have a low range by going to the 1.1v reference voltage on the Nano ATMega38P's ADC, bringing that to around 5mW.
    https://www.arduino.cc/reference/en/language/functions/analog-io/analogreference/

So when measuring 100mW, the 5mW resolution means we might see 95mW, 100mW, or 105mW.
If everything works perfectly and the ADC's are really 10 bit, this could be useful down to around 100mW.

Jerry, KE7ER



On Fri, May 11, 2018 at 02:36 pm, Kees T wrote:
OK, sounds good to me and eliminates the external ADC and S/H circuitry. I thought there was a problem syncing up the two Nano readings here also ? I really like those HSMS-2815 matched diodes. Where are you going to read the bias offset to subtract from the ADC reading ? ....may change over time.


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