SWR / Power meter calibration process

See also:
Do a calibration
Calbration summary

TrxAVR-Picastar Power/SWR meter overview 

TrxAVR-Picastar, on transmit, has a power meter whcih displays forward and reflected power and SWR.
Hobcat has a calibration facility for this power meter. This works via USB link.
Calibration profiles are stored in the PC and loaded via USB the TrxAVR-Picastar.

The TRxAVR board has two analog inputs, AD0 and AD1 which are dedicated to reflected and forward levels
from anSWR meter.
The SWR meter can be a design of your choice (and could be external).
Two homebrew options are:

Power meter calibration - Hobcat

Hobcat has separate forward and reflected power calibration windows / processes.

These are accessed via   Menu | Setup | SWR calibration | Forward power calibration
                         Menu | Setup | SWR calibration | Reflected power calibration

Note that TrxAVR-Picastar must be running (after its DSP load) and conencted by USB,
otherwise a 4-second timeout message appears and then the window closes.

The forward calibration screen is used to illustrate the software.
(The reflected screen differs only in having Mismatch watts and Mismatch seconds settings.)
If you are looking at this software for the first time, it would be useful to view and/or print the G3VPX default calibration information.
To view and print click Menu | Setup | SWR calibration | Power calibration summary
The calibration screen provides for th e following:
  • Create a new calibration (Data as entered)
  • Normalise into 32 ADC regularly spaced output points: 0,32,64,96 etc
  • Save the calibration to disc file
  • Transmit the calibration to TrxAVR-Picastar.
The processes are illustrated with screen shots:
Above is the inital appearance.


Loading a calibration file  

First let's load a calibration. Click on
The load dialog appears. Files are restricted to .fwd file. Your latest is selected by default.
If you haven't previously done a calibrationthen this will be G3VPX_default.fwd  located in
C:\Program files\hbradios\hobcat\caldata.
Click Open and the calibration below will display:
This shows the fourteen calibration points that I initially entered.

Note that PicAStar's 8.1 Tx drive level increments which are in 1dB steps and so this
determines the power levels of the calibration points.

Now click   or check 'Data normalised' in
Hobcat has effectively 'drawn a graph' using the initiall points and then produced calibration points at
thirty two ADC steps of 32 units.
(32*32 - 1 = 1023 - max ADC output for a 10 bit ADC) 
The normalisation process uses linear interpolation between the entered calibration points.
These 32 power levels are stored in TrxAVR-Picastar's 24LC512 EEPROM.
They appear in the rightmost columns of the ADC calibration printout.
TrxAVR-Picastar divides ADC output by 32 and uses the resulting number ( n = 0 to 31)  to look up the power level.
It actually also looks up the next adjacent power level (n+1) and uses simple linear interpolation between levels
n and n+1 for improved accuracy.