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
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
from anSWR meter.
The SWR meter can be a design of your choice (and could be external).
Two homebrew options are:
- The SWR bridge suggested
for the G3VPX PicSWR project: PICSWR
- The Peak SWR bridge
of the G3VPX PeakSWR project: PeakSWR Peak
reading is much better!!
Power meter calibration
Hobcat has separate forward
and reflected power calibration windows / processes.
These are accessed via
Menu | Setup | SWR calibration | Forward power calibration
| 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
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
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
- Transmit the calibration
The processes are illustrated
with screen shots:
Above is the inital
Loading a calibration file
First let's load a calibration.
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
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.
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
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.