Harris Platinum Z2 CD - Dac_Apc_Ref; Forward Factor and A;D_FWD_PWR

4-36 888-2406-002
WARNING: Disconnect primary power prior to servicing.
Overall System Theory
The following is a description of each of these parameters.
4.7.2.1 DAC_APC_REF
DAC_APC_REF [HOME, MORE, CONFIGURATION C,B] is a counter (internal
to the micro), whose output will be between 0 and 900, that is controlled by the
RAISE and LOWER buttons on the front panel. RAISE increases the count and
LOWER decreases the count. This number is then D/A converted to get the
APC_REF voltage which controls the APC loop discussed earlier in this section.
The correct APC_REF voltage for a specific DAC_APC_REF number can be
calculated from the formula:
APC_REF voltage = DAC_APC_REF * 0.004883
Example: For the nominal DAC_APC_REF value of 660, APC_REF Voltage = 660
* 0.004883 = 3.22V and can be measured at TP8 on the Master Controller.
This DAC value is used to control transmitter power level for all conditions where
the Master Controller is operational and the APC Loop switch S1-10 is closed.
4.7.2.2 Forward Factor and A/D_FWD_PWR
The Forward Factor is a calibration factor that is calculated during forward power
calibration. To check the Forward Factor press [HOME, MORE,
CONFIGURATION C,A,A]. All Z2 transmitters are factory calibrated with a
forward sample voltage of 8.9vdc at 2kW, as measured at TP9 on the Life Support
Board. This sample voltage is low pass filtered on the Master Controller and then A/
D converted in the micro for metering. This A/D value shows up as A/
D_FWD_PWR [HOME, MORE, CONFIGURATION C,B], which at 8.9V should
be very close to 806 (the A/D converter is 10 bit for 0 to 1024 range, but the input to
the A/D is not actually 8.9V). By setting the FWD PWR CAL screen to 2kW and
calibrating we are telling the micro that 806 represents 2kW. The micro now
calculates the Forward Factor by dividing the 2kW by the square of 806 or
Forward Factor = FWD PWR CAL (kW) / A/D_FWD_PWR
2
Forward Factor = 2kW / 806
2
= 0.003079
The A/D FWD PWR reading must be squared to make the voltage from the coupler
linearly track the transmitter power output (Power is the square of the voltage).