9/15/06
888-2586-001
4-49
WARNING: Disconnect primary power prior to servicing.
Section 4 Overall System Theory
Platinum Z16HD+
4.8.2.5
Reflect Factor and A/D_RFL_PWR
Reflect Factor is a calibration factor that is calculated during reflected power
calibration. To check the Reflect Factor press [HOME, MORE, CONFIGURATION, C,
A, C]. All Z16HD+ transmitters are factory calibrated by reversing the reflected coupler
(it now measures forward power) and adjusting for a reflected sample voltage of 2.1V at
210W, as measured at TP18 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_RFL_PWR [HOME, MORE, CONFIGURATION, C,
B], which at 2.1V should be very close to 700 (the A/D converter is 10 bit for 0 to 1024
range, but the input to the A/D is not actually 2.1V). By setting the RFL PWR CAL
screen to 210W and calibrating we are telling the micro that 700 represents 210W. The
micro now calculates the Reflect Factor by dividing the 210W by the square of 700 or
Reflect Factor = RFL PWR CAL (kW) / A/D_RFL_PWR
2
Reflect Factor = 210W/ 700
2
= 0.000429
The A/D_RFL_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). VSWR
protection is based on this A/D reading, which is directly proportional to a reflected
power level, not on a calculated VSWR reading.
The actual VSWR fault level in the transmitter is set at 1.7:1 (which is 470W). The
Foldback level is set at 1.30:1 (which is 115W).
4.8.3
VSWR Foldback
The Master Controller contains another DAC called VSWR_FB_LVL which stands for
VSWR Foldback Level. The output of this DAC is directly controlled by the setting in
the VSWR Foldback Screen [HOME, MORE, CONFIGURATION, B, A, D, D]. The
normal setting for this screen is 1.30. Using the formula below, this is equal to 115W for
7000W forward power.
RFL PWR = (VSWR-1 / VSWR+1)
2
* 7000W
RFL PWR = (1.30-1 / 1.30+1)
2
* 7000W = 115W
Therefore, the VSWR_FB_LVL DAC’s DC output voltage will be about 1.8V. This
voltage is sent to U7-12 as one input to the open loop gain differential amplifier. U7-13
is the calibrated voltage from the reflected coupler (from Life Support Board). As long
as there is little or no reflected power the voltage at U7-13 will be zero and the U7-14
output will be at the positive rail. This reverse biases diode CR1 which is connected to
the APC Voltage at U5-10. If the voltage from the coupler exceeds the
VSWR_FB_LVL, U7-14 will try to go to the negative rail (GND). This will forward
bias CR1 and the APC voltage will be pulled to ground through CR1 and R5. This in
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