4-40 888-2406-002
WARNING: Disconnect primary power prior to servicing.
Overall System Theory
A/D_RFL_PWR = SQRT(RFL_PWR / Reflect Factor)
A/D_RFL_PWR = SQRT(80W / 0.000123) = 806 (Fault) or
A/D_RFL_PWR = SQRT(44W / 0.000123) = 598 (Foldback)
VSWR protection is based on this A/D reading, which is directly proportional to a
reflected power level, not on a calculated VSWR reading.
4.7.3 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,D].
The normal setting for this screen is 1.35. Using the formula below, this is equal to
44W for 2kW forward power.
RFL PWR = (VSWR-1 / VSWR+1)
2
* 2kW
RFL PWR = (1.35-1 / 1.35+1)
2
* 2kW = 44W
Now using a formula from the previous section:
A/D_RFL_PWR = SQRT(44W / Reflect Factor) = 598
This says that when the output voltage from the reflected coupler is sufficient to
increase the A/D_RFL_PWR reading to 602, the reflected power is 44W. Therefore,
the VSWR_FB_LVL DAC is set to 602 and its dc output voltage will be about 1.8V.
This voltage is sent to U7-12 as one input to the open loop gain differential amplifer.
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 turn will reduce the reflected coupler output and the clamp will be
removed from the APC Voltage and the power will try to come back to normal.
Since this is happening extremely fast, once the reflected coupler output voltage
exceeds the VSWR_FB_LVL, the new APC loop will try to keep the 2 inputs to U7
the same, preventing the reflected power from exceeding 44W. Basically, a new
APC loop (using the output from the reflected coupler instead of the forward) has
been formed which takes precedence over the forward power loop.
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