3/6/08
888-2463-001
4-17
WARNING: Disconnect primary power prior to servicing.
Section 4 Theory of Operation
Diamond Series™
4.6.3.3.1 Bias Control
Both the PA pallet current and Preamp current circuits work the same, only the PA
Pallet circuit will be discussed here. The PA Pallets are adjusted for an open loop
current of 5 amps. U3 and R42 form the basis of the current sample circuit which
converts the current used to a voltage sample to operate the bias section of this circuit.
This voltage is applied to U4 which amplifies the difference of this proportional voltage
against an adjustable RF amp bias adjustment R59. The resultant voltage at U4-7
reduces the preset gate voltage to the level required to maintain the preset Pallet current
forming the closed loop. U2 provides a summation of the PA Pallet and Preamp current
samples for a Current Sample output at J1-3. This sample is also applied to comparator
U1 for over current fault sensing. U5 and Q1 make up the bias shutdown circuit which
will take the bias voltage to zero when the Driver is turned OFF or if there is a fault
condition.
Bias output voltage from U4 is monitored by U8 and compared to a voltage. In the
event of a FET failure, the bias voltage would go to the rail (maximum) and U8 will
send a FET failure pulse to the CPLD.
4.6.3.3.2 Temperature
Temperature protection is accomplished by a thermistor mounted on the heat sink. This
voltage is buffered by U2 and compared to an adjustable threshold voltage at U1, set by
R52. A Temp Fault signal will be sent to the CPLD if the thermistor temperature is
greater than the threshold.
4.6.3.3.3 AGC
The input and output RF samples are detected by two stage diodes to provide a voltage
to buffer op-amp U10. The second stage of the diode is for ambient temperature
compensation. The buffer output is a baseband response consistent with the average
power DTV signal and is adjustable by a potentiometer. U1 compares the detected RF
signal against an adjustable reference (R37) to generate an RF OK signal. When the
drive level falls below acceptable levels, U11 creates a shutdown condition. U5
compares the input and output detected signal and generates a control voltage for the
preamp attenuator circuit. R120 is used to adjust the level of gain in the AGC circuit.
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