AR 10047590, Operating And Service Manual

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Model 125S1G2z5
26 Rev A
The bias board provides the gate and drain voltages for RF FETs Q1 and Q2. The gate voltages are generated
by a control loop consisting of current monitors U1, U6 and op amps U2, U7. The output voltage of U2 or U7
(which is the RF FET gate voltage) changes as a function of the difference between the output of the current
monitor, which provides a voltage proportional to the RF FET drain current, and a reference voltage
Vref (~-2.82V). This reference voltage is mainly derived from the output of a voltage divider consisting of
resistors R9, R11 or R33, R35 and a potentiometer R10 or R34. The voltage divider input is connected to a -
5V regulated voltage. The reference voltage is also weakly dependent on VDDA which is connected to Vref
through a large pull up resistor R5 or R29. Since the RF FET gate voltage is dependent on Vref, the drain bias
current can be adjusted using potentiometer R10 or R34. A reference voltage of ~ -2.82V will result in an RF
FET gate voltage that corresponds to a maintained drain current of about 3.4A. This drain current in each RF
FET results in a total quad amp DC current consumption of about 6.8A.
The DC bias and fault voltages can be checked on the supply terminals on the side of the module. The normal
voltages on the terminals are: VDD (blue wire) +28V, VGG (violet wire) -8V and Fault (white wire with
black stripe) +5V. A low voltage on the Fault line indicates either an under or over current condition in one or
both RF FETs. U1 and U2 are 90° Quadrature couplers: U1 splits the input signal into two signals with a
phase difference of 90°; U2 combines the RF outputs from Q1 and Q2 and is connected to the output
connector J2. 50W termination resistors R36 and R37 absorb any difference signals and help to improve the
input and output VSWR of the module. The module has a gain of 10dB or greater and delivers approximately
75 watts of RF power. IC U3 or U8 is a dual comparator that senses if the drain current of the RF FET varies
above or below the normal operating range. If the drain current of either Q1 or Q2 should fall below 1A, or if
it should rise above ~ 3.75A, the output of U3 or U8 will drop low and the Amplifier Fault voltage at the
supply terminal will drop low.
3.2.4 A3 Two-Way Splitter
The Two-Way splitter splits the input signal into two equal-amplitude, equal-phase signals. The amplitude of
each signal is 3–3.5dB below the input signal when both outputs are terminated into 50Ω loads.
3.2.5 A10 Two-Way Combiner
A10 is a Two-Way combiner with impedance-matching. When the outputs of the four final amplifiers are fed
to the two-way combiner, the combined output will be approximately 3 dB above the output of a single final
amplifier.
3.3 POWER SUPPLIES (PS1, PS2)
Power supply PS1 supplies a +5VDC housekeeping supply for the control system assemblies A12
Control/Fault Board and A14 Interface Board.
PS1 also supplies -15 VDC at 2.5 amps. PS1 is a switching supply that automatically sets the AC input
circuits to the correct connections for the line voltage 100-240 VAC input ranges 47-440 Hz.
The -15 VDC at 2.5 amps power supply is fed to the A1 Pre-Amp, and A2 driver and A6–A7 Power Modules.
The +24 VDC at 7.5 amps power supply is fed to the B1 and B2 fan assemblies.
Power Supply PS2 is a single output supply which pr24VDC to the A1 Preamp, and +28 to the A2
driver and A6 - A7 power modules. Primary AC circuit protection is provided by the circuit breaker in the
Power Entry Module.