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Hall effect devices (HE1-HE4) to separately monitor the DC currents feeding each 1250 watt RF power
amplifier.
C. Front Panel Control Board
The front panel control board (Figure FA0600RX) has six parts, a frequency controlled, pulse-width
modulator, circuitry for output power control, monitoring circuitry for protection, frequency measuring
circuitry, metering circuitry, and remote interface circuitry.
The frequency controlled pulse-width modulator U1 provides the digital drive signals that run the four
1250 Watt power amplifiers. This circuit determines the RF output frequency and RF output power. This
IC also provides a soft-start function that slowly (1 second) ramps the power up to the setpoint when RF is
enabled. R35, R36, and C11 and the voltage applied to varactor diodes D54 & D64 determine the output
frequency. Voltage to the non-inverting input Pin 2 of U1 determines the PA drive pulse-width
(output power).
The power control circuitry smoothes output power and stabilizes it against line and load variations.
Control is achieved through U3D, the OP400 operational amplifier. This op-amp compares the delivered
power signal (from the power monitor board, FA0132RX) to that of the DC reference level (setpoint) at the
summation point of R24 & R32. The setpoint is from either the front panel potentiometer or from Pin 22(+)
& 9(-) of the rear panel ‘D’ connector as determined by LOCAL/REMOTE relay K2.
Monitoring circuitry for power amplifier protection is composed of five parts: RF output transistor over-
voltage control, over-current control, reflected power control, peak power control, and over-temperature
control. These separate functions are controlled respectively by U5D, U5A&B, U5D, U5C, and K3. In
each circuit, RF output is limited by reducing setpoint at summing junction R24 & R32 The over-
temperature circuit also disables RF output and stays latched until the RF enable signal from either front
panel toggle or rear panel Pins 17 and 4 is turned off.
The output frequency of the pulse-width modulator U1 is monitored by a frequency to voltage converter,
U8. The circuit is calibrated by pot R95.
Four-way meter switch S2C enables the user to monitor delivered power, reflected power, output
frequency, and setpoint. Op -amp U4B serves as a high input impedance buffer amplifier for the front panel
meter. Meter calibration is provided by R45.
Remote control analog interfaces are provided by op-amps U3B, U2C, U2A, & U2B. These op-amps
provide differential inputs (setpoint in) and outputs (delivered power, forward, & reflected power)
respectively. Opto-isolators ISO1D, ISO1C, ISO1B provide digital inputs (RF ENABLE,
REMOTE/LOCAL SELECT, DELIVERED/FORWARD POWER CONTROL) and ISO2B, ISO2A, &
ISO1A provide digital outputs (OVERTEMP, MAX POWER, RF OUTPUT ENABLED) respectively.
D. LF1250 Power Amplifier
There are four LF1250 power amplifiers (FA0003RX) in the CLF5000/PLL. This amplifier is an eight
device push-pull Class ‘D’ amplifier, with all devices mounted on a water-cooled copper heatsink. Each
device (transistor) is rated to produce over 300 Watts of power, giving a total potential output power of
2500 watts per amplifier. Four of these amplifiers are used in the CLF5000/PLL to give a large margin of
reliability and immunity to large load impedance fluctuations and gives the CLF5000/PLL the ability to
deliver large amounts of power into less than ideal load conditions. The output devices are FETs with
paralleled gates. The gate drive consists of two FET gate drive ICs U1 & U2. Peak voltage detector diodes
D9 & D10 are used by the front panel monitoring circuitry to limit peak voltages seen by the devices. The
output impedance of the LF1250 amplifier is about 15 ohms.
