SRK Electronics MFTX100, Руководство по эксплуатации и обслуживанию, Страница: 72 / 100

MFTX AM TRANSMITTERS
SRK Electronics 72
14.4 RF power amplifier
Each RF power amplifier generates 100W of RF power at 0% modulation.
The unique phase modulation system used by the MFTX range means that there
are actually two identical amplifiers per RF PA. These are driven by the RF+ and
RF- signals and their outputs summed in the transformer(s) of the output filter
assembly.
The RF+ and RF- signals are logic level squarewaves at the carrier frequency
with a phase difference that varies in proportion to the required instantaneous RF
output power.
These signals are amplified by U1 and 2 to approximately 20V p-p and coupled
into the gates of the output transistors by transformers T1 and 2.
Gate protection is provided by D1 to 8.
The output transistors (Q1-4) are configured as a full bridge with the two output
nodes connected to the primary of the relevant output transformer in the output
filter assembly.
The drain supply of the output transistors is a nominal 50V DC from the main
power supply. The current drawn from this supply is sampled by R5. This
sample is passed to the control unit to allow monitoring of PA current.
The DC supply is also fused to prevent serious damage if one or more of the
output transistors fails. This fuse is shunted by a resistor/LED combination to
indicate fuse failure.
Temperature measurement is provided by U3. This signal is passed to the
control unit for monitoring and protection.