MX800 Transceiver Technical Manual
© SPECTRA ENGINEERING 2006 Revision 4.2.3
126
8. Superseded Technical
Information
8.1 Power Amplifier Module
8.1.1 Technical Description
RF from the Exciter on CN1 is first attenuated by a 50-ohm pad, which is
used to provide good 50-ohm source impedance for the high power hybrid
amplifier IC1. The RF is amplified to between 5 and 13 watts at the hybrid
50-ohm output. The signal from the hybrid is then matched by a broadband
network to drive the low input impedance associated with the final transmit
power amplifier transistor TR1. The transistor's low collector impedance is
then also matched back to 50 ohms by a broadband matching network.
Trimmer capacitors enable adjustment of the power amplifier over a wide
bandwidth so as to maintain good conversion efficiency. Prior to transmission
a low loss 13 element elliptical low pass filter, filters out the unwanted
harmonics to less than -90dBc.
A dual directional coupler consists of coupled microstrip transmission lines
S5, S6 and S7 fabricated on the PCB artwork. The sampled RF energy is
rectified to provide a proportional DC voltage output on CN4-8 (FWD) and
CN4-5 (REFL).
TR2 serves to switch the DC supply to the Hybrid under control of the PTT
line from the Micro Controller on CN4-2. A thermistor TS1, physically
located on the PA heatsink to monitor the heatsink temperature, is connected
to the Micro Controller via CN4-4/6
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