Rohde & Schwarz M3SR Series 4100, Service Manual, Page: 54 / 138

R&S M3SR Series 4100 HF Transceivers Power Amplifier - Function
6124.9007.82.01 1.18
1.3.2.5 Power Amplifier - Function
The 150 W Power Amplifier consists of the following functional groups (see Fig. 1.7):
¾ Preamplifier
¾ Final Stage
¾ Harmonic Filter
¾ Control Board
¾ DC Power Supply
¾ Modem (Option)
The 150 W Power Amplifier amplifies the transmit signal from a +10 dBm level to 150 W peak envelope
power (PEP) or 100 W continuous wave (CW).
The transmit signal from the Frontend module is fed to the input connector X311. It passes the
Preamplifier, Final Stage and the Harmonic Filter. The antenna connector X30 is used for TX-out and
also for RX-in, if a separate receiving antenna is not available. In receive mode the Power Amplifier
routes the receive signal to the Frontend module via Harmonic Filter, Preamplifier and connector X311.
Transmit and receive signals are looped through connectors X313 and X314. If a Digital Selector is in-
stalled, it is connected to X313 and X314.
Two fans at the rear of the Power Amplifier provide air cooling for the modules in the radio.
Preamplifier
For transmit the HF signal (+10 dBm) is attenuated by 10 dB and is fed via a voltage-controlled
attenuator to an amplifier. The output from the amplifier is fed to the TX output through the Digital
Selector (optional). The amplifier can be switched on and off via the signal OP OFF. For receive the
received signal (RX) is directly fed backward to the HF IN/OUT connector also through the Digital
Selector (optional).
Final Stage
The HF signal from Preamplifier (approx. +13 dBm) is amplified by a two-stage wideband amplifier. The
wideband amplifier consists of a driver stage (20 dB gain) and a pushpull output stage (approx. 18 dB
gain). It can be switched on and off via the signal OP OFF. With the aid of an NTC resistor, the heat
sink temperature is measured. The resultant signal VREFTK compensates the bias for the power
transistors in the output stage according to the heat sink temperature. The following signals are routed
to the Power Amplifier Control Board for monitoring purposes: