What is the purpose of diodes after RF power amplifier and before LPF?

Question

I am planning to build an RF transmitter using an integrated PA module (RA30H1317M1). While studying schematics of similar transceivers, for example IC-2200H (that also has a PA module) I see all typical transmit chain components like the PA, filter, antenna connector, but I don't exactly know the purpose of diodes D27 and D12 (in the middle of the picture).

Are they perhaps to prevent "backward" energy flow reflected from the filter and antenna to prevent intermodulation in the PA? Are they necessary?

Answer 1

The diodes are part of a RF switch that switches between RX/TX modes of the transceiver (See also my answer to another question). This can be accomplished by changing the DC potential at the RX LINE side of the diodes.

You can see a RF filter of the TX LINE side consisting of R150 C174, C176 and L40. I'm sure there is a similar filter on the RX line side (connected to a switchable DC voltage source) not shown in the excerpt of the schematic.

• If the diodes are forward biased RF power can pass from TX LINE to COMMON LINE. The transceiver is transmitting.

• If the diodes are reverse biased RF power from TX LINE to COMMON LINE is blocked. The transceiver is receiving.

Answer 2

They are pin diode switches, when TX is enabled, current flows via R150,L40 forward biasing the diodes and effectively making them transparent to the RF power. Remove the bias (Or better reverse bias the diodes) and they present as small capacitors to the RF effectively isolating the PA from the aerial on RX.

I expect there is a similar arrangement elsewhere on the page to isolate the RX input from the transmitted power (I also expect there is a choke to ground somewhere on that green line to sink the bias current).

Regards, Dan.