Principles Of Operation; Transceiver Block Diagram; Receiver; T/R Switch - NORCAL NC2030 Manual

Principles of operation

Transceiver Block Diagram

Receiver

T/R switch

This circuit isolates the receiver from the transmit signal during when transmitting, and lets received
signals pass when receiving. Many transmit-receive T/R switches use clamping diodes to limit the
transmitted RF into the receiver front end. However, these diodes significantly degrade the receiver
third order intercept. Thus diodes have been replaced with MOSFETs. One MOSFET (BS123) is
placed in series with the in coming received signal, and a second (2N7002) is placed across the signal to
ground.
When receiving, the series MOSFET is turned on, while the shunt MOSFET is turned off. This lets the
received signal pass through un-attenuated. When transmitting, the series MOSFET is turned off,
blocking the transmitter signal from the receiver input, and the shunt MOSFET is turned on, further
clamping the transmitter input to the receiver.
A third MOSFET is used as an inverter to makes sure the series and shunt MOSFETs are turned on and
off oppositely from each other.
Norcal NC2030 v5 12-18-05
Page 109 of 128