Signal then passes from T1 via D2 and C1 to the gate of TR1, whose output goes to T2 via D5. Diodes D2 and D5 are conducting and diodes D3 and D4 are biased off. During reception the receiver power line is at + 12V and the transmitter line is grounded. The diodes D2-D5 are used to reverse the direction of amplifier gain between reception and transmission. The transformers act as impedance matching devices, together with resistors R3 and R4, and ensure that the mixer and the crystal filter are correctly terminated. The bi-directional amplifier consists of a field-effect transistor, TR1, four low capacitance switching diodes, D2-D5, two transformers, T1 and T2, two capacitors, C1 and C2, and five resistors, R1-R4 and R20. On the board this 50 ohm match is performed by the bi-directional amplifier. It is important, however that it is driven from 50 ohm sources and drives 50 ohm loads, otherwise its gain and intermodulation performance will suffer. Since the MD-108 is a passive device it is bi-directional, so no switching is necessary between transmission and reception. It requires a local oscillator power of about +7dBm (500mV rms) 1.
The MD-108 has a third-order intercept point of +15dBm and an insertion loss of 7dB. This transceiver uses the DC port as its RF port to permit operation at radio frequencies below 5MHz the 5MHz limit is unimportant on the other two ports. The MD-108 has three ports, each of 50 ohms impedance, an upper frequency limit of 500 MHz, a lower limit on two ports of 5 MHz and on the third of DC. The mixer is an Anzac MD-108 hot carrier diode ring mixer. There are also two integrated circuit supply voltage regulators, IC1 and IC10, consisting of a pair of 78L06 regulators. The circuit diagram of the transceiver may be broken down into six sections: the mixer, the bi-directional amplifier, the sideband filter, the receiver, the transmitter, and the sideband oscillators.
The transceiver board is capable of working at any frequency from 10 kHz to 500 MHz but does not contain the local oscillator, preselector, RF power amplifier and power supply. The transmitter and receiver sections are arranged so that no signal switching is required between transmit and receive, and the RF components are common to both. The transceiver, shown in Fig.1, consists of a single conversion superhet receiver with a 9MHz IF and a very efficient audio-derived AGC system. This web-page is maintained for the benefit of people who already have a G4CLF transceiver, and not to encourage anyone to build one. These circuits are virtually unobtainable today and this transceiver should not be attempted unless you have more than sufficient ICs to build it. It uses the Plessey Semiconductors' SL1600 family of integrated circuits for radio systems, which was made obsolete in the early 1990s. The G4CLF Transceiver is the heart of a simple, but high performance, SSB transceiver system developed in the Applications Laboratories of Plessey Semiconductors Ltd in the late 1970s and published in 1980.