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AN SSBTRANSCEIVER USING SL600 SERIES INTEGRATED CIRCUITS
This note describes the heart of a single sideband transceiver for use in the range of 10 kHz to 500 MHz. The unit is constructed on a single printed circuit board and requires only a local oscillator, linear amplifier, pre-selector, microphone and loudspeaker to complete the transceiver.
RECEIVER The receiver is a single conversion superheterodyne with a 9 MHz intermediate frequency. In order to improve the cross-modulation characteristics there is no RF amplification, high gain being provided by the IF stages. The input mixer is an ANZAC MD108 diode ring, which is used in order to obtain good large signal performance. The mixer input (pins 3 and 7) is obtained from a pre-selector, which prevents the image frequency being received. The local oscillator signal is applied to pin 8 of the MD1O8 at a level of about +7 dBm (500 mVl. A toroidal transformer is used to match the 50? output of the mixer to the crystal filter. IF amplification is provided by three cascaded SL612Cs, which are followed by an SL64OC used as a product detector. The carrier insertion oscillator is an FET which delivers 100 mV r.m,s. into the product detector. The audio amplifier, an SL630C, provides 100 mW output into a 40? loudspeaker; audio gain control is by means of a DC potential applied to pin
8 of the SL630 by a 5k? potentiometer in series with a 5k? resistor, as shown in Fig.2 Automatic gain control is provided by an SL621C a.g.c. generator, the transistor TR2 being used to buffer the output of the SL621C so that an 'S' meter may be driven from the emitter of TR2 if required. C16, C18 and C20 are kept low (4700pF) in order to retain the ignition suppression characteristics of the SL621C system.
TRANSMITTER The transmitter is also single conversion. It generates SSB at 9 MHz by the filter method using the same crystal filter as the receiver. An SL622C is used as the microphone pre-amplifier; this device has its own a.g.c. which automatically adjusts the output to suit the SL640 balanced modulator. The SL64OC is followed by an SL61OC amplifier which has ALC (derived from later stages of the transmitter( or a DC RF gain control applied to pin 7 of the SL61OC. Resistors R1 and R2 (see Fig.1 provide the correct match to the filter. The diode ring mixes the 9 MHz SSB with the local oscillator to produce the required output frequency (and of course the image frequency which afterwards must be removed). The carrier injection is provided by an FET oscillator, with 2 diode-switched crystals for upper and lower sidebands.
CONSTRUCTION The circuit board is single-sided, with two wire links on the top - one in the receive HT line the other the transmit HT line. The layout of this transceiver is critical; change of printed circuit design is not recommended as this could lead to instability Transformer T1 is made from four 2-inch lengths of 26 SWG wire twisted together, the twisted wire is then used to wind two turns on a CR 071-8A core. The ends are then separated and three windings connected in series for form the secondary the primary being the remaining winding. The board may be used as a receiver by omitting the following components: R1 to R5 inclusive, C1 to C13 inclusive and the three integrated circuits used in the transmitter. A 500? resistor should be connected between the crystal filter output and earth. The 100 mw audio output has been found adequate for most purposes but if extra power is required it could be obtained from a Plessey SL414 audio amplifier external to the board. In practice the
sensitivity of the receiver is satisfactory up to 30 MHz but above this frequency an RF amplifier is useful to take advantage of the lower atmospheric noise
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