PSK transceiver circuit

Posted on Feb 7, 2014

The receiver is a basic SA612 circuit. (U5 and U6) The LO for 20 meters is about 9 MHz and an IF of 5.068 MHz. The IF filter uses two crystals and has a peak response at about 5.070 MHz, so this matches up well with the 20 M PSK sub-band centered at about 14.071 MHz. The BFO frequency is adjusted to be below the IF frequency so Upper Side Band reception results. This was done because it was easier to pull the transmitter VXO higher than the receiver LO frequency to get the proper beat notes in the audio band than it was to go the other way around. The detected audio output of the BFO mixer is amplified by a simple LM386 amplifier (U2).

PSK transceiver circuit
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The transmitter uses another SA612 (U6) for mixing the BFO oscillator with a 9 MHz VXO signal produced by the SA612. The VXO uses two crystals in parallel for increased range, along with a series inductor. The frequency is voltage tuned using a 10 turn pot and 1N4756 Zener didoe used as a tuning diode, which gives enough capacitance swing to tune the VXO about 1.5 kHz, using HC-49US crystals. This tuning range matches the band width of the receiver's crystal filter pretty closely.

The transmitter mixer output is filtered by a double tuned circuit, using modified 10.7 MHz IF cans. The modification is simply breaking out the cap in the bottom of the can, so it can be retuned to 14 MHz using an external cap. A two stage transistor amp (Q4, Q6) brings the signal level up enough to drive the gate of a 74HC86 XOR gate (U4). The XOR gate is used to change the phase of the transmitted signal and to drive the gates of the three BS170 MOSFETs used for the power amplifier. Two gates are connected in parallel to improve drive to the PA gates.

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