This circuitry offers some features that are highly desirable when building a QRP (or ANY) transceiver. It also includes a combination of features that many might not always take the time to include in their projects. Considerable effort has been made to assure that in this design, the RIT (RX frequency offset control) voltage and the TX frequency offset control signals are immune to supply voltage variations. IC1 requires a regulated +6 V voltage for proper operation. This is achieved by using the highly regulated VFO power supply rail. The main power supply rail (+9 to +15 V unregulated) powers the relay coil and its transistor circuit. The separate power sources improves the stability.
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Some features included in this control circuit include:
Semi QSK with adjustable wide range break-in delay
Triggering and driving the Tx / Rx antenna relay
Sidetone oscillator with adjustable volume
RIT for driving a varactor diode located at the VFO
Dc offset bias during TX for a fixed 700 Hz VFO offset
When the key is pressed down, the antenna relay coil needs to be quickly energized. One of the sections within IC1 is assigned this task. The voltage on pin 13 determines if the output at pin 1 is at a logic high or low level. Since C3 is quickly discharged via a blocking diode, D2, when the key is pressed, IC1 pin 1 is set to a high output logic level. This level biases the VT1 into conduction, enabling relay coil current flow. When energized, K1 switches the antenna from the receiver input to the output of the TX.
To understand how this works, observe that voltage is applied to C3 via the variable resistance of control P2. This forms an RC timebase circuit. When a voltage is applied via the resistance of P2, energy will be stored in C3. As the amount of energy stored in C3 increases - the voltage measured across C3 will also rise. As this voltage steadily rises to a level beyond the threshold needed by IC1 pin 13, the output from IC1 pin 1 will revert back to its logic Low-level. Bear in mind that so long as the key is pressed, the voltage from P2 is shunted to ground and thus does not store a charge in C3....
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