Nachstimmschaltung for frequency control of VHF transmitters
For transceive operation of radios, the transmit signal must be conditioned by the receiver oscillator. It operates offset from the set frequency by the IF of the receiver, so in a 2m two-way radio for 144 ... 146 MHz with a (first) intermediate frequency of 10.7 MHz, for example in the range of 133.3 to 135.3 MHz. A common way is that the oscillator signal is mixed during transmit operation with a signal corresponding to the IF, in this example thus with a frequency of 10.7 MHz. However, unwanted mixing products can occur here. In addition, it is not easy to suppress the image frequencies. These would be in the aviation radio range of 122.6 to 134.6 . Such problems can be avoided if the transmitter has its own oscillator whose frequency is synchronized by the receiver oscillator.
For the circuit shown, a signal must be tapped from the transmitter and down-converted to 10.7 MHz with the signal of the receiver oscillator. This is then compared to the signal of a 10.7 MHz crystal oscillator. If these two signals deviate from one another, the transmitter oscillator is regulated to the correct frequency with a retuning voltage. This circuit is, so to speak, a primitive form of a PLL. It was taken from a Lorenz VHF SEM-57 radio. The windings of the two transformers are wound on small HF toroidal cores. With this arrangement for example the previously shown 48 MHz VFO could be controlled by using a variable capacitance diode instead of the tuning capacitor. In principle, the already existing receiver mixing stage could also be used for converting down to 10.7 MHz. The signal from the transmitter will process it due to its sensitivity, without having to connect to it from the transmitter. It is important that its supply voltage is not switched off during transmission.