A radio transceiver, capable of transmitting and receiving in any of a large number (7000.) of channels over a band of frequencies almost an octave wide in the u.h.f. spectrum, which includes a tunable frequency synthesizer operable over the band 225-400 MHz., in 25 KHz. channel spacings, a driver amplifier responsive to the synthesizer over the entire band, a filter of the multi-cavity resonator type continuously tunable over the band and a broad band power amplifier responsive to the output of the filter, amplitude modulation of carrier derived from the power amplifier being achieved at the driver amplifier in response to an error signal equal to the difference between a rectified version of the output of the power amplifier and an audio input signal. Since the cavity resonator presents a load to the driver amplifier which looks like a short circuit when the filter is not correctly tuned, the driver output stages, operating Class C, are bypassed during tuning, drive during tuning being achieved from Class A drivers of the Class C stages until the filter is completely tuned, and in the latter process rough tuning being achieved in terms of a comparison of power reflected from the input of the filter and issuing out of the filter, and fine tuning being achieved in response to phase comparison of the signals at the input and issuing out of the filter. The filter is tuned by a reversible drive motor, which runs the filter through its range. Reflected and output signal amplitudes at the filter are compared to derive a maximum output amplitude signal, which renders logic circuitry responsive to measurements of phase difference between input and output signals. Phase differences achieve plural zeros in passing into and out of the resonance frequency of the filter and logic circuitry is provided which causes repeated reversals of the motor, with progressively descreasing speed, in response to phase differences, as tuning passes from one side to the other of a true zero, until the motor comes to rest.