Patent GB-1518831-A (GB)
Title: Frequency stabilized single oscillator transceivers
IPC: H04B1/40
Filed: 1975-08-21
Abstract: 1518831 Transceivers; automatic frequency control UNITED TECHNOLOGIES CORP 21 Aug 1975 [29 Aug 1974] 34758/75 Headings H4L and H3A A transceiver system includes a pair of transceivers each comprising a single voltage-tunable solid state mocrowave oscillator 14 e.g. 40 GHz, an FM receiver 36 having the same IF e.g. 20 MHz in both transceivers, a resonant element 116 its resonance frequency in one transceiver differing from that in the other transceiver by the said IF value, an aerial 22 for transmitting and receiving, means for coupling power from the oscillator to the aerial, to the resonant element and to the input of the receiver 36, and frequency controlling means 126 responsive to the resonant element 116 for providing a tuning voltage for the oscillator, at least one transceiver of the pair having means for providing a tuning voltage controlled by an AFC voltage from the receiver in response to a received signal of predetermined strength. Analogue or digital frequency modulation for oscillator 14, which may be a varactor tuned Gunn oscillator, is applied at input 2, control over the sense of the modulation being provided by a switch 44 for either the master M or the slave S modes of operation. To produce frequency control for the oscillator a portion of the modulated output is coupled into a high Q cavity 116 via waveguide 114, the output after detection by a crystal 120 being a signal having the modulation frequency and amplitude proportional to the difference between the carrier frequency (f o ) and the cavity resonance frequency (f c ), Fig. 3 (not shown). This signal after amplification 124 (156,158), Fig. 2 (not shown), is applied to a phase sensitive demodulator 136 having a reference input 53 from the modulation source 2, the D.C. output at 138 being of polarity corresponding to the sense of the oscillator frequency error. A sweep and integrator circuit 140 comprises an integrating inverting amplifier (166), Fig. 2 (not shown), reciprocally coupled with a Schmitt trigger (178) so as to produce a symmetrical sawtooth generator, the sweep voltage being great enough to bring the oscillator frequency within the response characteristic of the cavity so that phase sensitive demodulator 136 produces an output. When this happens the production of the sawtooth sweep is inhibited and the D.C. control voltage applied to the oscillator becomes constant (b), Fig. 4 (not shown) at a value such that f o  differs from f c  by a small amount determined by the open loop gain of the amplifier (166). Amplifier (166) also functions as a low pass filter. A switch 132 provides for setting the sense of the control voltage as required for either master M or slave S modes. If the slave mode is enabled, and a received signal from the other transceiver is sufficiently strong, frequency control is taken over by the remote station in the following manner. An AGC signal produced in detector 36e of FM receiver 36 is applied to a threshold device 146 and a delay circuit 144, so that an output on 142 may disable amplifier 124 by switching on a transistor (162), and enable an AFC error input 42 by switching off a transistor (170). The AFC voltage derived from the receiver effectively replaces the output of phase sensitive demodulator 136. To ensure oscillator frequency stabilization during quiescent periods, substitute modulation may be applied at input 2. Since the local oscillator signal is provided by oscillator 14 which is modulated, the transmitter modulation must be cancelled at the receiver output. A connection from the source of modulation 2 passes through a delay unit 54, an amplifier 113 and a low pass filter 112, the delay of 54 being set so that the modulation signal passing through resistor 52 is in opposite phase to that coming from receiver 36. In order to produce cancellation of amplitudes an output of the receiver at 40 is applied to a phase sensitive demodulator 56 having the modulation as reference signal, and the output of 56 controls a variable gain amplifier 4 which determines the depth of modulation of oscillator 14.