Patent GB-1180351-A (GB)
Title: Improvements in Superheterodyne Receivers
IPC: H03B1/04
Filed: 1968-09-20
Abstract: 1,180,351. Superheterodyne circuits; automatic frequency control. C.I.T.-COMPAGNIE INDUSTRIELLE DES TELECOMMUNICATIONS. 20 Sept., 1968 [20 Sept., 1967], No. 44850/68. Headings H3A and H3Q. In a superheterodyne receiver in which the signal passes through at least two successive mixers each associated with a separate local oscillator, the frequency of the local oscillator for the first mixer is controlled in step-wise fashion in dependence on the frequency of the second local oscillator. In an embodiment, an intermediate frequency is produced in mixer 16, Fig. 1, utilizing as local oscillator a frequency synthesizer 19, mixer 16 being followed by I.F. filter 17 and amplifier 18. A preamplifier is provided in which the signal is changed in frequency in a first mixer 11, passed through fixed filter 12 and amplified in amplifier 13 and then reconverted to the original frequency in a second mixer 14, which is followed by signal frequency filter 15. The two mixers 11, 14 share a common local oscillator 21 which is tuned by means of a varactor diode 21a. In order to control the frequency of oscillator 21, its output is applied to a mixer 20, the second input of which is derived from the frequency synthesizer 19. The output from mixer 20 is passed through a filter 22 tuned to the desired fixed difference between the frequencies of the oscillators 21, 19 and amplified in amplifier 23. The output of amplifier 23 is applied to rectifier 24 and also to frequency discriminator 25 which feed terminals a, b respectively of logic analogue converter 30. Converter 30 applies logic signals to a step voltage generator 40, by way of terminals A, B, C, the step voltage generator feeding its output signal to control varactor diode 21a Logic analogue converter. When signal passes through filter 22, Fig. 1, detector 24 feeds a positive voltage via terminal a, Fig. 2, to the base of transistor 71: the transistor is then conductive and a " O " signal is applied from its collector to terminal B of step voltage generator 40 which then does not step. In the absence of a signal through filter 22, however, transistor 71 is non-conductive, a " 1 " signal is applied to terminal B and generator 40 generates a series of stepped voltages, driving the tuning of oscillator 21 until the desired signal passes through filter 22, whereupon stepping ceases. The tuning is then within the range covered by discriminator 25; if the output of this is positive and exceeds a predetermined value a bias is applied via diode chain 31 to the base of transistor 61, causing it to conduct. A voltage is then applied from its collector, via inverter 64, to terminal A of step voltage generator 40, causing it to step the tuning of oscillator 21 in such a sense as to reduce the mistuning. If, however, the discriminator output is negative and more so than a second predetermined limit diode, chain 32 is conductive to cut off transistor 51, which is normally conductive. A voltage is then applied from its collector to terminal C of the step voltage generator which causes stepping of the oscillator frequency in the opposite direction. When the output from discriminator 25 falls within the predetermined limits, there is no effect on transistors 51, 61 and the frequency of oscillator 21 remains constant.