Patent GB-863215-A (GB)
Title: Frequency generating stabilised system for a variable oscillator
IPC: H03L7/16
Filed: 1957-07-02
Abstract: 863,215. Automatic frequency control. COLLINS RADIO CO. July 2, 1957 [Nov. 23, 1956], No. 20890/57. Class 40(5) A frequency stabilized system for a variable oscillator 14 tunable by dials 10, 11, 12 hereto to within a small percentage of the frequency indicated on the dials (Fig. 1) to give either an "incremental" or "continuous" output, of the kind in which a frequency standard source 40 (including a crystal oscillator, Fig. 2, not shown) provides reference frequencies (100 k.c. and 1 k.c.) for two reference channels and "sidestep" frequencies (5 k.c. and 4.5 k.c.), both channels and the sidestep frequencies being used for "incremental" output. The number of incremental output frequencies which would be available in the absence of the provision of "sidestep" frequencies is multiplied by the number of such frequencies provided and the increments between the output frequencies are the same as those between the "sidestep" frequencies. Only the first reference channel is used for "continuous" output, the other reference frequency (250 k.c.) being obtained directly from source 40. The first reference channel comprises square wave generator 51, keyed oscillator 52, tunable filter 53, mixer M1, a tunable filter 54, a mixer M2, fixed filter 56, mixer M3, fixed filter 57 and a limiter and amplifier 58. Oscillator 52, triggered by pulses from generator 51, provides a spectrum of frequencies about its tuned frequency which are exact multiples (100 k.c. increments) of the 100 k.c. reference frequency harmonics, the oscillator output, the harmonics including the selected harmonic passing to filter 53, the oscillator and filter both being tuned by the output shaft 22 of transmission 21 and by a connection to band dial 10. An amplifier 61 provides the frequency selected by a band switch 28 having four positions to which provide respectively no output, output frequency of oscillator 14x1, x3, x7, and the selected frequency is heterodyned in mixer M 1   with the output of filter 53. The first order difference output frequency from mixer M 1  is selected by filter 54 tuned by shaft 22 over a range equal to that of oscillator 14. The unmultiplied output frequency of oscillator 14 and the output of filter 54 are heterodyned in mixer M 2  the first order difference frequency output of which is received by filter 56 having a band pass equal to the 100 k.c. reference frequency plus the maximum allowable error of oscillator 14 added to each end of its band. Interpolation oscillator 62 is variable over a range equal to the first channel reference frequency and is tuned by k.c. knob 12 in inverse relation thereto, and its output is heterodyned with output of filter 56 in mixer M 3  the output of which passes to filter 57 which provides a constant amplitude output. The second reference channel comprises a keyed oscillator 72 tunable over first'reference frequency range by k.c. knob 12 in inverse relation and triggered by the output of generator 71, the output frequencies being of lower increment (1 k.c.) than that of the first channel oscillator 52 and including the desired harmonic which is heterodyned with the output from interpolation oscillator 62 in mixer M 4 . Knob 12 tunes both oscillators 62, 72 thereby maintaining a fixed frequency difference between them, but if knob 12 is moved half-way between integral settings the frequency difference will increase by 0.5 k.c. and since the output frequencies of oscillator 72 are locked harmonically to the 1 k.c. reference frequency there will be no frequency change at the output of oscillator 72 but there will be two equal amplitude harmonics which will heterodyne to frequencies 1 k.c. apart in mixer 4 to provide output which may be received by either one of filters 74a, 75b switched by switch 73 actuated by knob 12, as it passes from an integral to a 0À5 k.c. setting, through cam 75 which alternately closes contacts 73a and 73b. Switch 73 is interlocked with "sidestep" switch 76 so that on integral setting of knob 12 the 5 k.c. reference frequency will be connected to mixer M 5  and an 0À5 k.c. steps the 4.5 k.c. frequency the sum of the inputs to mixer M 5 , being 250 k.c. for either switch position plus the frequency error of oscillator 62. Filter 77 and amplifier 78 provide the output of the second reference channel. A frequency discriminator 81 provides a coarse error indication and is tuned to the fixed frequency 250 k.c. and has its input connected to the first channel output while a phase detector 83 provides a fine indication and has one input 84 connected to the first channel output and the second input to either the second channel output or directly to the reference frequency (250 k.c.) depending on the position of switch 13. For "incremental" output, the output of the first channel will actuate frequency discriminator 81 which puts out a DC signal (through amplifier 91) to actuate a saturable reactor 92 to decrease the frequency error of oscillator 14 (coarse adjustment) until phase detector 83 comes into operation. One input to the detector is provided by the output of the second reference channel which remains fixed during the frequency regulation and is a stabilized 250 k.c. minus the frequency error of oscillator 62, and the other input by the first channel output. The regulating action is therefore as follows, the saturable reactor causes a frequency modulation of the first channel frequency by automatic control of oscillator 14, the phase detector 83 taking over operation when the discriminator input frequency differs by less than 500 c.p.s. from the 250 k.c. tuned frequency, the output of the detector being a signal representative of the difference frequency between its two input signals, said A.C. signal passing through filter 88 to modulate the oscillator frequency and thus the output of the first reference channel (through feedback loop 24, M 2 , 56, M 3 , 57, 58) so that the output from said first channel swings towards and phase locks with the output from the second reference channel (at 250À1 k.c.) so that the output frequencies are equal, when the frequency of oscillator 14 must equal exactly the setting on the knobs (3.1 m.c.). The error of the interpolation oscillator has no effect on the phase detector output because it is provided to both inputs. The desired outputis derived from mixer Ma where the output from oscillator 14 (through amplifier 24) is heterodyned with the required multiplied frequency. For "continuous" output, the switch 13 is moved to connect the 250 k.c. reference frequency direct from source 40 to one input of phase detector 83, the output of the system then having a slight "plus" error due to the slight error in oscillator 62. A decade system is used for indicating all output frequencies and by providing ten sidestep frequencies spaced at 0À1 k.c. increments rather than at 0.5 k.c. increments the output may be varied in 0À1 k.c. increments. A frequency standard source to provide these is described (Fig. 5, not shown). A modulated stabilized output may be obtained by the system shown in Fig. 6 to which a modulated subcarrier frequency is provided at terminal 111, the system replacing mixer Ma, the relevant part of switch 28 and filter 38, the single-output (which has, however, a fifth grounded contact) stabilized frequency of oscillator 14 being received at terminals 112(X1); 113(X3), 114(X7). Mixers Mc, Md, each receive the subcarrier and their other inputs. are connected to terminal 112, a tunable filter 120 connected to the output of mixer Mc selecting its first order difference frequency to provide the range 1À7-3À7 mc, filter 121 selecting the first order summed output frequency from mixer M D  to provide range 2.3 to 4.3 mc. The modulatedoutput frequency appears at terminal 110 connected to the pole of switch 129 coupled to shaft 17 and operated by the band knob, contacts 130, 131 being connected to filters 120, 121 respectively. The higher bands (outputs from mixers M E , M F , M G  through filters 122, 123, 124) use the modulated outputs of filters as inputs, the connections being as shown.