Patent GB-664195-A (GB)
Title: Improvements in and relating to apparatus for checking the frequency of high frequency electric oscillations
IPC: H03B21/02
Filed: 1949-03-03
Abstract: 664,195. Frequency measurements. BRITISH BROADCASTING CORPORATION and ROBERTS, W. A. March 2,1950 [March 3, 1949], No. 5891/49. Class 37. Apparatus for checking the frequency of high-frequency oscillations comprises a generator 10 of master oscillations of fixed frequency, a harmonic generator 11 for generating a raster of harmonics of that frequency, a harmonic selector 12 for selecting one of these harmonics, frequency dividers 13, 14, 15 for producing a further oscillation of frequency low compared with the master oscillation, a harmonic generator 17 for producing raster of harmonics of the further oscillation, means for applying this rasterto modulate the harmonic selected by the selector 12 so that a spectrum of discrete test oscillations is produced and applied to a mixer 18, to which is also applied from a terminal 19 the oscillation whose frequency is to be checked so that a beat oscillation is produced at a frequency dependent upon the difference between the frequency of the oscillation to be checked and one of the discrete test oscillations. For checking frequencies which are nominally adjusted to be multiples of 20, 10 or 5 kc/s., the oscillator 10 operates at 100 kc/s., the generator 11 generating harmonics of a high order. The frequency dividers 13, 14, 15 produce outputs of 20 kc/s., 10, kc/s., and 5 kc/s., which are applied to the fixed contacts of a switch 16 by which one frequency is selected and applied to the generator 17 which generates harmonics of a low order. The output of the mixer 18 is applied to a detector 20 for detecting beats, which are applied through a low-pass filter 21 and low-frequency amplifier 22 to an output terminal 23. It is assumed that the frequency to be checked is of the nominal value of 12.52 Mc/s. The switch 16 is set to the upper contact so as to apply the output of the divider 13 at 20 kc/s. to the harmonic generator 17. The selector 12 is adjusted to select a harmonic of 12.5 Mc/s. of the oscillator 10 which, when modulated by the output of the generator 17, has a lower sideband in the form of a series of frequencies equally spaced from one another by 20 kc/s., i.e. 12.48, 12.46, &c. Mc/s. and an upper sideband comprising frequencies of 12.52, 12.54, &c. Mc/s., also spaced at 20 kc/s. intervals. The error in the oscillator under test does not normally exceed 300 cycles/sec., so that the output from the mixer 18 contains beats of a frequency less than 300 c/s., which are detected by the detector 20. The low-pass filter 21 has a cut-off frequency of 2.5 or less kc/s., and hence beats between the output of the oscillator under test and frequencies other than 12.52 kc/s. are prevented from reaching the lowfrequency amplifier 22. The output from the amplifier 22 may be applied to a telephone receiver to make the beats audible, and the frequency of the oscillator under test may be varied to produce zero beat frequency. Alternatively the frequency of the oscillator 10 may be varied to obtain zero beat and the adjustment noted. The output of the divider 15 may be applied, by a switch 24, to the amplifier 22 so that the beats are made apparent as a 5 kc/s. tone whose amplitude varies at the frequency of the beat. The frequency of the oscillator 10 may be checked periodically against a remote frequency standard in a comparator 26. Specifications 596,741 and 598,203 are referred to.