Patent GB-749060-A (GB)
Title: Improvements in apparatus for electrical frequency measurement
Inventor: ELPHICK GEORGE STANLEY
IPC: G01R25/00
Filed: 1953-02-19
Abstract: 749,060. Frequency measurement. CINEMATELEVISION, Ltd. Feb. 9, 1954 [Feb. 19, 1953], No. 4685/53. Class 37. In frequency-measuring apparatus of the kind described in Specification 747,489 circuit means are provided which, when the maximum range of the component frequency determining circuit is exceeded and due to intrinsic imperfections of the low pass filter a difference signal persists at the output thereof, operate the selector to apply to the mixer the known signal of next highest frequency in sequence, whereby the filter output is reduced to zero and the mixer output signal falls within the range of measurement of the frequency determining circuit. Unknown frequency signals at input 1 (Fig. 1) are applied to a mixer circuit 2 where they beat with a signal of known frequency selected by a multipositional stepping switch 3 from a range of frequencies derived from a 1 Mc/s. crystal oscillator incorporated in the timing unit 8, by a harmonic selector 4. The beat frequency from the mixer is fed through a low pass filter to.a gate circuit 7, opened over 1 second intervals determined by timing pulses from a six decade counter incorporated in the timing unit which counts the cycles of the oscillator, to pass trains of pulses which are applied to a six decade counter 9 to 14 of limited range indicating the input frequency in a field of meters 15. When the frequency is outside the range of filter 5 and of the frequency counter, zero signal appears on the filter output whereby control device 6 is actuated to step the selector switch 3 to successively select different harmonic frequencies of 1 Mc/s. until a signal appears at the filter output which arrests the selector switch. To avoid errors arising from the non- instantaneous filter cut-off passing an appreciable signal which arrests the selector switch when the beat frequency exceeds the range of the counter, the recording by the counter of a value of 1 X 10<SP>6</SP> whereby its meter field is returned to zero impulses control device 6 and switch 3 to select the next higher harmonic frequency whereby the beat frequency is brought within the range of the counter. A further section of switch 3 selectively operates a circuit 16 controlling a meter adjacent to the meter field of counters 9 to 14 whereby the first or megacyclic digit of the measured frequency is displayed. In a modification (Fig. 3, not shown) the mechanical switch is replaced by four gate circuits in respective frequency channels from a harmonic selector to the mixer, sequentially and continuously opened by a counting ring circuit stepped by a timing circuit and arrested by a unidirectional potential pulse on arrival of a signal in the mixer output. As above, a further meter is added to the field of the frequency counter to indicate the selected harmonic and the first or megacyclic digit of the measured frequency, while the ring counter circuit is advanced another step by a pulse derived from the frequency counter when the latter passes through 10<SP>8</SP> back to zero, whereby the next higher harmonic is selected to display the true input frequency.