Patent GB-617438-A (GB)
Title: Device for measuring the frequencies of electric oscillations
IPC: G01R23/00
Filed: 1946-08-28
Abstract: 617,438. Frequency measurements. LABORATOIRES RADIOELECTRIQUES. Aug. 28, 1946, No. 25829. Convention date, Feb. 1, 1944. [Class 37] Relates to the system of frequency measurement in which the unknown frequency is lowered by successive heterodyning with standard frequencies of decreasing magnitude so that the resultant oscillation from each stage is lower than the basic standard frequency of that stage, and consists in including in each stage pre-tuned circuits to allow the first nine integral multiples of the basic standard frequency of that stage to be selected, the basic standard frequencies of successive stages being in the relation of successive powers of ten, and the pretuned circuits being followed by aperiodic amplifiers. A crystal oscillator 1, e.g. of frequency 100 kc/s. controls an oscillator 13 producing oscillations of 200, 300 ... 1000 kc/s.; oscillator 13 controls a similar oscillator 12 giving frequencies 2, 3 ... 10 mc/s. which in turn controls an oscillator 11 generating 20, 30 ... 100 mc/s.; similarly, oscillators 14, 15, 16 are controlled to give frequencies of 20, 30... 100 kc/s., 2, 3... 10 kc/s. and 20, 30 ... 100 c/s., respectively. Harmonic selectors 21 ... 26 comprise fixed tuned circuits which can be selected to give any one of the frequencies delivered by the associated oscillators 11 ... 16 and pass the selected oscillation through a meter 31 ... 36 to a mixer 41 ... 46. The unknown frequency Fx is applied over attenuator 2 to an amplifier 3 which is aperiodic for the range 0 ... 1 mc/s., but is tunable above this range, so that a meter 4 can indicate the order of frequency. Fx is applied to mixer 41 for successive heterodyning with 20, 30 ... 100 mc/s. delivered by oscillator 11 and selector 21, the difference frequency being passed through a low-pass amplifier 51 -having a cut-off at 1 mc/s., but tunable over the range 1 ... 10 mc/s., a voltmeter 61 giving a maximum indication when the difference frequency is below 10 mc/s. ; the frequency selected by 21 thus indicates the first digit of the unknown frequency Fx. The difference frequency is passed from 51 to the mixer 42 for successive heterodyning with the oscillations 2, 3 ... 10 mc/s. from 12 to determine the second digit; this process is repeated until the difference frequency from 56, which is less than 10 c/s. is measured by a frequency meter 5. By choosing always the lowest setting of the selectors 11 ... 16 to give a maximum in the meters 61 ... 66, the frequency is obtained by addition of the readings of the selectors 11 ... 16. Any of the selectors 21 ... 26 can be set to give zero frequency, the mixers 41 ... 46 and amplifiers 51 ... 56 then passing the unknown frequency through without heterodyning. An oscillograph 6 can be used to compare the frequency of the crystal oscillator with the frequencies developed by oscillators 12 ... 16. In place of the meter 5, the difference frequency from 45 may be increased to be between 1000 and 1100 c/s. and used to drive a synchronous motor operating a counter, for comparison with a counter driven by a synchronous motor fed with 1000 c/s. from oscillator 15.