Patent GB-742674-A (GB)
Title: Improvements in or relating to devices for the precise determination of frequency
IPC: G01R23/00
Filed: 1953-05-04
Abstract: 742,674. Frequency measurement. NATIONAL RESEARCH COUNCIL. May 4, 1953 [Feb. 2, 1953], No. 12388/53. Class 37. [Also in Groups XIX and XX] Apparatus for precisely determining the frequency of one or more sources of electromagnetic waves comprises a source of line frequency spectrum, whose output is combined in a first mixer with the source output to produce one or more beat frequencies with the line frequency spectrum, and a mechanical frequency-modulated oscillator whose output is combined with the beat frequency or frequencies in a second mixer to produce a second beat frequency or frequencies, which are applied to a narrow bandwidth selective circuit whose output when its frequency is coincident therewith operates an impulse circuit to stroboscopically illuminate a dial driven with the frequency modulated oscillator to indicate the precise frequency value of the input to the apparatus. Fig. 2 shows apparatus for precisely determining, e.g. to the nearest 1 Kc/s. the frequency of an oscillator 1 including an inductance 31 tuned by a capacitance 30 adjustable by a knob 2 to roughly set the frequency to e.g. the nearest 100 Kc/s., whose output is coupled through a cathode follower 33 to a heptode mixer 32. A generator of a line frequency spectrum comprises a pentode oscillator 38 frequency controlled by a crystal 37 energizing a double triode amplifier 39, over-driven to generate a range of harmonics of the crystal frequency, and supplies a pulse generator comprising a pentode 40 and a delay line 41 operative to generate pulses each containing a spectrum of harmonics of the crystal frequency at intervals of e.g. 100 Kc/s., extending over the entire range of the apparatus, and the desired frequency of the spectrum corresponding to the rough oscillator tuning is derived from the anode of the pulse generator through' crystal rectifier 43 and selected for application to the first mixer 32 by the R.F. transformer 34, 35 tuned by capacitor 36 ganged for operation by knob 2. The mixer output is connected over conductor 10 to a tuned I.F. amplifier 47 of e.g. 100 Kc/s. bandwidth feeding a second heptode frequency converter 52 whose oscillator circuit 55, 56, 57 is frequency swept over a predetermined range by a motor 13 continuously and cyclically varying the capaci- .tance of 57 together with the rotation of a pointer 15 over a dial 16. The output of converter 52 is supplied to an I.F. amplifier 58 of bandwidth, e.g. 9,000 c/s., rectified in the first section of double diode 61, amplified by pentodes 62, 63 in tandem, and applied to a trigger circuit comprising an inductance 64 in the anode circuit of pentode 63 and the second section of the double diode in series with the pentode anode, and a gas discharge tube 17 arranged to illuminate the pointer 15 and dial 16. Since the rough oscillator tuning also selects a corresponding one of the spectrum frequencies, the beat representing the oscillator frequency always lies within the bandwidth of I.F. amplifier 47 and once per sweep of the oscillator frequency of converter 52 a signal passes through the I.F. amplifier 58 at a position of the pointer 15 corresponding to the interpolation of the frequency of oscillator 1 as between adjacent 100 Kc/s. points set by knob 2. The rectified signal develops a sharp positive pulse across inductance 64 which excites tube 17 through the diode to give a momentary flash illuminating pointer 15 once per cycle of the sweep oscillator, to stroboscopically indicate the required frequency value on dial 16. For calibration, switch 11 enables the harmonic frequency spectrum from amplifier 39 to be injected into the output of mixer 32 through resistance 65, capacitance 45, and inductance 46. The frequencies of plural oscillators may be monitored simultaneously, and the indications may be displayed along a cathode-ray sweep produced electronically in synchronism with the oscillator sweep. The stroboscopic inductor may be a polystyrene pointer fixed radially to a hollow collar rotated over a dial synchronously with the variable condenser, the gas-discharge lamp being placed within and axially of the collar whereby the outwardly curved extremity of the pointer is stroboscopically illuminated as described (Figs. 3, 4 not shown).