Patent GB-829568-A (GB)
Title: Improved frequency measuring system
IPC: G01R23/02
Filed: 1956-06-12
Abstract: 829,568. Measuring bridges; frequency measurement. B. & H. INSTRUMENT CO. Inc. June 12, 1956 [June 15, 1955], No. 18134/56. Class 37. A frequency measuring system comprises a relay armature movable between two positions in response to predetermined successive excursions of an input signal circuit, means responsive to the pulsating signals from the relay armature in synchronism with the input signal to generate single polarity pulses of one form in amplitude and time duration for all input frequencies, and a bridge circuit including a portion of the pulse generation circuit means which is unbalanced solely in response to such pulses to a degree proportionate to the input frequency, which is thus indicated by the bridge unbalance. In Fig. 1 a sinusoidal voltage 12 is applied over terminals 10 and non- linear resistance 14 to energize coil 18 of a polarized relay 16 with a clipped sinusoidal voltage 20 so that armature 22 alternately touches contacts 24, 26 to alternately charge capacitance 30 from stabilized or battery voltage source 50 over half-wave rectifier 33 and resistance 40, and to discharge it over oppositely poled rectifier 32. The resultant square-wave voltage 60 at the armature is differentiated by resistance 40 in conjunction with capacitance 30 and half-wave rectifier 33 to a train of positive going voltage pulses corresponding in frequency to the input sinusoid, which appear across resistance 40 connected with resistances 42, 38, 44 in a bridge circuit energized from voltage source 50. Initially the bridge is balanced for zero input by adjusting the slider 48 of resistance 44, which is connected to the junction of resistances 38, 40 over a null galvanometer, and on application of the input sinusoidal voltage the bridge is unbalanced by the integrated value of the voltage pulses across resistance 40, and is rebalanced by adjustment of slider 48 to a point on a calibrated scale corresponding to the particular value of the input frequency. Rectifiers 32, 33 may be reversed to develop a negative going pulse train across resistance 40, and the galvanometer is switchable in series with a standard cell across a bridge resistance to enable voltage 50 to be adjusted to a predetermined value with zero input. In a modification (Fig. 2), the sinusoidal voltage 102 applied to terminals 100 is clipped by limiter triode 104 to waveform 114, which is squared by triode 116 to unidirectional waveform 124, which energizes coil 128 of unpolarized relay 126 whose armatures 130, 132 alternately touch contacts 134, 136 and 138, 140. Capacitance 146 is alternately charged from voltage source 148 over resistance 152 and bridge resistance 170, and discharged over resistance 160 to develop waveform 168, 166 at the respective armatures. A waveform 164 of unidirectional positive going pulses corresponding in frequency to the input sinusoid appears across resistance 162, and the bridge comprising resistances 162, 170, 172, 174 is unbalanced as before by the integrated value thereof as indicated by galvanometer 176 ; balance being restored by adjustment of slider 178 against a frequency calibrated scale. A polarized relay may be used in the circuit of Fig. 2 and an unpolarized relay in that of Fig. 1. In a further modification (Fig. 3, not shown), applicable to Fig. 1 or Fig. 2, the capacitance is charged on alternate half-cycles of the relay oscillation from a voltage source through a current regulating resistance and an arm of the frequency measuring bridge, and is discharged directly to earth on the remaining half-cycles. The bridge is completed by arms comprising variable zero balancing resistances and by a potentiometer whose variable tapping is adjustable for balance to positionally indicate the measured frequency, while the null galvanometer is switchable into two alternative conditions of sensitivity reading on distinct scales enabling the bridge to be balanced for frequency indication.by adjustment in succession of coarse and fine balancing resistances. Temperature variations may be compensated by utilizing temperature variable resistances in the bridge and a similarly variable charging capacitance.