Patent GB-598710-A (GB)
Title: Improvements in or relating to electric measuring apparatus
IPC: G01R23/00
Filed: 1945-09-06
Abstract: 598,710. Frequency measurements. STANDARD TELEPHONES & CABLES, Ltd., and SECKER, B. Sept. 6, 1945, No. 22993. Addition to 579,530. [Class 37] Apparatus for measuring the deviation from a specified frequency of the frequency of an alternating voltage generated by an oscillator comprises an equal ratio Wheatstone bridge circuit, means for connecting two dissimilar impedances which are equal at the specified frequency respectively in two adjacent arms of the bridge, means for connecting the output terminals of the oscillator to the input terminals of the bridge circuit, and means for indicating the value of that component of the output voltage of the bridge circuit which is in quadrature with the input voltage. As shown in Fig. 1, a Wheatstone bridge of the Wien type has two adjacent arms of which one contains a resistance 1 in series with a condenser 2 and the other a resistance 3 in parallel with a condenser 4. The other two arms are formed by two equal secondary windings 5, 6 of a transformer having a primary winding 7. The windings 5, 6 are closely coupled and in series aiding connection. An oscillator 8, the frequency of which is to be tested, is connected to the winding 7 and also through a phase changer 9 to the carrier wave input circuit of a balanced modulator 10. The output of the bridge is connected through an amplifier 11 to the signal wave input of a modulator 10, which comprises a transformer 12 connected to four rectifiers arranged as a ring modulator 13 and thence to a D.C. meter 14 shunted by two equal byepass condensers 15, 16 in series. The secondary winding of the transformer 12 is divided into halves between which is inserted a potentiometer 17. The output of the phase changer 9 is connected between the junction of the condensers 15, 16 and the slider of the potentiometer 17. The bridge impedances 1, 3, 2, 4 are such that the bridge is balanced at the standard frequency. If they are chosen for maximum sensitivity, the output voltage from the bridge is in quadrature with the input voltage and is proportional to the percentage frequency difference from the balance frequency of the bridge. If the phase changer 9 is adjusted to 90 degrees the meter 14 may be calibrated directly in frequency differences. In a modification, Fig. 2, a resistance 19 is included in series between the oscillator 8 and the winding 7, its terminals being connected through an amplifier 20 to the modulator 10. The voltage applied to the carrier wave input terminals is then proportional to the current supplied to the bridge from the oscillator instead of to the voltage. As under the maximum sensitivity condition the phase angle of the bridge impedance is 45 degrees, the voltages applied to the modulator 10 differ in phase by 45 degrees and the rectified current applied to the instrument 14 is about 70 per cent of that obtained if the two voltages were in phase. In order to adjust the circuit for various frequencies, the condensers 2, 4 may be variable and so ganged that the capacitance of the condenser 2 is double that of the condenser 4, the resistances 3, 4 being fixed and the resistance 3 being double the resistance 2.