GB690067A - An improved electric testing and measuring arrangement of the bridge type - Google Patents

Abstract

690,067. Bridge circuits. BRITISH INSULATED CALLENDER'S CABLES, Ltd. Jan. 18, 1952 [Jan. 19, 1951], No. 1533/51. Class 37. A bridge is fed by a source of beat frequency current derived from two oscillators, one of fixed frequency and one of variable frequency, and the output from the bridge is linked with the variable frequency oscillator to produce by a heterodyning process current of constant frequency. The bridge network 7 is arranged for testing a circuit element contained in one arm over a range of frequencies from 50 kc/s. to 6 mc/s. The source of current is a fixed frequency-oscillator 1 working at 10 mc/s. and a variable frequency oscillator 2 working in the range 10.05-16 mc/s. The outputs of the oscillators 1, 2 are combined in a mixer 3, the output of which includes a beat frequency in the range 50 kc/s-6 mc/s. The output from the mixer 3 passes through a low-pass filter 4 having a cut-off frequency of 7 mc/s., thus leaving only the beat frequency components in the range 50 kc/s.-6 mc/s. The output of the filter 4 is passed through a wide-band amplifier 5 to the primary winding of a transformer 6, the secondary of which is connected across one diagonal of the bridge network 7. The output from the other diagonal of the bridge is passed through a wide-band amplifier 8 to a mixer 9, which also receives current at frequency in the range 10.05-16 mc/s. from the variable oscillator 2 through a filter 10. This rejects current at a frequency of 10 mc/s. and thus prevents current from the oscillator 1 being fed to the receiver. The output from the mixer 9 is always at a frequency of 10 mc/s. and is passed through an intermediate frequency amplifier 11 and heterodyned with the output from a local oscillator 12 working at 10.001 mc/s. This is followed by a mixer detector 13 giving an output of 1 kc/s. which is passed through an amplifier 14 to a telephone 18 or an ammeter, or cathode-ray tube. In a modification an intermediate frequency in the receiver circuit is 35 mc/s. instead of 10 mc/s. in order to reduce the possibility of coupling with the fixed frequency oscillator 1. This is produced by heterodyning the output of the variable frequency oscillator 2 with a second fixed frequency oscillator and passing through a filter which rejects current at a frequency of 35 mc/s., which is passed to the mixer 9 and produces an output at a constant difference frequency of 35 mc/s. The oscillator 1 may be crystal controlled, and the rejection filters may also be of the crystal type.