1,008,461. Fault testing; impedance measurement. HER MAJESTY'S POSTMASTER GENERAL. March 29,1962 [March 29,1961],No. 11567/61. Heading G1U. Apparatus for locating faults in e.g. submarine cables comprises a B.F.O. having first and second variable frequency oscillators, with interconnecting means such that a current at the difference frequency is applied to a cable over the test terminals of the apparatus, metering means for indicating the sending end impedance of the cable as a function of the voltage developed across the test terminals, means for adjusting the frequency of the first oscillator to give a difference frequency corresponding to the frequency at which a first impedance irregularity appears in the impedance frequency characteristic of the cable means for adjusting the frequency of the second oscillator to give a difference frequency corresponding to the frequency at which a second adjacent irregularity occurs in the impedance/frequency curve and means for indicating the distance to the fault in accordance with the adjustment applied to the second oscillator. In Fig.3a a Colpitts search oscillator 11 incorporating pentode V 1 , switchably interchangable adjustable inductances L 1 , L 2 , L 3 and a tuning capacitance 13 incorporating a variable capacitance 14 (Fig. 3d) a zero set trimmer 15 and a preset trimmer 16 all in parallel, is capacitance coupled to the grid of a square low mixer triode V 3a , to the cathode circuit 23 of which is capacitance coupled the anode output of a calibrated Colpitts oscillator 12 comprising pentode V 2 , switchably interchangable adjustable inductances L14, L15, L16 and a tuning capacitance incorporating (Fig. 3c) long, medium, and stunt range measurement capacitors 19, 20, 21 with associated range adjustment capacitors. The difference frequency at the anode of the mixer is capacitance coupled through a low pass filter L23 and cathode follower V 3 b to a further low pass filter 24 feeding a resistance coupled constant current amplifier 28 whose cathode follower output V 6 a energizes the test terminals 25, 26 over a high series resistance and supplies an AGC circuit 27 operative on the input pentode V 4 . The intermediate pentode V 5 has a damped resonant circuit in its cathode return arranged so that the output/frequency characteristic at the lower end of the adjustment range approximates to the inverse impedance/frequency characteristic of a guttapercha telegraph cable. A monitor loud speaker is connectible across the output of V6a by press switch 30, and the voltage appearing across the test terminals is amplified by triode V6b and capacitance coupled to a voltage doubling rectifier, energizing meter M 1 through the slider of universal shunt calibration potentiometer VR1. The rectified voltage is also applied over potentiometer VR 2 and direct coupled amplifier V7a, V7b through halfwave diode 33 to an expanded scale meter M 2 backed off from the anode supply to give a scale magnification of 15:1 referred to meter M 1 for signals exceeding a preset noise level. Both meters are overload protected by shunting Zener diodes 35 and the system is operated from a transistorised stabilized power supply (not shown). The impedance/frequency characteristic of a faulty cable exhibits a series of peaks (Fig.1) and it is shown that the distance from the near end to the fault is given by x = K/F 1 - F 0 where F 1 , F 0 are the frequencies of two adjacent peaks and K is a constant equal to 1/2 the cable phase velocity and also that if K#L is a constant where L is the oscillator inductance the fault distance is a function only of the tuning capacitance of the calibrated oscillator, which may be directly graduated to show distance. In operation, a faulty cable is connected to the test terminals and the inductance selector switch is set to a K value corresponding to the type of cable. The oscillators are set to equality of frequency for zero setting using the loudspeaker, and the search oscillator is adjusted to pick up a suitable peak of the response using the meter indication. Thereafter, the calibrated oscillator is adjusted by variation of capacitances 19, 20, 21 strictly in that order covering in sequence of long, medium, and short ranges of distance, until the next adjacent peak of the impedance response is found as indicated by the meters. The fault distance in nautical miles away then be read off the appropriate dial. Calibrated continuously variable inductors may be used instead of switchable inductors, and the functions of inductors and capacitors may be exchanged. Variable capacitors 19, 20, 21 (Fig. 3) are adjustable over spindles 19 a , 20 a , 21 a (Figs. 4a, 4b) of which 19a carries a cam disc 39 radially slotted at 19a and an arm 41 engageable with an adjustable stop 42. Spindle 20a carries cam discs 43, 44 having radial slots 40b, 40c spaced 180 degrees apart and an arm 45 engageable with an adjustable stop 46, while spindle 21a carries a cam disc 47 with a radial slot 40d and an arm 48 engageable with an adjustable stop 49. Springloaded levers 51 and 53 pivoted at 50 and 54 respectively have inturned ends alternatively engageable with the slots 40a or 40b and 40c or 40d so that spindle 19a is initially free and is locked after 180 degrees indirectional relation to free spindle 20a which is in turn locked after 180 degrees rotation to free spindle 21a. Thus capacitors 19,20,21 are adjustable only in that order. The leading edges of slots 40a, 40c, are chamfered to allow resetting to the initial position by the reverse sequence.