Patent GB-783889-A (GB)
Title: Improvements in or relating to voltage comparator and like circuits
Inventor: BROWN THOMAS THEODORE
IPC: H03D3/08
Filed: 1954-09-10
Abstract: 783,889. Impedance; power factor measurement. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. June 6,1955 [Sept. 10, 1954], No. 26261/54. Class 37. [Also in Group XL (c)] A comparator circuit responsive to departures from a predetermined phase or amplitude relationship between two high-frequency voltages from a common source comprises a reactive potentiometer with resistance in one arm energized from the source to derive a fractional voltage proportional to one test voltage, a load circuit excited from the common source, means for deriving from the load circuit a voltage constituting the other test voltage, and rectifier means for combining the test voltages to produce a direct voltage representing the departure from the predetermined relationship; the resistance in the potentiometer arm being so dimensioned as to compensate aperiodically for the high-frequency effect of undesired impedance in the load circuit from which the other test voltage is derived. Fig. 6 shows a comparator circuit for developing a unidirectional voltage across load resistors 6, 7 representing the phase angle of an impedance 1, wherein the latter is supplied with a voltage VL and carries a current IL through the primary of current transformer 4, across the halves of whose centre-tapped secondaries appear voltages VA, VB respectively connected across resistances 6, 7 and common resistance 5 connecting the junction of 6, 7 to the centre tap. A potentiometer comprising capacitance 3 in series with capacitance 15 and resistance 14 across the input voltage VL develops a proportionate voltage VC across resistance 5 for vectorial combination with VA and VB, and rectification by diodes 8, 9 between the respective extremities of the transformer secondary and those of the resistances 6, 7. Normally for unity load power factor, and assuming resistance 14 to be zero (Fig. 1, not shown), the voltages VA and VB are equal, antiphased, and in quadrature with load current IL, while the vector sums VAC and VBC of VA, VB with VC are equal, so that the rectified voltage across resistances 6, 7 is zero, but deviation of the load phase angle from zero shifts the phase of VL with respect to IL so that a rectified voltage representing the phase angle in magnitude and sense appears across resistances 6, 7 (Figs. 2, 3, not shown). At high operating radio frequencies the damping of the secondary by the resistances of rectifiers 8, 9 shifts voltages VA and VB out of quadrature with voltage VC and current IL (Fig. 5) whereby a spurious unidirectional voltage is developed for unity load power factor. Resistance 14 in series with capacitance 15 is adjusted to a value such that the time constant of potentiometer arm 14, 15 is equal to that of the secondary of transformer 4 as damped by diodes 8, 9, and the phase of VC is then shifted so that it is in quadrature with voltages VA, VB at unity load power factor (Fig. 7) so that the unidirectional output is zero. Similarly in a load impedance measuring comparator (Fig. 8) comprising an impedance 1 energized over series resistance 10 from voltage VL with current IL, wherein the voltage drop across 10 is rectified by diode 12 and combined across tapped resistance 11 in opposition to the voltage developed by rectification in diode 13 of the proportion of VL appearing across capacitance 17 in series with capacitance 2 of a reactive potentiometer across VL, so that the unidirectional voltage at the tap represents the load impedance; the error due to the self-inductance 18 of resistance 10 at high operating frequencies is compensated by a resistance 16 in series with capacitance 17, such that the time constant of resistance 10 and stray inductance 18 is equal to that of 16 and 17 in series at the working frequency.