Phase and frequency comparator

1,003,601. Frequency and phase detectors. STANDARD TELEPHONES & CABLES Ltd. Sept. 8, 1961 [Sept. 9, 1960], No. 32345/61. Addition to 952,835. Heading H3A. In a phase detector as in the parent Specification producing also an output D.C. of polarity indicating the sense of any recurrence frequency error above a certain frequency error, the non-linear bi-directional element in series with storage capacitor 26 comprises a pair of parallel opposed diodes 28, 29 and these are back biased by a voltage varying with the amplitudes of the two inputs (normally bi-polar pulses) being compared (applied through capacitors 25, 26) to obviate errors due to amplitude changes of the inputs. This back bias may be self-produced by an R.C. circuit 30-32 energized by the mean rectified current through the diodes, and the output voltage derived from the junction of equal resistors 31, 32 or from a tapping of a single resistor in place of 31, 32. Alternatively one input may be applied through the storage capacitor 26 to the junction of 31, 32 and the D.C. output derived through a resistor from this junction. Alternatively 30 and 32 may both be short-circuited and the back bias applied to the other sides of the diodes by a parallel R.-C. circuit in series with resistors and a D.C. voltage variable with and derived from one at least of the two bi-polar pulse inputs.