A transistor quadrature suppressor for A.C. servo systems

If the response angle of the demodulator in an a.c. servo system is not zero, any phase error in the input carrier signal causes an error in the servo output. Correcting the response angle to zero eliminates the output error, but is often impracticable and does not significantly reduce the residual signal at balance. This signal may saturate the amplifier, so making the servo insensitive or even completely inoperative. Both effects are eliminated if a quadrature signal of appropriate magnitude is fed to the amplifier input so as to suppress the residual signal. If the phase error and hence the residual signal are not stable, the suppression must be controlled automatically, e.g. by a resistive element whose gain is varied by the residual signal itself. The paper describes a quadrature suppressor which uses four low-power transistors and three indirectly heated thermistors. Two of the transistors form a pre-amplifier, and two operate in switched mode as a demodulator, driving two of the thermistor heaters differentially. The corresponding thermistor beads act as a variable voltage divider which is used to control a suppression signal of precise quadrature phase. The transistors are all direct coupled, and the third thermistor provides d.c. feedback around the suppressor giving thermal stability and constant output resistance characteristics in ambient temperatures between 0 and 60° C. The device is analogous to a servo-driven slide-wire of moderate linearity, and will operate with a loop gain of several hundred without instability.