A modulator for controlling the frequency of the power applied to a gyroscope to rotate the seismic mass (wheel) of the gyro so as to compensate for movement of the gyro case about the spin axis of the gyro is disclosed. The modulator includes a slow acting phase-locked loop that includes: a high frequency (e.g., 20 MHz) crystal controlled voltage controlled oscillator (VCXO); a divider for dividing the VCXO frequency down by several decades (e.g., six decades); a phase comparator for comparing the relatively low frequency output of the divider with a low frequency reference signal; a loop filter for filtering the output of the phase comparator so as to give the loop its desired dynamic response characteristic and bandwidth; and, a summing amplifier for summing the output of the phase comparator with a rate signal having a voltage level linearly related to the rate of movement of the gyro case about its spin axis. The rate signal is obtained from a second gyro mounted so as to sense movement about the spin axis of the gyro whose power frequency is being modulated. The power frequency control signal is derived from the output of the VCXO via a divider, which may form part of the divider of the slow acting phase-locked loop.
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