A gravimeter or inertial sensor system, comprising: a variable frequency signal source (100, 101, 102), configured to provide a first input signal from a first port and a second input signal from a second port, the first and second input signals having a desired frequency; a resonant sensor (103; 200) connected to the first port of the variable frequency signal source to receive the first input signal, the resonant sensor having a resonant frequency, an amplitude or a phase response that is dependent on a physical parameter to be measured; a phase comparator or an amplitude comparator connected to an output of the resonant sensor and to the second port of the variable frequency signal source to receive the second input signal, and configured to compare a phase or an amplitude of an output signal from the resonant sensor with a phase or an amplitude of the second input signal from the variable frequency signal source to provide a phase or amplitude comparator output signal; and a controller connected to the phase comparator or amplitude comparator, the controller configured in a first mode to connect to the variable frequency signal source and to control the desired frequency of the first and second input signals from the variable frequency signal source based on a value of the phase or amplitude comparator output signal to lock the frequency of the first and second input signals to a resonant frequency of the resonant sensor, and in a second mode, after the frequency of the first and second input signals has been locked to the resonant frequency of the resonant sensor, to disconnect from the variable frequency signal source and to record an open loop output signal indicative of the physical parameter to be measured based on the response of the resonant sensor. a variable frequency signal source (100, 101, 102), configured to provide a first input signal from a first port and a second input signal from a second port, the first and second input signals having a desired frequency; a resonant sensor (103; 200) connected to the first port of the variable frequency signal source to receive the first input signal, the resonant sensor having a resonant frequency, an amplitude or a phase response that is dependent on a physical parameter to be measured; a phase comparator or an amplitude comparator connected to an output of the resonant sensor and to the second port of the variable frequency signal source to receive the second input signal, and configured to compare a phase or an amplitude of an output signal from the resonant sensor with a phase or an amplitude of the second input signal from the variable frequency signal source to provide a phase or amplitude comparator output signal; and a controller connected to the phase comparator or amplitude comparator, the controller configured in a first mode to connect to the variable frequency signal source and to control the desired frequency of the first and second input signals from the variable frequency signal source based on a value of the phase or amplitude comparator output signal to lock the frequency of the first and second input signals to a resonant frequency of the resonant sensor, and in a second mode, after the frequency of the first and second input signals has been locked to the resonant frequency of the resonant sensor, to disconnect from the variable frequency signal source and to record an open loop output signal indicative of the physical parameter to be measured based on the response of the resonant sensor.