A method of stabilizing a variable filter for an analog electromagnetic signal against circuit oscillation includes the steps of: providing a signal loop comprising a signal input, a signal output, and a plurality of variable circuit elements connected in the signal loop, the plurality of variable circuit elements comprising an adjustable resonator and an adjustable gain block, the signal loop having a variable frequency response that is characterized by a central frequency, a frequency passband, a response Q, and an operating point and a resonator response curve that are plottable in a Cartesian s-plane having an origin, a real axis, and an imaginary axis; and maintaining stability of the variable filter within an operating range by controlling the adjustable resonator and the adjustable gain block such that, in the Cartesian s-plane, the resonator response curve satisfies an orthogonality stability condition.

Systems, devices, and methods including a first band-pass filter configured to receive and filter a detector signal where the first band-pass filter has a central frequency of 2f; a second band-pass filter configured to receive and filter the detector signal where the second band-pass filter has a central frequency of 1f; a first logarithmic amplifier (Log Amp) configured to apply the filtered detector signal from the first band-pass filter; a second Log Amp configured to apply the filtered detector signal from the second band-pass filter; a differential amplifier configured to subtract the applied signal from the first Log Amp from the applied signal from the second Log Amp; and an Anti-Log Amplifier configured to determine an inverse logarithm of the subtracted signal from the differential amplifier.

Systems, devices, and methods including a first band-pass filter configured to receive and filter a detector signal, where the first band-pass filter has a central frequency of 2f; a second band-pass filter configured to receive and filter the detector signal where the second band-pass filter has a central frequency of 1f; a first logarithmic amplifier (Log Amp) configured to apply the filtered detector signal from the first band-pass filter; a second Log Amp configured to apply the filtered detector signal from the second band-pass filter; a differential amplifier configured to subtract the applied signal from the first Log Amp from the applied signal from the second Log Amp; and an Anti-Log Amplifier configured to determine an inverse logarithm of the subtracted signal from the differential amplifier.

Systems, devices, and methods including a first band-pass filter configured to receive and filter a detector signal, where the first band-pass filter has a central frequency of 2f; a second band-pass filter configured to receive and filter the detector signal, where the second band-pass filter has a central frequency of 1f; a first logarithmic amplifier (Log Amp) configured to apply the filtered detector signal from the first band-pass filter; a second Log Amp configured to apply the filtered detector signal from the second band-pass filter; a differential amplifier configured to subtract the applied signal from the first Log Amp from the applied signal from the second Log Amp; and an Anti-Log Amplifier configured to determine an inverse logarithm of the subtracted signal from the differential amplifier.

A signal processing circuit has a first signal loop with a first signal processing block and a first feedback path that extends around the first signal processing block, the first signal processing block having a frequency dependence that causes the first signal loop to generate a passband. A second signal processing block is downstream of the first signal loop. A second feedback path extends from downstream of the second signal processing block to upstream of the first signal processing block. In operation, the first feedback path reinforces a signal in the passband and the second feedback path conditions the signal at an output downstream of the first signal processing block.