A system for detecting the amplitude of radio frequency energy includes: an antenna for receiving the radio frequency energy; a modulator, responsive to a reference frequency signal, for pulse modulating the received radio frequency energy at the reference frequency; a detector for converting such pulse modulated signal to a detector output signal having a low frequency component representative of the amplitude of the received radio frequency energy, in summation with DC bias current, and a high frequency component at the reference signal; and a high pass or band pass filter fed for the detector output signal for passing the high frequency components and for removing the low frequency component. A phase detector, with or without a subsequent IF amplifier, is fed by the reference frequency and the high frequency components for producing an output representative of the high frequency components. A low noise amplifier is fed by the antenna and has a bias fed by the reference frequency signal for modulating the received radio frequency energy at the reference frequency.