Phase error correction system - is for phase keyed signals and has second product demodulator controlling mixing oscillator frequency

The phase-error correction system, for phase-keyed signals, shifts the phase of the IF signal by an additional 90 deg. (L2) after it has been delayed after leaving the mixer. The 90 deg. shifted IF signal is then passed to a product demodulator (M2) with the undelayed, unshifted IF signal. The undelayed unshifted IF signal is also fed to another product demodulator (M1) with the delayed unshifted IF signal. The output of the first product demodulator is filtered and used to control the frequency of the mixing oscillator (0). The advantage lies in using relatively cheap components to correct for frequency fluctuations due to temp. ageing, and transit time changes.