phase detectors and comparators
phase detectors and comparators are circuits or instruments that provide a DC output voltage proportional to the difference in phase between two RF signals. They include local oscillator (LO), radio frequency (RF), and intermediate frequency (IF) components. A mixer combines signals from the local oscillator with signals from a preamplifier, producing two additional outputs: one at the input signal frequency minus the local oscillator frequency, and one at the sum of these frequencies. An IF amplifier processes the outputs of the mixer and provides band pass filtering, allowing only a selected range of frequencies to pass. Typically, double-balanced mixers are used with RF phase detectors and comparators. The mixer’s LO and RF terminals are the inputs to the phase detector. The IF terminal is the output.
Performance specifications for RF phase detectors and comparators include input frequency range, power level, LO/RF isolation, maximum DC output voltage, and maximum DC offset output voltage. Input frequency range is measured in megahertz (MHz) and includes minimum and maximum frequencies. Power level is measured in decibels per meters (dBm) and represents the required RF power that devices apply to the LO and RF ports. LO/RF isolation is the attenuation of the LO signal when measured at the RF port with the IF port properly terminated. Maximum DC output voltage occurs when the two signals (RF and LO) are in phase or 180° out of phase. Maximum DC offset output voltage is the amount of voltage present during zero-voltage conditions. Normally, zero-voltage is achieved when there is only one input, or when the two input signals are 90° out of phase.