RF Phase Detectors and Comparators

RF 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. 

There are several mounting styles for RF phase detectors and comparators. Surface mount technology (SMT) adds components to a printed circuit board (PCB) by soldering component leads or terminals to the top surface of the board. Through hole technology (THT) mounts components by inserting component leads through holes in the board and then soldering the leads in place on the opposite side of the board. Flat pack (FPAK) devices have flat leads and are available in a variety of body sizes and pin counts. Connectorized devices attach with coaxial or other types of connectors. They provide exceptional thermal and electrical performance and are available in cavity-up and cavity-down configurations. Waveguide assemblies consist of a hollow metallic conductor with a rectangular or elliptical cross-section. Some conductors contain solid or gaseous dielectric materials. 

RF phase detectors and comparators use several types of connectors. Bayonet Neil-Concelman (BNC) connectors are used in applications to 2 GHz. Threaded Neil-Concelman (TNC) connectors are similar in size to BNC connectors, but feature a threaded coupling nut for applications that require performance to 11 GHz. Miniature coaxial (MCX) connectors provide broadband capability through 6 GHz and are used in applications where weight and physical space are limited. Ultra high frequency (UHF) connectors are designed with non-constant impedance for use in comparatively low voltage and low frequency applications. Subminiature-A (SMA) connectors directly interface the cable dielectric without air gaps. Subminiature-B (SMB) connectors snap into place and are used for frequencies from DC to 4 GHz. Subminiature-P (SMP) connectors are rated to 40 GHz and, depending on detent type, can withstand from 100 to 100,000 interconnect cycles. Other connectors for RF phase detectors and comparators include MMCX, Mini-UHF, Type F, Type N, 1.6/5.6, and 7-16 connectors.

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