Introduction to Bi-phase Modulators

A bi-phase modulator is a component of many complex IQ and quadrature modulator systems, and performs the task of two-state phase modulation (between 0 and 180 degrees) and is often paired with a continuously variable attenuator to provide both amplitude and phase modulation.. Bi-phase modulators can be used directly to produce binary phase shift keying (BPSK) modulation, or can be used as a component of a vector modulator by using multiple out-of-phase bi-phase modulators. Two 90 degree out-of-phase bi-phase modulators can be directly used to create a quadrature modulator that performs quadrature phase shift keying (QPSK).

More complex modulation is possible using trigonometric function input to achieve an arbitrary phase with two bi-phase modulators. For instance, a vector modulator can be implemented using two 90 degree out-of-phase bi-phase modulators (using a 90 degree hybrid at the input and a power combiner at the output), resulting in continuous phase and amplitude modulation (IQ modulation). The inputs to the I and Q ports at the bi-phase modulators (usually IQ mixers in this case) must be varied using sine and cosine functions with carefully matched peak amplitude.

An IQ vector modulator comprised of a 90 degree hybrid, an in-phase combiner, bi-phase modulators, and phase invariant attenuators.

In this arrangement, bi-phase modulators replace the use of mixers and filters to realize complex modulation circuitry. Additionally, bi-phase modulators can be used as components of frequency modulated continuous wave (FMCW) radar, in phased array antennas, as to produce the feed-foward linearization technique with a vector modulator replacing an attenuator/phase shifter cascade in the nulling loop (providing enhanced performance). Even old-style radar decoys used phase varying circuits to return a radar signal that was modified in phase in such a way to present the target as being in a different position.

An example circuit of a double-balanced bi-phase modulator comprised of two hybrids, an integrated driver, and two single-balanced bi-phase modulators.

If a double-balanced bi-phase modulator is configured so that the magnitude of the vector remains fixed, then an IQ vector modulator can be used to provide a desired phase shift with constant amplitude. In this case the I input would need to follow the cosine of theta, and the Q input would need to follow sine of theta. Bi-phase modulator based IQ modulators can also be used as frequency translators and anti-jamming circuits in electronic counter-measure (ECM) applications.

Several key performance factors of bi-phase modulators are frequency range, insertion loss, phase error, VSWR, impedance, switching speed, control logic, RF input power range, environmental performance and packaging. In the case where mixers are used as bi-phase modulators to create IQ modulators, the conversion loss, isolation between ports (Lo to RF, Lo to IF, and RF to IF), the linearity (1 dB compression point and two-tone third order intercept point) are also important performance factors to consider along with the frequency range of each port (IF, RF, LO).

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