Abstract: A method and apparatus to characterize a modulator without requiring access to modulator local oscillators. A periodic signal is input so that at least two tones are output, and the envelope of the filtered output is detected. The modulated output signal is generally differentially filtered before envelope detection, and the measured amplitude and phase of the envelope of the differentially filtered modulator output frequencies, combined with the known filter characteristics, are used to deduce the amplitude and phase of the modulated output. The modulator input signal frequencies may advantageously be controlled to substantially limit the output to frequencies which differ from a reference frequency by odd multiples of a delta frequency, reducing intermodulation components. In this special case the modulator output may be deduced from a squared envelope signal without a need for differential filtering. The deduced output, compared to the known input, characterizes the modulator at the input frequencies.

Abstract: A broadband high frequency device is capable of determining linear combinations of broadband high frequent signals, in particular determining a differential signal, from input signals, being applied substantially simultaneously. One embodiment is based on a plurality of broadband microwave structures, arranged such that symmetry properties can be exploited. The output signal of this differential coupler is proportional to the difference of 2 input ports. This signal is made using a hybrid structure. A pair of broadband impedances are used to scale the 2 incoming signals with 2 complex coefficients. A symmetric structure of two identical broadband directional couplers and a symmetric power combiner are used afterwards to obtain the differential signal. The high bandwidth of both the terminations, the directional couplers and the power combiner make it possible to obtain a differential coupling factor and a common mode rejection ratio which is frequency insensitive over several decades.