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Problem and approach
The demands placed on modern systems for the transmission of broadband signals with high signal quality are becoming more stringent in both civil and military applications. Crucial features for the performance of these systems are a constant attenuation characteristic and a linear phase characteristic versus frequency in the useful band.
Especially for microwave applications, a wideband baseband signal at the transmitter end is upconverted to a high-frequency signal. At the receiver end, the high-frequency signal is downconverted to the baseband. Relative or absolute group delay is a measure of phase linearity versus frequency. The relative or absolute group delay measurement of the mixers or converters involved is therefore of major importance. The relative group delay can be characterized using the reference mixer or golden mixer technique, provided that the local oscillator of the mixer or at least the reference frequency can be accessed. However, numerous applications require different approaches, especially in the field of aerospace and defense, where neither the local oscillator nor a reference frequency can be accessed.
This paper describes a technique developed by Rohde & Schwarz for the network analyzers of the R&S®ZVA family: It allows high-precision group delay measurements of converters and mixers even without access to the local oscillator or the reference signal. This technique uses a two-tone signal to stimulate the mixer or converter. The phase difference between these two carriers is measured at the input and the output of the DUT (device under test) to calculate the group delay from the phase shift caused by the DUT.