Novel Phase Detector Measurement Procedure Using Quasi-Synchronized RF Generator

TL;DR

The paper addresses the challenge of characterizing phase detectors at microwave frequencies without expensive synchronized generators or frequent VNA usage. It introduces a measurement procedure that uses two quasi-synchronized RF generators sharing a $10$ MHz reference, combined with an Arduino-based control loop and simple signal-routing hardware, and alternating calibration with data acquisition to compensate phase drift. The approach yields correctly referenced $I\times I$ and $Q\times I$ curves for a 360° switched dual-multiplier detector over 3–8 GHz (1 GHz steps), achieving a final error of $\pm 2^\circ$ in frequency and input power, and enabling potential operation without a VNA in many cases. The method offers significant cost and complexity reductions while maintaining usable measurement accuracy, with practical applicability to similar phase-detector topologies and instrumentation setups.

Abstract

This paper presents a new procedure for phase detector measurements that allows the use of generators that share a 10 MHz reference oscillator but do not synchronize in phase, in other words, quasi-synchronized RF generators. The objectives are taking advantage of the benefits of using two generators but recovering lower-cost generators that have worse synchronization performance and opening the door to the possibility of using a very simple control element based in Arduino Uno and cheaper instruments. The new procedure is characterized by continuously alternating calibration and measurement sequences to make up for the phase drift of quasisynchronized generators and guarantee a maximum phase error specification (+-1 grade in this paper). Data acquisition has been divided in two stages: measurement of detector curves without phase reference (in-phase and phase-shifted) and measurement of reference data. All the data is later combined to obtain correctly referenced in-phase detector curves. The technique can be reproduced with other equivalent instrumentation. The novel procedure that allows compensation for errors (amplitude, phase shift, mismatching, etc.) is detailed, and its relation to the required measurement accuracy is amply discussed. The proposed technique is applied to characterize a phase detector based on in-phase and phase-shifted multiplication from 3 to 8 GHz with 1 GHz step. Measurements have a final maximum error of +-2 grade for both frequency and calibrated input power, according to the accuracy specifications of the VNA used to calibrate the signal distribution network, added to the +-1 grade specified in this new procedure.

Figures (11)

• Figure 1: Phasor representation of two signals of frequency $w$ and $w$ + $\Delta w$ ($w=2\cdot \pi\cdot f$).
• Figure 2: Representation of the amplitude phasors at the input of the RF combiner. It shows the effect of amplitude ($\Delta A$) and phase ($\Delta\theta_S$) deviation on both the maximum power at the output ($\theta_M =$ 0º), and the minimum power at the output ($\theta_M =$ 180º).
• Figure 3: Simplified diagram of the calibration and measurement system.
• Figure 4: (a) Photograph showing devices and equipment used for calibration and measurement. (b) Photograph showing the connection from the switches to the power combiner and the phase detector (cells 2 and 3 of the measuring array). The SA and the oscilloscope are out of frame.
• Figure 5: $I\times$$I$ and $Q\times$$I$ output signals of the detector to establish the sign convention, $\theta_{Q\times I}$ - $\theta_{I\times I}$. (a) 90º phase shift ($\beta$ = 0º) and (b) 130º phase shift ($\beta$$\approx$ 40º).