Chapter Five - Spectrum and Network Analyzers

Abstract

Spectrum analyzers automate and improve tunable radio frequency (RF) voltmeters. The heart of the spectrum analyzer is the mixer and local oscillator (LO). The LO is a voltage-controlled oscillator (VCO) that produces an output frequency that is proportional to an applied input-control voltage. In the case of a spectrum analyzer, the input voltage is a ramp and hence the voltage changes as the ramp voltage rises. Because most VCO circuits have a quadratic relationship between control voltage and frequency, it may be necessary in some cases to alter the tuning voltage waveform from a linear ramp to a shape that makes the sweep of the VCO output frequency look linear. A network analyzer examines incident, reflected, and transmitted signals through a circuit or device and displays the magnitude and phase of these signals. A spectrum analyzer, on the other hand, measures only one channel and displays magnitude and frequency. A network analyzer consists of a three-channel RF receiver and a display. The incident signal is considered to be the reference signal and hence it is called the “R-channel.” The other two channels receive the reflected signal on the A-channel and the transmitted signal on the B-channel. The chapter concludes with a discussion of network analyzers.