Spectrum and Network Analyzers

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.