Understanding RF Instrument Specifications: Part 2

This is the second of a 3-part tutorial designed to help everyday engineers understand both basic and advanced RF instrument specifications. Part 1 covered generic RF instrument specifications, such as: instantaneous bandwidth, frequency range, tuning speed, phase noise, and VSWR. This article covers five specifications that are particularly important to RF signal generators: frequency tolerance, output level accuracy, output power range, intermodulation distortion, and modulation bandwidth. Each specification is described according to technical details and its importance in various applications.

When choosing an RF signal generator, it is important to consider many specifications in addition to the ones listed below. For that reason, please refer to Part 1 for additional specifications to consider.

Frequency Tolerance (Resolution and Accuracy)
For RF signal generators, frequency tolerance describes the frequency accuracy of a center frequency or carrier. More specifically, the frequency tolerance describes the maximum deviation of the instrument from the desired center frequency. Thus, it is specified in Hertz. Generally, this specification applies to both vector signal generators and vector signal analyzers. Often, it can be measured by a frequency counter.

Frequency tolerance is determined by a variety of components. More specifically, an instrument's local oscillator (LO) has the greatest affect on frequency tolerance. Because RF synthesis of an LO is based on phased-locked-loop (PLL) circuitry, the precision of its crystal oscillator (usually a VCXO or OCXO) can significantly affect the frequency accuracy of the LO. Thus, the accuracy of the reference source is also typically specified in parts per billion (PPB).

For RF signal generators, the frequency tolerance can depend on one of several factors (Figure 1). For generators which implement direct upconversion, frequency tolerance is solely dependent on the frequency accuracy of the LO. For a superheterodyne architecture (shown below), the frequency tolerance is affected by both the LO and the frequency accuracy of the intermediate frequency (IF) signal.