Using Noise for RF Receiver Built-In Test Applications

Microwave thermal noise has long been employed for built-in test (BIT) of receivers and radiometers. Today's compact solid-state noise sources are broadband and extremely stable making them very useful signal sources for a variety of tests. What is not commonly known is how many different types of BIT systems using noise currently exist. This article identifies these uses at the system architecture level and describes the differing attributes of the noise source required for each.

Perhaps the most common application using noise is to test the RF spectral gain/loss response of the front-end transmission line and microwave control componentry from the antenna through the RF receiver. System designers will sometimes use a switch to alternate between antenna and noise source; other times a directional coupler is used. The noise signal is required to have a bandwidth at least as wide as the receiver's bandwidth. The receiver then detects the noise signal, which, in theory, is flat over the frequency band. If the detector is swept over frequency, the system will have the spectral data of its front end. The easiest way to see this in action in the laboratory is to simply use a spectrum analyzer (essentially a broadband swept receiver) and a front-end filter.