Planar Microwave Engineering A Practical Guide to Theory, Measurement, and Circuits

INTRODUCTION

Oscilloscopes and spectrum analyzers are ubiquitous pieces of test equipment in any RF laboratory. The reason, of course, is that it is useful to study signals in both time and frequency domains, despite the fact that both presentations theoretically provide equivalent information.

Most electrical engineers are familiar with basic operational principles of lower frequency oscilloscopes. However, an incomplete understanding of how probes behave (particularly with respect to grounding technique) is still remarkably widespread. The consequences of this ignorance only become worse as the frequency increases and so, after a brief review of a conventional low-frequency scope, our primary focus will be the additional considerations one must accommodate when using scopes at gigahertz frequencies. Also, because the sampling oscilloscopes commonly used at high frequencies have subtle ways of encouraging “pilot error,” we'll spend some time studying how they work and how to avoid being fooled by them. High-speed sampling circuits are interesting in their own right, so these types of scopes give us a nice excuse to spend a little bit of time examining how samplers function.

Another amazing instrument is the modern spectrum analyzer (with cost approximately proportional to the square of amazement), which is capable of making measurements over a wide dynamic range (e.g., 80–100 dB SFDR) and over a large frequency span (e.g., near DC to 20 GHz in a single instrument).