Originally posted Mar 25, 2013
Knowing when to look and when to shut your eyes
Measuring a pulsed or bursted signal with a spectrum analyzer will yield a valid measurement but it may not be the measurement you’re looking for. Pulsing a simple CW signal, for example, will produce a spectrum display with a nearly infinite number of sidebands whose spacing varies with the pulse period. Similarly, the envelope of the sidebands will vary with the duty cycle of the pulse and can be used to determine average carrier amplitude.
The pulse spectrum is valuable information but in many cases the sidebands will interfere with detecting and measuring important signal characteristics such as spurious, intermodulation, adjacent channel power, etc. The desired measurement for many R&D and compliance testing tasks is what the signal spectrum would be if the signal was not pulsed at all. This measurement goal applies both to pulsed CW signals and to digital modulation that is in the form of bursts or frames. That includes just about every digitally modulated signal these days!
Some answers to this measurement problem appeared in the late 1980s when digital control of spectrum analyzer local oscillators (some fully synthesized) and video data processing allowed them to choose exactly when to measure and when to “shut their eyes” and stop sweeping and/or ignore measurement results. Later developments enhanced the triggering capabilities that are so critical to the success of time-gated measurements.
A newly-published article from Agilent’s Bob Nelson describes how to optimize these time-gated measurements and explains their background. The article is from the March 20, 2013 issue of MicroWaves & RF magazine and titled “Optimize Time Gating in Spectrum Analysis” and is available at http://mwrf.com/test-amp-measurement/optimize-time-gating-spectrum-analysis. The article is the latest in a series by Bob, one of Agilent’s long-established measurement experts.
A different article describes recent work to add intelligent compensation for non-ideal LO behavior in signal analyzers, further improving the accuracy and convenience of these measurements. LO dynamics have always been a challenge in achieving the same accuracy from gated measurements as from non-gated ones. To solve these issues Agilent’s signal analyzers implement intelligent pre-sweep, post-sweep and LO back-up as shown below:
This timing diagram shows typical LO settling phenomena and their effects on the frequency output of the IF chain. These settling effects can cause measurement errors in some gated LO spectrum measurements.
You can find this article “Bringing New Power and Precision to Gated Spectrum Measurements” in the August 2007 issue of High Frequency Electronics magazine and at http://highfrequencyelectronics.com/Archives/Aug07/HFE0807_Zarlingo.pdf
Two extra terms for better understanding: “Gated video” is sometimes called “gated data” because the sweep is continuous but the measurement data is gated to display only what is valid from the gate interval. “Gated LO” is sometimes called “gated sweep” because the analyzer’s sweep starts and stops with the gate. Perfecting these starts and stops is described in the article and graphic above.
Lastly, there is one big condition for these measurement approaches (pulse spectrum and time gating) to produce valid results. The signal under test must be repeating, with any time-varying characteristics repeating consistently pulse-to-pulse. Violate this at your measurement peril!
Well, perhaps that’s a bit too restrictive. We know that many signals these days are transient and some vary with modulation or multipath or delay spread or Doppler or antenna scanning. We have plenty of tools and techniques to measure them precisely and they’ll be discussed in future posts.