When you are working on your design at the component level, you need to understand how each component will behave when used in its final design. For example, in most transceivers, there are amplifiers to boost LO power to a mixer and there are filters to remove signal harmonics. You may need to measure these components to verify they perform the same as their simulation models. Tracking generators provide a cost-effective way for spectrum analyzer users to expand their testing to cover more component-level characterization. They stretch your instrument budget by expanding the capabilities of your spectrum analyzer and provide effective scalar transmission measurements.
Traditional spectrum analyzers display amplitude versus frequency, making them an excellent tool for taking power-based measurements to characterize carrier power, sidebands, harmonics, and spurs. These are essential measurements for characterizing your transmitter designs. But you’ll likely need more information about the input-output relationship between the ports of your device when performing component-level characterization. For example, you will probably want to measure insertion loss, pass/stop band of a component, or gain of an amplifier.
When characterizing components by measuring S-parameters, like S21 – the power transferred from port 1 to port 2, you probably think about using a network analyzer. At its most simplified form, you can think of a network analyzer like a tracked source and receiver. The network analyzer’s source stimulates your device under test, and the receiver measures the output. This allows you to compare what is going into your device under test to what is coming out of the device. While this is a perfect use of a network analyzer, acquiring an additional benchtop measurement tool is costly.
If you think about a signal analyzer at its most basic form, it is a receiver. If you could add a tracked generator to this receiver, you would have the basics of a network analyzer. Hence, one of the great benefits of a tracking generator! It does just this.
A tracking generator is a signal source whose RF output follows the tuning of the spectrum analyzer. The tracking generator uses the swept LO from the spectrum analyzer and mixes that with a stable, fixed oscillator. If we tune that oscillator to the center frequency of the IF filter in the spectrum analyzer and use the difference mixing product, then the output frequency of the tracking generator will equal the input frequency of the spectrum analyzer. The span of the spectrum analyzer and the tracking generator are matched and synchronous. This provides precise tracking to enable you to make single-channel, scalar network analysis with your spectrum analyzer.
Figure 1. A tracking generator connected to a handheld spectrum analyzer to measure the two-port transmission of filters and amplifiers, including insertion loss, amplifier gain, and filter pass-band.
While a tracking generator can’t replace a network analyzer for deep-dive analysis of S-parameters, it can provide some of the same capabilities of a network analyzer at a fraction of the cost. This is perfect if you are looking for a cost-effective way to perform component-level characterization. Adding a tracking generator to your spectrum analyzer enables you to measure insertion loss, pass/stop band of a component, or gain of an amplifier.
For instruction on how to set up a tracking generator with a spectrum analyzer in three steps, read Making Stimulus/Response Measurements Using the N9322C Basic Spectrum Analyzer and Tracking Generator Application Note.