Spectrum analyzer tracking generator

- tracking generators can be used with many spectrum analyzers to enable them to be used as a scalar RF network analyzer.

A tracking generator provides spectrum analyzers with additional measurement capability beyond that of the basic spectrum analyser.

The tracking generator enables some basic network measurements to be made, therby providing additional capability beyond basic spectrum analysis.

In view of this a tracking generator considerably extends the applications for which a spectrum analyzer can be used, making them more flexible and versatile.

Tracking generator basics

Normally spectrum analyzers are what may be termed passive instruments, making measurements of signals applied to them. Typically they may be used for measuring the spectra of oscillators, transmitters or other signals in RF systems. They measure signals in the frequency domain rather than the time, and this makes them ideal for looking at many RF signals.

In their basic form, analyzers are not able to make response or network measurements. These types of measurements require signals to be applied to a particular device or network under test, and then measuring the response or output.

In order to make a network measurement like this, it is necessary to have a source to stimulate the device under test, and then a receiver is needed to measure the response. In this way it is possible to make a variety of network measurements including frequency response, conversion loss, return loss, and other measurements such as gain versus frequency, etc..

There are two items of test equipment that can be made to make these stimulus-response measurements. Possibly the most obvious type of test equipment is an RF network analyzer and the other is a spectrum analyzer with a tracking generator. If phase information is required, then it is necessary to use a vector network analyzer, but it possible to use a spectrum analyzer tracking generator arrangement for many other measurements. As many laboratories will already use a spectrum analyzer, the tracking generator approach is particularly attractive. In addition to this, tracking generators are incorporated into many spectrum analyzers as standard. This means that it is possible to use these test instruments to make many network measurements at no additional cost.

What is a tracking generator?

A spectrum analyzer tracking generator operates by providing a sinusoidal output to the input of the spectrum analyzer. The by linking the sweep of the tracking generator to the spectrum analyzer, the output of the tracking generator is on the same frequency as the spectrum analyzer, and the two units track the same frequency.

Block diagram for the tracking generator for a superheterodyne swept spectrum analyzer
Spectrum Analyser and Tracking Generator Diagram

If the output of the tracking generator was connected directly to the input of the spectrum analyzer, a single flat line would be seen with the level reflecting the output level of the tracking generator.

If a device under test, such as a filter is placed between the output of the tracking generator and the input of the spectrum analyzer, then the response of the device under test will alter the level of the tracking generator signal seen by the spectrum analyzer, and the level indicated on the analyzer screen. In this way the response of the device under test will be seen on the analyzer screen.

Using a tracking generator

Using tracking generators is normally very easy. As a tracking generator is either built into the spectrum analyzer, or is manufactured as an external option for a test instrument, then there are few issues with their use. However there are a few standard precautions to remember when using one:

  • Adjust tracking generator to centre of analyse passband:   There is often an adjustment for the tracking oscillator to trim its frequency. Before using the tracking generator, it is wise to adjust the frequency trim adjustment to ensure that it is on exactly the same frequency as the spectrum analyzer. This is achieved by maximising the reading on the spectrum analyzer display.
  • Calibrate system using direct connection:   To ensure that any cable losses are known, it is always wise to replace the device under test with a back-to-back connector, or other short connecting line. In this way, the system will reveal any losses which it may be possible to "calibrate out".

When using a spectrum analyzer tracking generator it is possible to make many measurements very easily. A few precautions, when making the measurements will enable inaccuracies to be counteracted, and reliable measurements made.

By Ian Poole

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