Oct 1, 2008 12:00 PM, By Jay M. Jacobsmeyer, P.E.

Spectrum analyzers are indispensable, but they have limitations

At its most basic level, the spectrum analyzer is a frequency-selective voltmeter calibrated to display the rms value of a sine wave. The most familiar output of the spectrum analyzer is a display of amplitude versus frequency, which is useful for a variety of RF measurements, including channel power, emission mask compliance, spurious products, intermodulation interference, noise, adjacent channel occupied power, radiated interference, modulation index, and occupied bandwidth. When equipped with a tracking generator, the spectrum analyzer also can be used to collect stimulus/response measurements of filters, isolators, couplers, amplifiers, or other active and passive devices. The spectrum analyzer is probably the most important single test instrument available to the land mobile radio technician or engineer.

Our treatment of the spectrum analyzer proceeds in four parts: theory of operation, instrument limitations, typical measurements and common pitfalls.

Theory of operation

The most popular form of the spectrum analyzer is a swept frequency superheterodyne receiver employing a variable resolution bandwidth filter, envelope detector, log amplifier and variable video bandwidth filter as shown in Figure 1. Typically, the output of the sample and hold circuit is sampled by an A/D converter, and the resulting digital signal is displayed on an LCD.

Instrument limitations

The spectrum analyzer is an ultrawideband, superheterodyne receiver with high dynamic range. Wide bandwidth and high dynamic range are difficult to achieve without sacrificing sensitivity, and spectrum analyzers have notoriously bad noise figures. A high-performance spectrum analyzer will have a noise figure of 22 dB, while low-cost units often have noise figures greater than 30 dB. In comparison, a typical LMR repeater has a noise figure of 6 dB, so a spectrum analyzer is generally a poor choice for measuring weak signals. One can improve the sensitivity of the spectrum analyzer with an external or internal preamplifier at the expense of degraded dynamic range.

A second limitation of the spectrum analyzer is its extremely wide front end. When measuring over-the-air signals at antenna sites, the analyzer's front end is easily overdriven by unwanted signals, unless a bandpass filter is used to isolate the signal of interest. Note that the instrument salesman sometimes gets to specify the instrument's dynamic range, and he knows the bigger the number, the better. Consequently, the dynamic range specified is often the difference (in dB) between the 1 dB compression point and the displayed noise floor using the narrowest resolution bandwidth available in the instrument. This advertised dynamic range can be as high as 130 dB. A better definition of dynamic range is the difference in amplitude between two input signals and the resulting third-order intermodulation (IM) product. Usually, this is a much smaller number for the same instrument — typically 70-80 dB for input signals at -30 dBm — but far more useful.

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