SPECTRUM ANALYZER

G8AWB RAY LAWRENCE HAS PROVIDED THIS ARTICAL AS A TASTER INTO THE USES OF AN SA AND WHAT IT WOULD COST TO BUILD A WORKING EXAMPLE.

11 Dec 2010

The Spectrum Analyser

During a recent discussion the subject of using a Spectrum Analyzer (SA) for transmitter testing was raised.
To most amateurs the Spectrum Analyser is an expensive, exotic and complicated piece of equipment well out of their reach. This perception is unfortunate, as the SA is a very useful and powerful tool for RF work, both for the constructor and the operator. The cost of ownership need not be a great as many people fear, but more on that later.
First, What is Spectrum Analysis? To answer that question let’s first look at a close relative of the Spectrum Analyzer and a device which is familiar to most - the Oscilloscope. The ‘scope displays the amplitude of a waveform with respect to time. However, it can not distinguish between the various frequency components of a complex waveform, it simply sums them and displays the result; this is known as Time Domain measurement.
The Spectrum Analyser on the other hand displays the amplitude of waveforms with respect to frequency, and can display the individual frequency components of complex waveforms; this is known as Frequency Domain measurement.

Figs 1 and 2 below show how the two instruments view a signal.
Oscilloscope Spectrum Analyzer



So what is a Spectrum Analyzer? At low frequencies a technique called Fast Fourier Transform (FFT) is used, now days this is implemented in DSP. This kind of display can be seen in some consumer electronic products, such as Hi Fi sets. It is also used in some Amateur radio equipment.

At RF (above about 100 kHz) the technique used is basically a superhet receiver, which is tuned by sweeping the local oscillator. The L.O. sweep rate is synchronized with the display sweep rate, thus the horizontal deflection on the display is directly proportional to the signal frequency; Vertical deflection depends on the input signal amplitude. The tuning range of the L.O determines the input frequency range of the instrument and can extend into the Ghz region in commercial units.


Fig 3 below is simplified block diagram of a typical Spectrum Analyzer. As can be seen it similar to a conventional superhet receiver with the following notable differences. The L.O. is driven by a ramp (sawtooth) generator, which also drives the horizontal trace on the display. Manual adjustment of the centre frequency is provided. The scan width is adjusted by varying the amplitude of the ramp voltage. The detector provides a logarithmic output to allow large differences in received signal levels to be presented on the display. There is no R.F. amplifier, the input signal is applied the first mixer usually via a switched attenuator and a L.P.F.
The fact there is no selectivity before the first mixer requires extensive shielding to minimise leakage and thus false signals being presented on the display. This requirement plus the very wide frequency coverage is a major factor contributing to the high cost of commercial Spectrum Analyzer’s.

So what is the alternative? While the price of pre-used commercial Spectrum Analyzer’s is a fraction of their original cost they are still too expensive for most folk. However over the years there have been a number of articles in the Radio Amateur Press on the construction of a Spectrum Analyzer for amateur use using an oscilloscope for the display. In addition there are a number of kits available, which eases construction. Most of these are from the U.S.; the costs depend on the sophistication. A kit based on the Spectrum Analyzer featured in The Radio Communications Handbook is available in the U.K. This kit consists of just three small PCB’s and all parts, except the case, and allows the amateur to own a 100 MHz Spectrum Analyzer for about £100; or less if you have a reasonable stocked junk box. Using converters can extend the frequency range. While these instruments do not have the same level of sophistication and performance of commercial units, they don’t they carry the same price tag either; but they are adequate for most amateur applications providing you recognise their limitations.
A Spectrum Analyzer has a multitude of uses around the shack; band monitoring, alignment of filters, antenna measurements, transmitter and receiver fault finding and alignment, to name a few.

Try one to check the output of your transmitter, but be prepared for a big surprise!




G8AWB 201000