Radio Frequency Spectrum Analyser

Spectrum analysis is the analysing of signals in terms of their frequencies. The spectrum analyser can measure the frequencies present in a complex signal or the frequencies resulting from modulation on a carrier.

The spectrum analyser is a measurement instrument that displays frequency components of a signal. Each frequency component contained in the input signal is displayed as a signal level corresponding to that frequency. The vertical scale displays the amplitude of each component, and the chosen frequency band is displayed horizontally.

Spectrum analyser (SA) just like any other electronic test instrument is used in design test and maintenance of Radio Frequency circuitry and equipment. A spectrum analyser operate in same way as an oscilloscope as a basic tool used for observing signals. The only difference between a Spectrum analyser and an Oscilloscope is that the former looks at signals in the frequency domain while the latter looks at signals in the time domain.

Features of a Spectrum Analyser

• Frequency tuning range –measurement range of the analyzer so that all of the frequency components are measured.
• Frequency accuracy and stability –to be more stable and accurate than the signal to be measured.
• Sweep width –the frequency band over which the unit can sweep without readjustment.
• Sensitivity and or noise figure – to observe very small signals or small parts of large signals.
• Phase noise –a signal with spectral purity greater than that of the analyser conversion, oscillators cannot be characterized.
• Sweep rate –maximum sweep rate is established by the settling time of the filter that sets the resolution bandwidth.
• Dynamic range –the difference between the largest and smallest signal the spectrum analyser can measure without readjustment.

Types of Spectrum Analysers

We have two types of Spectrum analysers:

1. Analog Spectrum analyser –uses either a variable band-pass filter whose mid-frequency is automatically tuned (shifted, swept) through the range of frequencies of which the spectrum is to be measured or Superheterodyne receiver where the local oscillator is swept through a range of frequencies.
2. A Digital spectrum analyser computes the Discrete Fourier Transform (DFT), a mathematical process that transforms a waveform into the components of its frequency spectrum.

You can also read: The Fundamentals of Oscillators in Communication Systems

Operation of a Superheterodyne Spectrum Analyser (SPA)

The Superheterodyne analyser operates on the same principle as AM or FM radio receivers. In Superheterodyne, the incoming RF signal is first moderately amplified with user-adjustable gain factor and then sent to the one input of a mixer which acts like an analog multiplier.

The output signal of a local oscillator goes to the other input of the mixer and then the output of the mixer is the sum and difference frequency between the RF and local oscillators. This signal then goes to the intermediate frequency (IF) port of the mixer. The IF signal is subsequently band filtered, rectified and sent to the vertically deflecting plates of the Cathode Ray Tube (CRT) in the SPA.