What is a Spectrum Analyzer?

The spectrum analyzer is an electronic measuring instrument that is widely used for testing and maintaining radio frequency (RF) circuits. As the name suggests, it analyzes the frequency spectrum. It is similar to an oscilloscope. The only difference is that, it provides the signal amplitude in the frequency domain, while the oscilloscope provides the signal amplitude in the time domain.

The frequency domain of the signal provides information like the harmonics present in the signal and noise levels affecting the signal at particular frequencies. The frequency domain display of the spectrum analyzer also provides the capability to view the power levels of the signal at different frequencies.

All these measurements are important from the point of view of RF circuit-testing and maintenance.

The spectrum analyzer renders a plot or trace of the amplitude of the signal, against the frequency of the signal. The horizontal axis of the device is calibrated for frequency, with the higher frequencies on the right hand side of the display. The vertical axis is calibrated for amplitude with the amplitude increasing in the upper part of the display.

The frequency axis is calibrated in a linear or logarithmic scale. Logarithmic scale facilitates a wider frequency range to be plotted on the horizontal axis. For the device with a logarithmic scale, the amplitude scale is calibrated typically as 10 dB per division.

Though the logarithmic scale provides a wider range of frequencies, it does not give the absolute value of the signal. The linear scale, on the other hand, permits only a small band of frequencies to be displayed. However, it provides the absolute value of the signal amplitude.

The analog spectrum analyzer uses a bandpass filter. A bandpass filter is a filter that permits the passage of only a particular band of frequencies. The mid-frequency of this band is automatically tuned in the device, as the range changes.

The disadvantage of a fully analog device is that the stability of the center frequency and frequency bandwidth is poor. The logarithmic displays also give errors in reading the decibel levels of the signal.

The digital spectrum analyzer uses the Fast Fourier Transform (FFT) method to give the frequency components. This is an improved version of an analog device. The amplitude accuracy of the signal is improved. The amplitude accuracy is of two types, viz., absolute accuracy and relative accuracy.

The absolute accuracy is the relative amplitude as per the standards set by the national laboratories. The relative amplitude accuracy is the ratio of two measurements of the same amplitude and it is not related to the national standards.

The digital spectrum analyzer also preserves the related phase information and amplitude information of the signal in the particular frequency. Nowadays, the digital filter is of two types; one which measures only the amplitude of the signal and the other which also measures the phase of the signal.

This device is also called the vector signal analyzer. The device which gives both the amplitude and phase information is called the hybrid spectrum analyzer, and it allows the analysis of the signal at higher frequencies with a great accuracy.

The price is mainly determined by the range of frequency that it allows to view. Further, the bandwidth resolution and stability of frequencies, are the other criteria considered to add to its cost. Although, this device has a complex circuitry and complicated working, it displays considerable information about a particular signal.