Spectrum Analyzer Tutorial

A spectrum analyzer shows signals in the frequency domain, whereas an oscilloscope shows signals in the time domain. Basically a spectrum analyzer is a tunable radio that shows the magnitude (amplitude) of radio signals at different frequencies. It is similar to an oscilloscope, but with the front end of the oscilloscope replaced by a wideband receiver. You may read my previous post, “Oscilloscope Tutorial”, as the back end circuitry is similar.

By analyzing the spectrum of a audio, or radio, signal, then frequency, power, distortion, harmonics, bandwidth, and other spectral components of the signal can be observed that are not easily detectable on an oscilloscope.

A spectrum analyzer “front end” can be purchased and used with a X-Y oscilloscope to make a fully functioning spectrum analyzer. 

Fast Fourier Transforms (FFT) analyzers are not included in this post but will be discussed in a later post.

The diagram below is a 1GHz spectrum analyzer built on the oscilloscope model in “Oscilloscope Tutorial”. There are differences as follows.

1. A spectrum analyzer will generally have a 50 ohm input impedance. It also has a 75 ohm mode, but requires a 50-75 ohm matching pad at the front end for accurate measurements at 75 ohms.
2. The vertical deflection amplifier of an oscilloscope (assuming a cathode ray tube as opposed to an LCD display) has to be high voltage and high speed. In a spectrum analyzer, the vertical deflection amplifier has to be high voltage, but can be low speed.
3. The horizontal deflection amplifier of an oscilloscope (assuming a cathode ray tube as opposed to an LCD display) has to be high voltage and high speed. In a spectrum analyzer, the horizontal deflection amplifier has to be high voltage, but can be low speed.
4. Although a spectrum analyzer is built for a 50 ohm, or 75 ohm, input, it also has the option for an active FET probe to make high impedance (10 megaohms) measurements without loading down the circuit being measured.

The diagram below is split into two sections for better viewing.

If measurements are made in a 75 ohm system, using a 50-75 ohm minimum loss pad, then a button must be pressed on the spectrum analyzer to tell it that it is measuring a 75 ohm system, because the minimum loss pad has 5.7dB of insertion loss, and the spectrum analyzer can take this into account automatically if it knows that the minimum loss pad is being used.

Notice the intermediate frequency (IF) resolution filter selections available in a spectrum analyzer. In a previous post, we questioned the response of a bandpass filter to a repeated impulse. If the bandpass resonates at 2.1 MHz, Linear Transform Invariant (LTI) theory says that only 2.1MHz will come out of the 2.1MHz bandpass. However inverse Laplace transform of a 2.1 MHz bandpass filter will show a natural decaying frequency at 2.088 MHz. By using a sufficiently narrow IF resolution filter, it can be determined experimentally if the response of a 2.1 MHz bandpass filter, to a periodic impulse, is 2.1 MHz or 2.088MHz.

An innovation to the block diagram above is a gating switch that allows a spectrum to be analyzed in a given window. Ordinary spectrum analyzers do not have such an option, but if you are transmitting a 50 MHz RF burst to a resonant circuit, and only want to look at the spectrum of the decaying pulse in the 50 MHz resonant circuit, then it is necessary to block the time interval in which the higher power 50 MHz burst is being transmitted.

The minimum loss schematic is shown below.

A high impedance FET probe kit is shown below.

A typical FET probe schematic is shown below. The circuitry must be located in the probe tip, and not at the cable end that plugs into the spectrum analyzer.

If possible, it is extremely important to use the following attachment when using a FET probe. The black lead is a ground connection and is adjacent the signal input. This limits the distance between ground and the signal being measured, otherwise many false signal artifacts will appear if the ground point is located at any appreciable distance from the signal point.

Resolution bandwidth

The resolution bandwidth filter or RBW filter is the bandpass filter in the IF path. It determines how close two signals can be and still be resolved by the analyzer into two separate peaks. Adjusting the bandwidth of this filter allows for the discrimination of signals with closely spaced frequency components. A higher RBW causes a higher measured noise floor.

Video bandwidth

The video bandwidth filter or VBW filter is the low-pass filter directly after the envelope detector. The video bandwidth determines the capability to discriminate between two different power levels. This filter is used to “smooth” the display by removing noise from the envelope. If VBW is less than RBW, the following relation for sweep time is used:

Here t is the sweep time, k is a dimensionless proportionality constant, f − f is the frequency range of the sweep, RBW is the resolution bandwidth, and VBW is the video bandwidth.

The circuit in the initial block diagram is functional only, and if a 3 Hz resolution bandwidth is really required, the analyzer may require an additional down conversion to 455 KHz.