A Beginning Tutorial on Spectrum Analysis (Part-2)

This is the second part of the blog. If you missed out on the first part, I recommend you read the first part here before reading this one.

Spectrum Analyzer Block Diagram

Spectrum Analyser Block Diagram

The major components in a spectrum analyzer are the RF input attenuator, mixer, IF (Intermediate Frequency) gain, IF filter, detector, video filter, local oscillator, sweep generator, and LCD display. Before we know how these pieces work together, let’s get a fundamental understanding of each component individually.

RF Input Attenuator and IF Gain Circuit

RF Attenuator and IF Gain Circuit

The RF Input Attenuator is a step attenuator located between the input connector and the first mixer. It is also called the RF attenuator. This is used to adjust the level of the signal incident upon the first mixer. This is important to prevent mixer gain compression and distortion due to high-level and/or broadband signals.

The IF Gain is located after the mixer but before the IF, or RBW, filter. This is used to adjust the vertical position of signals on the display without affecting the signal level at the input mixer. When changed, the value of the reference level is changed accordingly. Since we do not want the reference level to change (i.e. the vertical position of displayed signals) when we change the input attenuator, these two components are tied together. The IF gain will automatically be changed to compensate for input attenuator changes, so signals remain stationary on the display, and the reference level is not changed.

Mixer Circuit

Mixer Circuit

A Mixer is a device that converts a signal from one frequency to another. Therefore, it is sometimes called a frequency-translation device.

By definition, a mixer is a non-linear device (frequencies are present at the output that were not present at the input). The local oscillator signal (Flo) is applied to one port of the mixer and the signal to be converted (Fsig) is applied to the second port. The output of a mixer consists of the two original signals (Fsig and Flo) as well as the sum (Flo+Fsig) and difference (Flo-Fsig) frequencies of these two signals. In a spectrum analyzer, the difference frequency is the frequency of interest. The mixer has converted our RF input signal to an IF (Intermediate Frequency) signal that the analyzer can now filter, amplify and detect to display the signal on the screen. We will see how this is done shortly.

IF Filter Circuit

IF Filter Circuit

The IF filter is a bandpass filter that is used as the “window” for detecting signals. Its bandwidth is also called the resolution bandwidth (RBW) of the analyzer and can be changed via the analyzer's front panel. By giving you a broad range of variable resolution bandwidth settings, the instrument can be optimized for the sweep and signal conditions, letting you trade-off frequency selectivity (the ability to resolve signals), signal-to-noise ratio (SNR), and measurement speed.

We can see from the above image that as RBW is narrowed, selectivity is improved (we can resolve the two input signals). This will also often improve SNR. The sweep speed and trace update rate, however, will degrade with narrower RBWs. The optimum RBW setting depends heavily on the characteristics of the signals of interest.

Detector Circuit

Detector Circuit

The analyzer must convert the IF signal to a baseband or video signal so it can be viewed on the instrument’s display. This is accomplished with an envelope detector which then deflects the display on the y-axis or amplitude axis. Many modern spectrum analyzers have digital displays which first digitize the video signal with an analog-to-digital converter (ADC). This allows for several different detector modes that dramatically affect how the signal is displayed. The Positive-Peak Detector Modecaptures and displays the peak value of the signal throughout one trace element. This mode is good for analyzing sinusoids but tends to over-respond to noise when no sinusoids are present. Similarly, the Negative-Peak Detector Mode captures the minimum value of the signal for each bin.

In Sample Detection Mode, a random value for each “bin” of data (also called a trace element) is produced. This detector mode is best for computing the RMS value of noise or noise-like signals, but it may miss the peaks of burst signals and narrowband signals when the RBW is narrower than the frequency spacing of the bins. For displaying both signals and noise, a detector mode called the Normal Detector Mode(or sometimes the Rosenfeld detector) is used. In this mode, if the video signal is monotonically increasing or decreasing during the period representing one trace element, then it is assumed that a spectral component is being measured, and positive-peak detection is used. If the signal level is changing non-monotonically during this time (i.e. it rose and fell), then it is assumed that noise is being measured, and tracepoints alternate between positive- and negative peak detection. When a minimum value is displayed, the maximum value is saved and compared to the maximum value for the next trace element. The higher of the two values is displayed. This technique provides a better visual display of random noise than peak detection yet avoids the missed-signal problem of sample detection.

Video Filter

Video Filter

The Video Filter is a low-pass filter that is located after the envelope detector and before the ADC. This filter determines the video amplifier bandwidth and is used to average or smooth the trace seen on the screen. The spectrum analyzer displays signal-plus-noise so that the closer a signal is to the noise level, the more the noise makes the signal more difficult to read. By changing the video bandwidth (VBW) setting, we can decrease the peak-to-peak variations of noise. This type of display smoothing can be used to help find signals that otherwise might be obscured in the noise.

Sweep Generator Circuit

Sweep Generator Circuit

The Local Oscillator is a Voltage Controlled Oscillator (VCO) which in effect tunes the analyzer. The Sweep Generator tunes the LO so that its frequency changes in proportion to the ramp voltage. This also deflects the display screen from left to right, creating the frequency domain in the x-axis.