a Rigol DSA 815 spectrum analyzer

Spectrum analyzer

A spectrum analyzer is a device measuring and displaying a signal's strength (amplitude) across a frequency range. While an oscilloscope shows how a signal changes overtime, a spectrum analyzer reveals the "frequency fingerprint" hidden in the signal.

A spectrum analyzer shows the frequency domain, whereas an oscilloscope shows time domain.

Resolution bandwidth

[3] A spectrum analyzer, using the heterodyne principle, sweeps a BPF or a IF filter, across the spectrum of interest and continually measures and displays the signals' power in the BPF.

The filter's bandwidth is called the resolution bandwidth (RBW).

Ideally, we set the bandwidth as narrow as possible, in order to you the finest freq resolution. The tradeoff is sweep time. The narrower the resolution bandwidth, the longer the sweep time, but if you sent the RBW too wide, you won't see signals that are close to one another.

heterodyne principle: signal processing method mixing 2 different freqs together in a nonlinear device to make new freqs
sweep: moving/scanning across something gradually (across frequencies in this case)

[4]

Note: The analyzer's BPF is represented graphically as the high curve, but is NOT the curve itself, which is instead gain/power being passed by the filter, (low and high freq blocked, center freq passed, as a band-pass filter) .

Features

The following spectrum analyzer model used is the Rigol DSA 815.

The Frequency (Freq) button

a spectrum analyzer with the span set at 9.57 MHz with 4 harmonics

Note: The center freq (10 MHz in this case) is NOT the center of the stop freq (5.2 MHz in this case), but the center of the "stop" + "start" freq. This means that portion of the freq range isn't visible onscreen, i.e., freq values below the stop freq and beyond the start freq.
- E.g., center freq: 10 MHz = (14.78+5.22)/2, IO 10 MHz ≠ 14.78/2 = 7.39 MHz

In the screen above, center freq = 10 MHz and span = 9.57 MHz, meaning the display shows freqs from:

Start freq = 10 MHz − (9.57 MHz / 2) ≈ 5.22 MHz, i.e., it starts from 5.22 MHz
Stop freq = 10 MHz + (9.57 MHz / 2) ≈ 14.78 MHz, i.e., it stops at 14.78 MHz
- start and stop frequency: lowest and highest displayed frequency (x-value) (left and right edge respectively)

The Span button controls the frequency range's width displayed onscreen.

Span: total freq range visible across the screen (from start to stop freq).
E.g., This screen has the span at 9.57 MHz, hence the visible signal on the whole graph is 9.57 MHz wide.
- Note: Not to be confused with stop freq, the value that the lowest visible x-value starts at onscreen.
Full Span shows the widest possible frequency range of the instrument.
Zero Span sets span to 0 Hz, so you stop sweeping in frequency and instead see signal vs time (acts like an AM/time-domain view).
Zoom In narrows the span around the current center frequency (more detail, less range).
Zoom Out widens the span (more range, less detail).
Last Span goes back to the previous used span setting.

The Trace/PF button controls how the analyzer captures, processes, and displays the signal's measurement data.

The Peak button automatically places a marker on the highest-amplitude point in a visible freq spectrum, instantly calculating and displays the max freq and power level (amplitude) of the signal.

The calculation is done by converting a continuous time signal x(t) to a digital signal x[n] via an ADC, then calculates the discrete freq spectrum X[k] via the Fast Fourier Transform formula:

N = total number of sampling points (window size)
n = current time-domain index (0 ≤ n ≤ N)
k = current freq bin index (0 ≤ k ≤ N)

By being connected to a function generator, the analyzer can shown several peaks (curves), referred to as the 3rd, 5th, and 7th peaks or odd-order harmonics of the first peak/harmonic.

Note: A "peak" ≠ "harmonic". A peak is a visual curve, while a harmonic is a specific signal's physical property.

The BW/Det (Bandwidth/Detector) button controls how signals are processed and displayed, with the Bandwidth and Detector settings involved.

The RBW button sets the controls the internal BPF, adjusting how much freq detail the analyzer displays.
- A narrower RBW ("filter curve") or lower RBW increases freq resolution. It narrows the filter so we can distinguish two frequencies very close to another. It also lowers the noise floor, revealing hidden low-power signals, but increases the sweep time (screen update is slower).
- A wider RBW ("filter curve") or lower RBW provides a faster sweep time and gives a broad overview of the spectrum. But, the filter blend closely spaced signals together, and a higher noise floor may obscure weaker signals.

RBW (resolution bandwidth) : smallest freq difference analyzer can separate between 2 signals. Lower RBW = sharper view or close signals are seen separetely, Higher RBW = blurry view or close signals merge together.