Spectrum analyser
An electrical spectrum analyser. It shows the signal reaching it against frequency: each tone as a peak at its rms level, over the noise floor in its resolution bandwidth.
Ports
One input, in. It has no outputs.
The model
Each tone of amplitude \(\lvert c\rvert\) shows as a peak at its rms level, in dB of a volt (or an amp, for a current):
\[L_\text{tone} = 20\log_{10}\frac{\lvert c\rvert}{\sqrt2}\ \text{dBV}.\]The noise shows as its density times the resolution bandwidth \(b\):
\[L_\text{noise}(f) = 10\log_{10}\big(S(f)\, b\big)\ \text{dBV}.\]So narrowing the resolution bandwidth lowers the noise floor and leaves the tones where they are, as on a real analyser. The frequency axis is log or linear, between two frequencies set or, at 0, automatic. The vertical axis runs down from a top level, at so many dB a division, or automatic.
Settings
| Field | Default | Meaning |
|---|---|---|
scale | "log" | Frequency axis: "log" (Log), "linear" (Linear) |
rbw | 10 | Resolution bandwidth (Hz) |
fmin | 10 | From (Hz; 0: auto) |
fmax | 1000000 | To (Hz; 0: auto) |
dbdiv | 0 | dB per division (0: auto) |
ref | 0 | Top (dB) |
From the tool's own tables, file format version 1: the names are those in a .jekray file.
Readouts
Its panel draws the spectrum, which can be saved as a picture (PNG) or as its numbers (CSV).
Assumptions
- An ideal analyser: no noise floor of its own, no spurs, and no input attenuator to set.
- Tones are drawn as peaks at their frequencies, not swept through the filter’s shape.
Related
- Oscilloscope.
- Optical spectrum analyser, for the light.