Short note: Spectrum analyzer

A spectrum analyzer displays signal amplitude versus frequency (frequency domain), whereas an oscilloscope shows amplitude versus time.

Superheterodyne (swept-tuned) type:

RF in-->[Atten]-->[Mixer]-->[IF filter]-->[Log amp]
                     ^         (RBW)          |
                     |                    [Detector]
                [Swept LO]                    |
                     ^                  [Video filter]
                     |                     (VBW)
              [Ramp generator]                |
                     |                        v
                     +---->[ Display: X=freq, Y=amp ]

Working:

1. The input attenuator sets a safe level for the mixer.
2. A swept local oscillator, driven by a ramp, mixes with the input; each input frequency fin passes the fixed IF filter when fLO − fin = fIF.
3. The IF filter bandwidth is the resolution bandwidth (RBW): it decides how close two signals can be and still be seen apart.
4. A log amplifier and envelope detector give amplitude in dB; the video filter (VBW) smooths noise.
5. The same ramp drives the horizontal axis, so the screen shows amplitude vs frequency.

Key specifications: frequency range, RBW, sweep time, dynamic range, sensitivity (displayed average noise level), phase noise.

Uses: measuring harmonics, spurious signals, modulation spectra, intermodulation, noise, EMI testing and occupied bandwidth. Modern analyzers also use FFT and real-time processing.