Working principle of analog spectrum analyzer

An analog spectrum analyzer displays the magnitude of the frequency components of an input signal versus frequency, i.e. it is a frequency-domain oscilloscope. Two analog forms exist: the filter-bank (real-time) analyzer and the swept-tuned superheterodyne analyzer, which is the common one.

Swept superheterodyne spectrum analyzer

 RF in
  |
 [Attenuator]->[LPF]->( X )->[IF BPF]->[Log IF amp]
                      Mixer   (RBW)         |
                        ^              [Envelope det]
                        |                   |
                      [VCO]           [Video filter]
                        ^                   |
                        |                   v
 [Sawtooth gen]---------+-----> X   CRT  <- Y

Working principle:

1. Input attenuator and LPF: the attenuator keeps the mixer within its linear range; the low-pass filter removes image frequencies above the analyzer's range.
2. Sweep generator: a sawtooth (ramp) voltage is produced. It does two jobs: tunes the VCO linearly across a frequency range, and drives the horizontal (X) deflection of the CRT. Hence the horizontal position of the spot always corresponds to the frequency being analyzed.
3. Mixer: multiplies the input with the VCO output, producing sum and difference frequencies. When $f_{LO}-f_{in}=f_{IF}$ for an input component, that component falls in the IF passband.
4. IF filter: a narrow band-pass filter at fixed IF. Its bandwidth is the resolution bandwidth (RBW) and decides how close two components can be and still be seen as separate.
5. IF amplifier (often logarithmic): amplifies the IF signal; the log characteristic gives a dB display with wide dynamic range.
6. Envelope detector: converts the IF signal amplitude into a DC level proportional to the strength of that input component.
7. Video filter: a low-pass filter that smooths the detected output and reduces displayed noise.
8. Display: the detected voltage drives vertical (Y) deflection. As the ramp sweeps, each component produces a vertical line at its frequency with height equal to its amplitude.

Example: an AM signal at 1 MHz with 5 kHz tone shows three lines: the carrier at 1 MHz and sidebands at 0.995 and 1.005 MHz.

Filter-bank (real-time) analyzer

The input is applied to many fixed, adjacent narrow band-pass filters in parallel, each followed by a detector. An electronic switch scans the detector outputs onto the display. It shows all components at the same time (good for transients) but has limited resolution and range.

Important trade-offs and uses

• Narrow RBW gives fine resolution and a lower noise floor but requires a slower sweep (sweep time $\propto$ span/RBW$^2$).
• Uses: measuring harmonic distortion, modulation index and sidebands, occupied bandwidth, spurious emissions, noise and interference, and filter responses.