Question E4A02:
Which of the following parameters does a spectrum analyzer display on the vertical and horizontal axes?  

Choices:  
A: Signal amplitude and time  
B: Signal amplitude and frequency  
C: SWR and frequency  
D: SWR and time    

Correct Answer is  B: Signal amplitude and frequency  

A spectrum analyzer shows how signal energy is distributed across frequency. Its display is:

- Horizontal axis: frequency (Hz)
- Vertical axis: signal amplitude, typically power level in dBm or dBµV

Why B (Signal amplitude and frequency) is correct:
- A spectrum analyzer sweeps or FFTs across frequency and plots the amplitude (power) present at each frequency bin. This is literally the definition of a spectrum.

Why the other choices are incorrect:
- A (Signal amplitude and time): That’s an oscilloscope display. Oscilloscopes plot voltage (amplitude) vs time.
- C (SWR and frequency): SWR vs frequency is measured by an antenna/network analyzer. A plain spectrum analyzer does not directly measure SWR; with extra fixtures it can show return loss vs frequency, but the native axes are still amplitude vs frequency.
- D (SWR and time): Not a standard measurement for a spectrum analyzer.

Useful formulas and rules of thumb:
- Power in dBm: 
  $$P_{\mathrm{dBm}} = 10 \log_{10}\!\left(\frac{P_{\mathrm{mW}}}{1\,\mathrm{mW}}\right)$$
- Voltage–power (for a 50 Ω system):
  $$P = \frac{V_{\mathrm{rms}}^2}{R}, \quad V_{\mathrm{rms}} = \sqrt{P R}$$
  Example: 0 dBm = 1 mW into 50 Ω → $V_{\mathrm{rms}} \approx \sqrt{0.001 \times 50} \approx 0.224$ V.
- dB quick facts:
  - +10 dB = ×10 power
  - +3 dB ≈ ×2 power
  - For voltages: $20 \log_{10}(V_2/V_1)$ (so +20 dB = ×10 voltage)
- Display tips:
  - Vertical scale is usually in dB per division (often 10 dB/div).
  - Horizontal scale is frequency span (set by start/stop or center/span).