Question E4B09:
Which of the following can be measured by a two-port vector network analyzer?  

Choices:  
A: Phase noise  
B: Filter frequency response  
C: Pulse rise time  
D: Forward power    

Correct Answer is  B: Filter frequency response  

Correct answer: Filter frequency response

Why it’s correct:
- A two-port vector network analyzer (VNA) measures complex S-parameters across frequency: $S_{11}, S_{21}, S_{12}, S_{22}$.
- For a filter, the frequency response is given by the transmission coefficient $S_{21}(f)$ (magnitude and phase). Sweeping frequency and plotting $|S_{21}(f)|$ shows passband, stopband, ripple, and insertion loss; the phase of $S_{21}$ gives phase response and group delay.

Useful formulas:
- Transmission magnitude (in dB): $$|S_{21}|_{\text{dB}} = 20 \log_{10}|S_{21}|$$
- Insertion loss (passive filter): $$IL = -|S_{21}|_{\text{dB}} \quad (\text{positive dB for loss})$$
- Return loss from input reflection: $$RL = -20 \log_{10}|S_{11}|$$
- VSWR from reflection coefficient $\Gamma = S_{11}$: $$\text{VSWR} = \frac{1+|\Gamma|}{1-|\Gamma|}$$
- Group delay (from phase of $S_{21}$): $$\tau_g = -\frac{d\phi}{d\omega}$$

Why the others are incorrect:
- Phase noise: A VNA does not measure oscillator phase noise (noise sidebands around a carrier). That requires a phase-noise analyzer or spectrum analyzer with phase-noise capability.
- Pulse rise time: A VNA works in the frequency domain. While some VNAs can compute a time-domain response via math (TDR/TDT), measuring an actual pulse’s rise time is a time-domain job for a fast oscilloscope.
- Forward power: A VNA primarily measures ratios (S-parameters) and normalizes to internal references; it’s not an absolute power meter. Accurate forward power is measured with a power meter/directional coupler, not a standard two-port VNA measurement.