Question E6E07:
What type of transmission line is often used for connections to MMICs?  

Choices:  
A: Miniature coax  
B: Circular waveguide  
C: Parallel wire  
D: Microstrip    

Correct Answer is  D: Microstrip  

Correct answer: D — Microstrip

Why D is correct:
- MMICs operate at UHF/microwave frequencies and are mounted on substrates/PCBs with a ground plane.
- Microstrip is a planar, printed transmission line that provides controlled impedance (commonly 50 Ω), minimal lead inductance, and easy integration right up to the MMIC pads.
- It supports compact, distributed matching networks and is the standard interconnect on RF/microwave PCBs.

Why the others are incorrect:
- A: Miniature coax
  - Useful for module-to-module or board-to-board connections, but too bulky for on-chip or very short MMIC interconnects and not planar/integrated like microstrip.
- B: Circular waveguide
  - Large, rigid, and used for high-frequency, low-loss runs—not for PCB-level IC connections.
- C: Parallel wire
  - Radiates strongly at microwave frequencies, has poor shielding, and offers poor impedance control on PCBs; MMIC ports are typically unbalanced and expect microstrip/CPW.

Useful formulas and rules of thumb:
- Effective permittivity of microstrip:
  $$\epsilon_{eff} = \frac{\epsilon_r + 1}{2} + \frac{\epsilon_r - 1}{2}\left(1 + 12\frac{h}{w}\right)^{-1/2}$$
- Characteristic impedance (Hammerstad approximations):
  - For $w/h \le 1$:
    $$Z_0 \approx \frac{60}{\sqrt{\epsilon_{eff}}}\,\ln\!\left(\frac{8h}{w} + \frac{w}{4h}\right)$$
  - For $w/h \ge 1$:
    $$Z_0 \approx \frac{120\pi}{\sqrt{\epsilon_{eff}}\left(w/h + 1.393 + 0.667\ln(w/h + 1.444)\right)}$$
- Guided wavelength on microstrip:
  $$\lambda_g = \frac{\lambda_0}{\sqrt{\epsilon_{eff}}}$$
- Rules of thumb:
  - Design traces for $Z_0 \approx 50\ \Omega$ near MMIC ports.
  - Keep interconnects as short as practical compared to $\lambda_g$ unless intentionally using distributed matching.