Question E6E12:
Why are DIP through-hole package ICs not typically used at UHF and higher frequencies?  

Choices:  
A: Excessive dielectric loss  
B: Epoxy coating is conductive above 300 MHz  
C: Excessive lead length  
D: Unsuitable for combining analog and digital signals    

Correct Answer is  C: Excessive lead length  

Correct answer: C — Excessive lead length

1) Why C is correct
- DIP through-hole packages have long leads that add parasitic inductance and capacitance. At UHF, these parasitics are no longer “small” and they detune, delay, and mismatch circuits.
- Lead inductance rule of thumb: about $1\ \text{nH/mm}$ (≈ $20\ \text{nH/in}$).  
  Example: a 5 mm lead is ≈ $5\ \text{nH}$.
- Inductive reactance grows with frequency:
  $$X_L = 2\pi f L$$
  At $1\ \text{GHz}$, $L = 5\ \text{nH}$ gives $X_L \approx 2\pi(10^9)(5\times 10^{-9}) \approx 31\ \Omega$ per lead—large enough to cause serious impedance errors.
- Pins also have a few pF to ground/adjacent pins, giving capacitive reactance:
  $$X_C = \frac{1}{2\pi f C}$$
  At $1\ \text{GHz}$ and $C = 2\ \text{pF}$, $X_C \approx 80\ \Omega$. These L and C can resonate and cause instability or loss.
- Through-hole leads and longer trace paths act like stubs/antennas at UHF, making impedance control difficult. Surface-mount (short leads, ground paddles) is preferred.

2) Why the other choices are incorrect
- A: Excessive dielectric loss  
  Not the primary limitation. The dominant problem is series lead inductance and package parasitics, not dielectric loss of the molding compound.
- B: Epoxy coating is conductive above 300 MHz  
  False. Epoxy remains an insulator; it does not become conductive at UHF.
- D: Unsuitable for combining analog and digital signals  
  Mixing analog/digital is a design issue, not inherently prevented by DIP packaging or tied to UHF frequency operation.

3) Useful formulas and rules of thumb
- Lead inductance: ≈ $1\ \text{nH/mm}$ (≈ $20\ \text{nH/in}$).
- Inductive reactance: $$X_L = 2\pi f L$$
- Capacitive reactance: $$X_C = \frac{1}{2\pi f C}$$
- Wavelength: $$\lambda = \frac{c}{f}$$  
  At $300\ \text{MHz}$, $\lambda \approx 1\ \text{m}$; even short leads become significant reactive elements at UHF.