Question E7H13:
Which of the following is a technique for providing highly accurate and stable oscillators needed for microwave transmission and reception?  

Choices:  
A: Use a GPS signal reference  
B: Use a rubidium stabilized reference oscillator  
C: Use a temperature-controlled high Q dielectric resonator  
D: All these choices are correct    

Correct Answer is  D: All these choices are correct  

Why D is correct
- All three listed methods are standard, proven ways to obtain the very high frequency accuracy and stability required for microwave LOs and transverters:
  - GPS-disciplined references lock a local 10 MHz (or similar) standard to GPS timing for superb long-term accuracy.
  - Rubidium (Rb) atomic standards provide excellent holdover stability and low aging as a primary 10 MHz reference.
  - Temperature-controlled high-Q dielectric resonator oscillators (DROs/OCXOs) give very low phase noise and good short/medium-term stability, often phase-locked to a reference.

Why A–C alone are not the best choices
- A, B, and C are each correct techniques, but each is only one valid method. Since the question asks which technique(s) can provide the needed performance and all three do, the most complete answer is D: All these choices are correct.

Quick notes on each technique
- A: GPS signal reference (GPSDO)
  - Locks to GPS 1 PPS or 10 MHz for long-term accuracy on the order of $10^{-12}$ (averaged).
  - Excellent absolute frequency accuracy; short-term phase noise depends on the local oscillator and disciplining loop.
- B: Rubidium stabilized reference
  - Atomic standard with very good stability (typical short-term Allan deviation $10^{-11}$ to $10^{-12}$).
  - Good holdover without sky view; modest aging (e.g., $\sim10^{-10}$/day) can be steered by GPS for long-term accuracy.
- C: Temperature-controlled high-Q dielectric resonator
  - High-Q resonators yield low phase noise; temperature control reduces drift.
  - Often used as a low-noise local oscillator and then phase-locked to a 10 MHz reference for absolute accuracy.

Useful formulas and rules of thumb
- Frequency error from stability (ppm):
  $$\Delta f = f \times \text{ppm} \times 10^{-6}$$
  Example at $f = 10\ \text{GHz}$:
  - $1\ \text{ppm} \Rightarrow 10{,}000\ \text{Hz}$ error
  - $0.1\ \text{ppm} \Rightarrow 1{,}000\ \text{Hz}$ error
- PLL multiplication and phase noise:
  $$f_{\text{LO}} = N\, f_{\text{ref}}, \quad \frac{\Delta f_{\text{LO}}}{f_{\text{LO}}} \approx \frac{\Delta f_{\text{ref}}}{f_{\text{ref}}}$$
  Phase noise increases with multiplication approximately by:
  $$L_{\text{out}} \approx L_{\text{in}} + 20\log_{10} N\ \text{(dBc/Hz)}$$
- Practical takeaway: For microwave LOs, start with a very stable/accurate 10 MHz (GPSDO or Rb), then use low-noise oscillators (e.g., temperature-controlled high-Q DRO/OCXO) and PLL chains to reach the microwave frequency while preserving stability and minimizing phase noise.