Question E7E09:
What occurs when the input signal levels to a mixer are too high?  

Choices:  
A: Spurious mixer products are generated  
B: Mixer blanking occurs  
C: Automatic limiting occurs  
D: Excessive AGC voltage levels are generated    

Correct Answer is  A: Spurious mixer products are generated  

Correct answer: A — Spurious mixer products are generated

Why A is correct:
- A mixer is a nonlinear device. At proper drive levels it produces mainly the desired sum and difference:
  $$f_{IF} = |f_{LO} - f_{RF}|$$
- When driven too hard, higher-order nonlinear terms grow, creating intermodulation and spurious outputs at many combinations:
  $$f = m f_{LO} \pm n f_{RF},\quad m,n \in \mathbb{Z^+}$$
- Common troublesome third-order spurs include:
  - $$2f_1 - f_2,\quad 2f_2 - f_1$$
- These “spurs” raise the noise floor and create false signals that can land in-band.

Why the others are incorrect:
- B: Mixer blanking occurs
  - “Blanking” refers to noise blankers or pulse blanking circuits, not a normal behavior of mixers under high drive.
- C: Automatic limiting occurs
  - Limiting is a function of limiter stages (often in FM IF strips), not mixers. Overdriven mixers compress and distort, they don’t “limit” cleanly.
- D: Excessive AGC voltage levels are generated
  - AGC is derived in detector/IF circuits and applied to variable-gain stages. While strong signals can increase AGC, the characteristic effect inside an overdriven mixer is generation of spurious products.

Useful formulas and rules of thumb:
- Ideal products: $$f_{SUM} = f_{LO} + f_{RF},\quad f_{DIF} = |f_{LO} - f_{RF}|$$
- Spur family: $$f = m f_{LO} \pm n f_{RF}$$
- Third-order IMD products (most problematic): $$2f_1 - f_2,\quad 2f_2 - f_1$$
- Keep signals well below the 1 dB compression point $P_{1\text{dB}}$ and maintain margin to the third-order intercept (IP3). Rule of thumb: keep desired input levels at least 10–15 dB below $P_{1\text{dB}}$ to minimize spurs and compression.