What cancels out MOST local oscillator noise?

What cancels out MOST local oscillator noise?

• A. Unbalanced Crystal Mixer
• B. Single-ended Mixer
• C. Balanced Crystal Mixer (XTAL)
• D. Mixer with preamplifier

Answer: (C)

The key idea is that a balanced mixer rejects signals that appear the same on both signal paths. In a balanced crystal mixer, the LO drive is split and applied to two mixer arms with opposite phase. Any LO-related noise or leakage that shows up on both arms becomes a common-mode signal and cancels out when the two mixer outputs are combined, so the LO noise is greatly reduced at the output. In contrast, unbalanced or single-ended mixers lack that symmetry, so LO noise tends to appear in the final output. A mixer with a preamplifier would actually amplify LO noise along with the wanted signal rather than cancel it, offering no advantage for LO noise suppression. So, the balanced crystal mixer provides the strongest reduction of local oscillator noise through its differential, symmetric design.

The key idea is that a balanced mixer rejects signals that appear the same on both signal paths. In a balanced crystal mixer, the LO drive is split and applied to two mixer arms with opposite phase. Any LO-related noise or leakage that shows up on both arms becomes a common-mode signal and cancels out when the two mixer outputs are combined, so the LO noise is greatly reduced at the output.

In contrast, unbalanced or single-ended mixers lack that symmetry, so LO noise tends to appear in the final output. A mixer with a preamplifier would actually amplify LO noise along with the wanted signal rather than cancel it, offering no advantage for LO noise suppression.

So, the balanced crystal mixer provides the strongest reduction of local oscillator noise through its differential, symmetric design.