What is the difference between a single balanced and a double balanced mixer in terms of spurious performance?
Balanced Mixer Types
Mixer balance refers to the symmetry of the diode configuration and its effect on suppressing unwanted mixing products. An unbalanced (single-ended) mixer has no inherent spurious suppression and produces all possible m×fLO ± n×fRF mixing products at the IF port. Each level of balance adds a degree of spurious suppression.
The single-balanced mixer uses a transformer (balun) at either the LO or RF port to drive two diodes in a push-pull configuration. This cancels either even LO harmonics (LO-balanced) or even RF harmonics (RF-balanced) at the IF output. LO-to-IF and RF-to-IF leakage is reduced by 20-30 dB compared to unbalanced.
The double-balanced mixer (DBM) uses baluns at both LO and RF ports, driving four diodes arranged in a ring or star. All even-order products of both LO and RF are suppressed: 2fLO, 4fLO, 2fRF, 4fRF, 2fLO±2fRF, etc. This dramatically cleans up the IF spectrum, leaving only the desired products (fRF±fLO) and odd-order intermodulation products (3×1, 1×3, etc.).
Frequently Asked Questions
How much spur suppression?
Single-balanced: 20-35 dB suppression of even-order products of the balanced port. Double-balanced: 30-50 dB suppression of all even-order products. The actual suppression depends on the balance (amplitude and phase matching between diodes and balun arms).
What about triple-balanced mixers?
Triple-balanced mixers use two double-balanced mixer rings and additional transformers, providing isolation and spurious suppression on all three ports simultaneously. They achieve the widest bandwidth (multi-octave on all ports) but have higher conversion loss (8-10 dB) and require more LO power.
When is single-balanced sufficient?
When the simplified spurious spectrum is acceptable and cost/size are priorities. Single-balanced mixers are smaller, require less LO power (3 dB less than DBM), and are easier to integrate at mmWave frequencies where four-diode ring configurations become impractical.