Mixer Spur
Understanding Mixer Spurs
Every mixer generates an infinite set of output frequencies at m*fLO +/- n*fRF. Most of these are far from the IF frequency and are easily filtered. However, some products (particularly those with small m and n) may fall near or within the IF passband for certain input frequencies, creating false signals that cannot be filtered.
Spur Suppression by Mixer Type
- Single-ended: All products present. No inherent spur suppression.
- Single-balanced: Even-order LO products suppressed (m even = 0).
- Doubly-balanced: Even-order products suppressed for both LO and RF. Only odd x odd products present.
- Triple-balanced: Best spur performance. Additional port isolation.
Critical Mixer Spurs
- (1,1): Desired product. fLO - fRF = fIF.
- (2,2): 2fLO - 2fRF = 2*fIF. In-band for wideband IF.
- (3,1): 3fLO - fRF. Can fall in-band for specific frequency plans.
Frequently Asked Questions
What is a mixer spur?
A mixer spur is an unwanted output at m*fLO +/- n*fRF from the mixer's nonlinearity. Low-order spurs are strongest. If they fall in the IF passband, they create false signals. Doubly-balanced mixers suppress even-order spurs.
How do you predict mixer spurs?
Use a spur chart that plots all significant m*fLO +/- n*fRF products vs input frequency. Where the spur lines cross the IF passband, problems occur. Choose IF and LO frequencies to avoid crossings in the operational RF range.
How to minimize mixer spurs?
Use doubly-balanced mixers (suppress even-order). Apply adequate LO drive level. Filter the IF output. Choose IF/LO frequencies strategically. Use higher-IP3 mixers (reduce all spur levels). Add LO and RF filtering.