Mixer Spurious Response
Understanding Mixer Spurious Responses
Mixer spurs are a critical consideration in receiver design. An unplanned spur falling at the IF frequency creates a false signal that degrades dynamic range and can mask weak desired signals.
Spur Analysis
- (1,1) = f_RF - f_LO: The desired IF product. Conversion loss 5-8 dB.
- (2,1) = 2*f_RF - f_LO: Second-order RF harmonic mix. Suppressed by double-balanced mixer.
- (1,2) = f_RF - 2*f_LO: Second LO harmonic mix. Suppressed by double-balanced mixer.
- (3,1) = 3*f_RF - f_LO: Third-order RF spur. Always present. Typically -30 to -40 dBc below desired.
Spur Avoidance
- Use double-balanced mixer (removes all even-order products).
- Choose IF frequency to avoid m,n product frequencies for m,n up to 5.
- Use pre-selection filtering to limit RF harmonics entering the mixer.
Frequently Asked Questions
What are mixer spurs?
Mixer spurs are unwanted outputs at f = m*f_RF +/- n*f_LO. The (1,1) product is desired; all others are spurs. Double-balanced mixers suppress even-order products. Proper IF planning avoids in-band spurs.
How do I find mixer spurs?
Create a spur chart: plot all m*fRF +/- n*fLO products (m,n = 1 to 5) as lines on a frequency vs RF frequency plot. Identify which lines cross the IF band. Choose IF to minimize crossings in the RF tuning range.
What is the worst spur to worry about?
The (2,2) product is often problematic because it is a low-order product that is not suppressed by single-balanced mixers. The (3,1) and (1,3) products are always present. Check the mixer datasheet for specific spur levels.