Spur Chart
Understanding Spur Charts
Every mixer produces an infinite set of spurious products at frequencies m x f_LO +/- n x f_RF, where m and n are integers. Most of these are at frequencies far from the IF and are easily filtered. However, some products (especially low-order ones) may fall in the IF passband for certain RF input frequencies, creating false responses.
How to Use a Spur Chart
- Plot lines for each significant spur product (m x f_LO +/- n x f_RF) vs RF frequency.
- Draw horizontal lines at the IF passband boundaries.
- Wherever a spur line crosses the IF passband, a spurious response exists at that RF frequency.
- Choose IF and LO frequencies that minimize crossings in the operational RF range.
Key Spur Products
- (1,1): f_LO - f_RF = desired IF. This is the wanted product.
- (1,-1): f_LO + f_RF. Image response.
- (2,1): 2f_LO - f_RF. Second-order spur.
- (1,2): f_LO - 2f_RF. Half-IF spur.
Frequently Asked Questions
What is a spur chart?
A spur chart plots mixer intermodulation products vs RF frequency to show which spurious products fall in the IF passband. It is essential for receiver frequency planning, helping engineers avoid in-band spurs by choosing optimal IF and LO frequencies.
Why do mixers create spurs?
Mixers are intentionally nonlinear devices. While the desired product is f_LO - f_RF, the nonlinearity also produces m x f_LO +/- n x f_RF for all integer m, n. Higher-order products are weaker but can still cause problems if they fall in-band.
How do you minimize mixer spurs?
Choose IF frequency to avoid spur crossings in the operational RF band. Use higher-performance mixers (doubly-balanced mixers suppress even-order spurs). Add pre-mixer filtering. Design the LO frequency plan to push critical spurs out of band.