Free RF Mixer Intermodulation Spurs Tool
Map all $(m \cdot f_{RF} \pm n \cdot f_{LO})$ intermodulation products up to 5th order. Identify critical in-band IF spurious responses and compare high-side vs low-side LO injection.
📻 Frequencies & IF Passband
📊 Intermediate Frequency & In-Band Spurs
| Order (m, n) | Formula | Spur Freq | Threat |
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RF Mixer Intermodulation Spurs & Spectral Planning
RF frequency mixers are non-linear elements (diodes or Gilbert cell multiplier transistors) that multiply an RF input by a Local Oscillator (LO) to translate frequency. In addition to the desired sum or difference intermediate frequency, the non-linear transfer function generates infinite spurious intermodulation products: $$f_{spur} = |m \cdot f_{RF} \pm n \cdot f_{LO}|$$ Where $m$ and $n$ are non-negative integers ($0, 1, 2, 3...$). The order of the spur is defined as $m + n$.
Most Dangerous Mixer Spurious Responses
- Half-IF Spur ($2 \times 2$): An input at $f_{RF} - \frac{f_{IF}}{2}$ creates a second harmonic that beats with the second harmonic of the LO, producing a spurious tone landing directly inside the IF passband that cannot be filtered out by post-mixer IF bandpass filters.
- RF Feedthrough ($1 \times 0$): Direct leakage of the high-power input RF signal across the mixer die to the IF port.
- LO Leakage ($0 \times 1$): Local oscillator energy bleeding into the IF stage, potentially saturating sensitive IF amplifiers.
Frequently Asked Questions
Why can in-band mixer spurs not be removed with an IF filter?
Because they fall within the exact passband of the desired intermediate frequency signal. Once an intermodulation product is generated inside the IF filter bandwidth, no filter can distinguish it from the real converted signal. It must be prevented by RF pre-selection filtering or LO frequency planning.
What is the difference between Low-Side and High-Side LO injection?
Low-Side injection uses f_LO < f_RF, which preserves the spectral orientation of the RF signal (no spectrum inversion). High-Side injection uses f_LO > f_RF, which inverts the upper and lower sidebands, but can dramatically shift troublesome intermodulation spurs out of the band.
How does mixer IP3 relate to spur levels?
High third-order intercept (IP3) mixers (e.g. Level 17 or Level 23 diode ring mixers) have larger dynamic range and significantly attenuate odd-order intermodulation spurs (such as 2x1 and 3x1), but require higher LO drive power (+17 dBm to +23 dBm).