How do I calculate the spurious products of a mixer using the spur chart or intermodulation table?
Mixer Spur Analysis
Every mixing product m×fRF ± n×fLO that falls within the IF passband creates a spurious response: a false signal that appears at the IF output as if a real signal were present at that RF frequency. The receiver will detect and process this spurious signal, potentially causing false detections, interference, or data errors.
| Parameter | Passive Diode | Active FET | Subharmonic |
|---|---|---|---|
| Conversion Loss/Gain | 5-9 dB loss | 0-10 dB gain | 8-12 dB loss |
| LO Drive Level | +7 to +17 dBm | -5 to +5 dBm | +5 to +13 dBm |
| IP3 (typical) | +15 to +30 dBm | +5 to +20 dBm | +10 to +20 dBm |
| Noise Figure | 5-9 dB (= conv. loss) | 8-15 dB | 9-14 dB |
| LO-RF Isolation | 25-45 dB | 15-35 dB | 20-40 dB |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
Frequently Asked Questions
How do I choose IF to avoid spurs?
Select the IF frequency so that no low-order (m+n ≤ 4) spur falls at the IF for any RF frequency within the intended receive band. This often requires iterating through candidate IF frequencies and checking the spur chart at each. Software tools automate this analysis.
What about the image spur?
The image (m=1, n=1 with the opposite sign) is the most significant spur. It's not really a spur but the fundamental response at the image frequency. Image rejection (preselector or image-reject mixer) handles this separately from the spur chart analysis.
Do digital IFs change the spur analysis?
Yes. In receivers with wide IF bandwidth (digital IF), more spurs can potentially fall within the IF passband. The spur analysis must cover the entire IF bandwidth, not just the center frequency. This often drives the choice toward higher IF frequencies and wider spur-free ranges.