How does the choice of IF frequency affect spurious response performance?
Spurious Response and IF Planning
Every mixer generates output products at all combinations of m×fRF ± n×fLO, where m and n are integers. Most of these products are weak (especially for high m and n), but some can be strong enough to create false signals at the IF frequency. The receiver then processes these false signals as if they were real, creating ghost signals, interference, or desensitization.
| Parameter | Superheterodyne | Direct Conversion | Digital IF |
|---|---|---|---|
| Image Rejection | 60-90 dB (filter) | 30-50 dB (mismatch) | N/A (digital) |
| DC Offset | No issue | Major issue | No issue |
| LO Leakage | Low | High | Low |
| Integration | Difficult | Easy (single chip) | Moderate |
| Dynamic Range | 80-120 dB | 60-90 dB | 70-100 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
Frequently Asked Questions
How do I generate a spur chart?
Plot lines for each m×fRF ± n×fLO product on axes of RF frequency versus IF frequency. The intersections with the horizontal line at the chosen IF frequency reveal spurious response frequencies. Software tools automate this for all relevant m,n combinations.
What mixer order matters?
Products with m+n up to 5 are generally significant for Level 7 and Level 13 (+7 dBm and +13 dBm LO) mixers. For Level 17 and higher, spurious products with m+n up to 7 may need consideration. The mixer's spur table in the datasheet specifies relative levels for each product.
Does the LO injection side matter?
Yes. High-side and low-side injection produce different spur patterns. In some cases, switching injection sides moves critical spurs away from the IF passband. Some receivers use frequency planning that alternates injection sides across the tuning range to minimize worst-case spurs.