How do I select the LO frequency to avoid spurious products falling in band?
Spur-Free Frequency Planning
The frequency plan is the most critical design decision in a superheterodyne receiver. A poor choice of IF and LO frequencies creates spurious responses that degrade the receiver's selectivity and dynamic range. Spur analysis must consider the entire input bandwidth (not just the center frequency) because the mixer generates spurs at every RF frequency within the preselector passband.
| 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
What if I cannot avoid all spurs?
If no frequency plan is completely spur-free: choose the plan where the unavoidable spurs are highest order (weakest amplitude). A 3×2 spur is typically 40+ dB below the desired signal and may be tolerable. Additionally, increase the mixer's balance (use double-balanced or triple-balanced) to suppress even-order spurs.
Does the preselector help?
Yes. A narrow preselector filter reduces the range of fRF values that the mixer sees, eliminating many potential spur responses. The narrower the preselector, the fewer spurs can reach the mixer. This is why high-performance receivers use narrow tracking preselectors despite their added complexity.
What about digital IF receivers?
Digital IF receivers with wide IF bandwidth accept more potential spurs into the IF passband. The spur analysis must cover the entire digitized bandwidth. Some digital receivers use oversampled ADCs and digital filtering to reject spurs that fall within the ADC bandwidth but outside the signal bandwidth.