What is the difference between single conversion and double conversion in a frequency translation scheme?
Conversion Architecture
Single conversion: fIF = |fRF - fLO|. The image frequency is 2×fIF from the desired RF. For wide tuning receivers (where fRF varies), the preselector must track the tuning to maintain image rejection. This tracking filter adds complexity and is difficult to implement with sharp rejection at microwave frequencies.
| 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 |
Frequently Asked Questions
What about triple conversion?
Triple conversion adds a third mixing stage, providing even more flexibility in spur management and image rejection. Used in high-performance spectrum analyzers and military receivers where the dynamic range and spurious-free range requirements exceed what double conversion can achieve.
How does zero-IF fit in?
Zero-IF (direct conversion) is essentially single conversion with fIF = 0. It eliminates the image problem entirely (the image is the desired signal's mirror) at the cost of DC offset, 1/f noise, and I/Q imbalance challenges. Zero-IF is standard in modern cellular and WiFi transceivers.
Can I mix the architectures?
Yes. Low-IF architectures use a first conversion close to zero-IF (fIF = 1-10 MHz) to avoid DC offset while keeping the image close enough for digital image rejection. This hybrid approach combines some advantages of both zero-IF and superheterodyne.