How do I select between high side and low side LO injection for a given receiver architecture?
LO Injection Side Selection
The choice between high-side and low-side LO injection affects the receiver's spurious response, image location, and tuning behavior. In a sweeping or tunable receiver, the LO must track the desired RF frequency, and the choice of injection side determines the LO frequency range and the relationship between LO tuning direction and RF frequency direction.
| 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
Can I use both sides simultaneously?
Not in a standard mixer. But in an image-reject mixer architecture, both sidebands are downconverted simultaneously with the image being cancelled by quadrature processing. This effectively uses both injection sides but with the image suppressed.
What about zero-IF (direct conversion)?
Zero-IF sets fLO = fRF (fIF = 0), placing the IF at DC/baseband. This eliminates the image problem entirely but introduces DC offset, 1/f noise, and I/Q imbalance challenges. Zero-IF is standard in modern cellular and WiFi transceivers.
How does injection side affect phase noise?
It does not fundamentally change the LO phase noise contribution. Both sides mix the LO phase noise to the IF identically. However, a higher LO frequency (high-side injection) may be more difficult to generate with low phase noise, creating a practical difference.