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.
Spectral inversion occurs with high-side injection because the mixing process reverses the frequency ordering: the highest RF frequency converts to the lowest IF frequency and vice versa. For FM and SSB signals, this inversion must be corrected in the demodulator. For OFDM and many digital modulation formats, spectral inversion is handled by the digital baseband processing.
Spur planning is critical for multi-conversion receivers. Each combination of LO harmonics and RF harmonics (m×fLO ± n×fRF) can produce a response at the IF. Choose the LO injection side that places the most troublesome spurs (low-order, like 2×1 and 1×2) outside the receiver's RF preselected bandwidth. Spur analysis software or a mixer spur chart guides this selection.
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.