What are image frequencies and how do I reject them in a superheterodyne receiver?
Image Rejection
The image frequency problem is fundamental to all superheterodyne receivers. The mixer performs downconversion: fIF = |fRF - fLO|. But this same equation has two solutions for fRF: fRF = fLO + fIF (desired) and fRF = fLO - fIF (image), or vice versa depending on LO injection side. Both frequencies produce output at the same IF, and the mixer cannot distinguish between them.
| 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
Frequently Asked Questions
How much image rejection do I need?
The image rejection must be sufficient to keep the strongest possible image-band signal below the receiver's desired signal sensitivity. For commercial receivers: 40-60 dB is typical. For simple systems with no strong interferers at the image frequency: 20-30 dB may suffice.
What IF frequency minimizes the image problem?
Higher IF = easier image filtering but more expensive and higher-loss IF components. Common trade-offs: first IF of 70 MHz (VHF receivers), 140-170 MHz (UHF/SHF), or 1-2 GHz (microwave/mmWave receivers). Some modern receivers use zero-IF (direct conversion) to eliminate the image problem entirely.
What is the Hartley image-reject mixer?
Two mixers driven by the same RF signal but with LO phases shifted by 90°. The IF outputs are recombined with a 90° hybrid. Image signals cancel at the output; desired signals add. The cancellation is limited by the amplitude and phase balance: 1 dB amplitude and 5° phase imbalance limits rejection to about 20 dB.