How do I design an image reject mixer to eliminate the need for a preselector filter?
Image Reject Mixer
The Hartley image reject mixer is the simplest IRM architecture. The RF signal drives two identical mixers. The LO is split through a 90° hybrid, providing 0° and 90° LO drive to the two mixers. The IF outputs pass through a 90° phase shifter on one path, then both IF paths are summed. The desired signal adds constructively (0° + 0° = doubled signal), while the image signal adds destructively (0° + 180° = cancellation).
| 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 do I improve rejection beyond 30 dB?
Digital I/Q correction in the baseband DSP measures the I/Q imbalance and applies compensation. This can improve rejection to 40-60 dB. Self-calibration using an internally generated test signal automates the correction across frequency and temperature.
Can I use an IRM for wideband receivers?
Yes, but the I/Q balance must be maintained across the entire RF bandwidth. Wideband 90° hybrids (Lange couplers, branchline couplers) achieve 1-2° phase and 0.3-0.5 dB amplitude balance over octave bandwidths. Digital correction relaxes these requirements for wideband operation.
What about the IF 90° phase shifter?
The IF 90° phase shifter must provide accurate 90° delay across the entire IF bandwidth. Polyphase networks provide wideband 90° splitting with ±1° accuracy. RC-CR phase-split networks work for narrowband IFs. Digital implementation (Hilbert filter) is exact but requires digitization of both IF outputs.