How do I design a system to minimize third order intermodulation products near the desired signal?
IM3 Minimization Strategies
The most effective IM3 reduction comes from preventing strong interferers from reaching nonlinear stages. A preselector filter that attenuates an interferer by 20 dB reduces the IM3 product by 40 dB (because IM3 grows as the cube of the input power when one of the two tones dominates). This multiplicative benefit makes filtering far more effective than improving amplifier linearity.
| Parameter | Class A | Class AB | Class F/Doherty |
|---|---|---|---|
| Max Efficiency | 50% | 50-78% | 70-90% |
| Linearity | Excellent | Good | Moderate (needs DPD) |
| P1dB Backoff | 0-3 dB | 3-6 dB | 6-10 dB |
| Complexity | Low | Low | High |
| Common Use | Test, small signal | General PA | Base station, broadcast |
Compression Behavior
Where filtering is insufficient (because the interferers are near the desired signal), high IIP3 components are essential. The system IIP3 is dominated by the stage with the worst IIP3 referred to its input. For most receivers, this is the first mixer, which handles the highest signal levels after LNA gain. Passive mixers with +25 to +30 dBm IIP3 are preferred in demanding environments.
- 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
Efficiency Trade-offs
Frequency planning avoids placing the desired receive channel at frequencies where IM3 products from known strong signals could fall. This is particularly important in multi-system sites where transmitters from different services share tower-top or co-located equipment. A spur chart analysis identifies all dangerous frequency combinations.
Frequently Asked Questions
Should I increase IIP3 or add filtering?
Filtering first, always. Each dB of filtering reduces IM3 by 2-3 dB (depending on which tone is filtered), while improving IIP3 by 1 dB reduces IM3 by only 1 dBc. Filtering is also passive and does not add noise, while higher IIP3 amplifiers may have higher noise figures.
What about digital cancellation?
Digital IM cancellation is possible if the interferer signals are known. The receiver digitizes the interferers, computes the expected IM3 products, and subtracts them from the received signal. This technique can provide 20-30 dB IM3 reduction but requires significant DSP resources and knowledge of the interferer characteristics.
How does gain distribution affect IM3?
More gain before a nonlinear stage increases the signal level at that stage, worsening IM3. Reducing LNA gain or adding inter-stage attenuation reduces IM3 at the expense of noise figure. The optimum balances the noise figure and IM3 requirements specific to the application.