What is cross modulation distortion and how does it differ from intermodulation distortion?
Cross Modulation Mechanism
Cross modulation is a third-order nonlinear effect, like IM3, but its manifestation is different. While IM3 creates discrete products at new frequencies (2f1-f2), cross modulation impresses the modulation pattern of a strong signal onto a weak desired signal at the desired signal's own frequency. This makes it undetectable by simple spectral analysis and impossible to remove by filtering.
| 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 |
- 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
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
- Margin allocation: include sufficient design margin to account for manufacturing tolerances and aging effects
Frequently Asked Questions
Is cross modulation related to IP3?
Yes. Both arise from the same third-order nonlinearity (a3 coefficient). A device with high IP3 will have low cross modulation. The cross modulation coefficient can be calculated from IP3: cross mod (%) = 75·P_interferer²/(OIP3²), where powers are in watts.
When is cross modulation worse than IM?
Cross modulation dominates when a strong AM signal is present near a weak desired signal. IM3 dominates when two strong signals of similar level are present. In receivers with AGC, cross modulation can be more problematic because AGC responds to the total power including the interferer.
How do I test for cross modulation?
Apply a modulated strong signal at an adjacent frequency and a CW desired signal. Measure the AM appearing on the desired signal output. The cross modulation ratio is the ratio of induced AM depth to the interferer's AM depth, expressed in dB.