What is the difference between in-band and out-of-band spurious emissions?
Spurious Emission Categories
Every transmitter produces unwanted emissions beyond the desired signal. These fall into two categories with different implications and regulatory requirements. In-band emissions (close to the carrier) are inevitable byproducts of modulation and amplifier nonlinearity. They affect only the transmitter's own signal quality and are regulated by the communication standard (3GPP, IEEE 802.11, etc.).
| 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 |
Frequently Asked Questions
What are typical regulatory limits?
FCC Part 90 (land mobile): -36 dBc at 12.5 kHz offset. 3GPP (cellular): -45 dBc ACLR for the first adjacent channel. FCC Part 97 (amateur): -43 dBc beyond necessary bandwidth. ITU spurious domain: -60 dBc or -36 dBm, whichever is less restrictive.
What is the spurious emission domain?
The spurious domain begins at a defined frequency offset from the carrier (typically 2.5× the necessary bandwidth per ITU). Within this domain, all discrete emissions must be below the spurious emission limit. Between the channel edge and the spurious domain is the out-of-band domain, governed by the emission mask.
How do harmonics fit in?
Harmonics (2f0, 3f0, etc.) are typically in the spurious domain and must meet the general spurious emission limit. A 2 GHz transmitter's second harmonic at 4 GHz must be below -60 dBc or -36 dBm. Harmonic filters with 40-60 dB rejection at 2f0 are standard on most transmitters.