Harmonic Filter
Understanding Harmonic Filters
Every power amplifier generates harmonics. Even a well-designed Class-A amplifier produces harmonics at compression, and Class-B/C/E amplifiers generate significant harmonic content by design. Harmonic filters are mandatory in any transmitter to suppress these harmonics below regulatory limits.
Harmonic Levels
Typical unfiltered PA harmonic levels: 2nd harmonic -15 to -25 dBc; 3rd harmonic -20 to -35 dBc; higher harmonics decrease progressively. Regulatory limits (FCC Part 97, ETSI, etc.) typically require harmonics to be -43 to -60 dBc or below absolute thresholds.
Harmonic Filter Design
- Lowpass filter: Most common. Passes the fundamental and rejects all harmonics. Cutoff between f0 and 2f0.
- Notch filter: Specific rejection at one harmonic. Used when only one harmonic is problematic.
- Absorptive filter: Absorbs harmonic energy rather than reflecting it back to the PA, preventing PA performance interaction.
Required rejection at 2nd harmonic:
Reject > PA_2nd_harmonic_level - specification
Example: 2nd harmonic = -15 dBc, spec = -60 dBc
Required filter rejection at 2f0: > 45 dB
LPF cutoff: 1.2 x f0 to 1.5 x f0 typical
5th-order Chebyshev: ~60 dB at 2f0
Frequently Asked Questions
What is a harmonic filter?
A harmonic filter suppresses the harmonic frequencies (2f0, 3f0, etc.) generated by power amplifiers. It is typically a lowpass filter placed after the PA with a cutoff between the fundamental and second harmonic, passing the signal while rejecting all harmonics.
Why are harmonic filters required?
Harmonics can interfere with other radio services operating at those frequencies. Regulatory bodies (FCC, ETSI) set strict limits on harmonic emissions, typically -43 to -60 dBc or absolute power limits. A harmonic filter ensures compliance.
What filter type is best for harmonic suppression?
A lowpass filter with cutoff between f0 and 2f0 is the most common and effective approach. It rejects all harmonics simultaneously. A 5th-order Chebyshev provides about 60 dB rejection at 2f0, sufficient for most applications.