Harmonic
Understanding Harmonics
All real amplifiers and oscillators generate harmonics to some degree. The fundamental signal at frequency f0 passes through a nonlinear transfer function, producing output components at 2f0, 3f0, 4f0, etc. These spurious signals can interfere with other services operating at those frequencies and must be filtered before reaching the antenna.
Harmonic Sources
- Power amplifiers: Nonlinear gain compression generates harmonics. Higher drive levels = stronger harmonics.
- Oscillators: Most oscillator waveforms are not pure sinusoids; harmonics are typically -15 to -30 dBc.
- Mixers: Generate harmonics of both RF and LO signals in addition to desired mixing products.
- Frequency multipliers: Deliberately generate harmonics (e.g., doubler produces 2f0 from f0 input).
Harmonic Filtering
Lowpass filters after the power amplifier suppress harmonics before the antenna. The filter must pass the fundamental frequency with low insertion loss while providing adequate rejection at 2f0 and above. For transmitters, harmonic suppression of -40 to -60 dBc is typically required.
Second harmonic: 2f0 (typically -20 to -30 dBc)
Third harmonic: 3f0 (typically -30 to -45 dBc)
Harmonic distortion:
THD = sqrt(P_2nd^2 + P_3rd^2 + ...) / P_fund x 100%
Frequently Asked Questions
What are harmonics in RF?
Harmonics are integer multiples of a signal's fundamental frequency, generated by nonlinear behavior in amplifiers, oscillators, and mixers. The second harmonic is at twice the fundamental frequency, the third at three times, and so on. They are unwanted signals that must be filtered out.
Why are harmonics a problem?
Harmonics can interfere with other radio services operating at those frequencies. For example, a 900 MHz transmitter generates a second harmonic at 1800 MHz, which falls in another cellular band. Regulatory agencies set strict limits on harmonic emissions from transmitters.
How do you suppress harmonics?
Lowpass or bandpass filters placed after the amplifier remove harmonics before the signal reaches the antenna. Operating the amplifier below compression reduces harmonic generation. Balanced (push-pull) amplifier configurations inherently suppress even-order harmonics.