Clipping
Understanding Clipping
Every active RF component has a finite linear dynamic range bounded by the noise floor at the low end and by gain compression at the high end. When the input signal is large enough to push the output into the compression region, the transfer function deviates from a straight line and the output waveform no longer faithfully reproduces the input. At mild compression (0.5 to 1 dB), the distortion manifests primarily as third-order intermodulation products that fall near the desired signal. As the drive increases further, the output waveform peaks flatten against the transistor's knee voltage and supply rail, producing the characteristic clipped waveform.
The spectral consequences of clipping are severe. A clipped sinusoid contains odd-order harmonics (3rd, 5th, 7th) whose amplitudes depend on the clipping depth. For a symmetrically hard-clipped sine wave with 50% duty cycle at full clip, the third harmonic sits at -9.5 dBc, the fifth at -14 dBc, and the seventh at -16.9 dBc. For modulated signals such as OFDM or QAM, clipping produces both in-band distortion (raising EVM) and out-of-band spectral regrowth (degrading ACPR). In receiver chains, ADC clipping is particularly damaging because it raises the wideband noise floor, desensitizing the receiver to all signals in the digitized bandwidth, not just the one that caused the clipping.
Clipping Distortion Equations
THD = √(V2² + V3² + V4² + ...) / V1 × 100%
Hard-Clipped Sine 3rd Harmonic Level:
a3 = (4/π) × [sin(3θc)/3 - sin(θc)] (Fourier coefficient)
Crest Factor (PAPR):
CF = Vpeak / Vrms ; CFdB = 20log(CF)
Where Vn = amplitude of nth harmonic, V1 = fundamental amplitude, θc = clipping angle. OFDM CF is typically 10 to 12 dB; after CFR, 6 to 7 dB.
Clipping Types and Characteristics
| Clipping Type | Mechanism | Harmonic Profile | THD at 3 dB Overdrive | Application |
|---|---|---|---|---|
| Hard clipping | Rail-limited (supply/ADC) | Strong odd-order, sharp rolloff | 15 to 25% | Digital limiters, ADC |
| Soft clipping | Gradual gain compression | Lower harmonics, broader regrowth | 5 to 12% | Class AB amplifiers |
| Asymmetric clipping | Unequal positive/negative limit | Even + odd harmonics | 10 to 20% | Single-supply amps |
| ADC clipping | Full-scale code saturation | Wideband noise floor rise | 20 to 40% (severe) | Digital receivers |
| Intentional (CFR) | Digital peak limiting | Controlled, filtered regrowth | <2% (managed) | 5G/LTE transmitters |
Frequently Asked Questions
What causes clipping in RF amplifiers?
Clipping occurs when the input signal drives the amplifier beyond its 1 dB compression point (P1dB), where the output can no longer increase linearly with input. The transistor's drain or collector voltage swings into the supply rail or knee region, flattening the waveform peaks. Overdriving by 1 dB beyond P1dB typically raises THD from under 1% to 3 to 5%, while 3 dB of overdrive can push THD to 10 to 30%.
How does ADC clipping differ from amplifier clipping?
ADC clipping is always hard clipping: the converter output saturates at the maximum or minimum digital code with no gradual compression. A 14-bit ADC with 2V peak-to-peak full-scale range clips any input exceeding plus or minus 1V, producing severe distortion that raises the noise floor by 20 to 40 dB across the entire Nyquist band. Standard practice is to keep the maximum signal 6 to 10 dB below ADC full-scale.
How do you prevent clipping in an RF receiver?
Prevention requires managing signal levels at every gain stage. Use front-end attenuators or limiters to keep signals 10 dB below P1dB, implement AGC loops with fast attack times (under 1 microsecond), size ADC dynamic range to exceed maximum signal plus crest factor (10 to 12 dB for OFDM), and apply crest factor reduction (CFR) in transmitters to reduce PAPR from 10 to 12 dB down to 6 to 7 dB.