Measurements, Testing, and Calibration Advanced Measurement Topics Informational

How do I measure the AM-AM and AM-PM distortion of a power amplifier?

Q: Do AM-AM and AM-PM change with frequency?

Yes. The gain compression and phase shift vary across the PA's operating bandwidth. A PA that is well-matched at band center may have different compression characteristics at band edges (due to the frequency-dependent matching network response). For wideband PAs (> 10% bandwidth): AM-AM and AM-PM should be measured at multiple frequencies across the band to capture the frequency-dependent behavior. DPD algorithms must model this frequency dependence for wideband correction.

Measuring the AM-AM (amplitude-to-amplitude) and AM-PM (amplitude-to-phase) distortion of a power amplifier characterizes the nonlinear relationship between the input signal amplitude and the output amplitude (AM-AM) and output phase (AM-PM). These measurements are essential for predicting the amplifier's distortion of complex modulated signals (QAM, OFDM) and for designing digital pre-distortion (DPD) linearization. The AM-AM measurement involves: sweeping the input power level from small signal to well beyond compression (typically -30 dBm to +10 dBm at the DUT input) and recording the output power at each input level. The AM-AM curve shows: linear region (output increases 1 dB for each 1 dB input increase), compression region (output increases less than 1 dB per dB input), and saturation (output is nearly constant regardless of input increase). The AM-PM measurement involves: recording the phase of the output signal relative to the input signal at each input power level. The AM-PM curve shows: constant phase in the linear region, and phase shift (typically 1-10 degrees from small signal to saturation) in the compression region. The measurement is performed using: a VNA in power sweep mode (the VNA sweeps the source power while measuring both the magnitude and phase of S21; the magnitude gives AM-AM and the phase gives AM-PM) or a signal generator and vector signal analyzer (the generator sweeps the input power while the VSA measures the output amplitude and phase simultaneously).

Category: Measurements, Testing, and Calibration

Updated: April 2026

Product Tie-In: VNAs, Probes, Chambers, Signal Generators

Power Amplifier AM-AM and AM-PM Characterization

AM-AM and AM-PM distortion are the primary characterizations of a power amplifier's static nonlinearity. They completely describe the amplifier's instantaneous (memoryless) nonlinear transfer function and are the basis for behavioral modeling and digital pre-distortion.

Measurement Methods

VNA power sweep: The simplest method. Set the VNA to a fixed frequency and sweep the source power. Measure S21 magnitude (AM-AM) and S21 phase (AM-PM) vs. input power. Advantages: calibrated, accurate, fast. Limitations: CW measurement only (does not capture memory effects)
Two-tone measurement: Apply two closely spaced tones and measure the output spectrum. The AM-AM and AM-PM can be extracted from the amplitude and phase of the fundamental and intermodulation products. Captures some memory effects
Modulated signal method: Apply a wideband modulated signal (e.g., OFDM) and capture the input and output waveforms simultaneously using a VSA. Extract the AM-AM and AM-PM by plotting the instantaneous output amplitude and phase vs. the instantaneous input amplitude. This is the most realistic measurement and captures memory effects

AM-AM/AM-PM Measurement Parameters

AM-AM: G(P_in) = P_out(P_in) - P_in [dB] (gain vs. input power)
AM-PM: phi(P_in) = phase(S21) vs. P_in [degrees]
P1dB: input power where gain compresses by 1 dB
Typical AM-PM: 1-5 degrees from small signal to P1dB
GaN PA: AM-PM typically 2-8 degrees from linear to saturation

Common Questions

Frequently Asked Questions

Why does AM-PM matter?

AM-PM conversion causes spectral regrowth (adjacent channel leakage) and EVM degradation in modulated signals, similar to AM-AM compression. For some modulation formats (e.g., FM, GMSK): AM-PM is the dominant source of distortion because the signal has constant amplitude but varies in phase. For QAM signals: AM-PM rotates the constellation points as the signal amplitude fluctuates, creating errors. DPD must correct both AM-AM and AM-PM to achieve the best linearity.

What AM-PM specification is typical?

For linear applications (LTE, 5G base stations): AM-PM < 2-3 degrees over the operating power range. For saturated applications (radar, FM): AM-PM of 5-15 degrees is common and may not be a concern if the signal is constant-envelope. For DPD-linearized PAs: the uncorrected AM-PM can be 5-10 degrees, but the DPD corrects it to < 1 degree at the system output.

Do AM-AM and AM-PM change with frequency?

Yes. The gain compression and phase shift vary across the PA's operating bandwidth. A PA that is well-matched at band center may have different compression characteristics at band edges (due to the frequency-dependent matching network response). For wideband PAs (> 10% bandwidth): AM-AM and AM-PM should be measured at multiple frequencies across the band to capture the frequency-dependent behavior. DPD algorithms must model this frequency dependence for wideband correction.

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