What does a load pull measurement produce?

Load pull produces constant-value contours on the Smith chart for each measured parameter. Power contours: concentric circles showing constant output power. The center of the innermost contour is Z_opt_power, the impedance for maximum output power. Efficiency contours: typically offset from power contours. Z_opt_eff (maximum PAE/drain efficiency) is usually at a different impedance than Z_opt_power, often at higher impedance. Linearity contours (ACPR/EVM): optimal linearity impedance is usually between the power and efficiency optima. The PA designer must choose a compromise impedance that meets power, efficiency, and linearity targets simultaneously. Load pull data is essential for Doherty PA design, where the main and peaking amplifier impedances must modulate with power level.

What are the types of load pull systems?

Passive mechanical tuner: a motorized tuner with one or more probes that create controllable reflections in a transmission line. Maury Microwave and Focus Microwaves are the main manufacturers. Advantages: handles high power (100W+), no linearity issues. Limitations: maximum VSWR is limited by tuner loss (typically |Gamma| 1 (negative resistance) for stability analysis. Advantages: covers entire Smith chart, faster measurement. Limitations: linearity of the injection amplifier can be an issue. Hybrid: combines passive tuner for fundamental with active injection for harmonic control. Modern systems support real-time load pull with modulated signals (5G NR waveforms).

Why is harmonic load pull important?

The impedance at harmonic frequencies (2f0, 3f0) significantly affects PA performance. Harmonic terminations shape the voltage and current waveforms: Class F: presents short at 2nd harmonic, open at 3rd. Creates square voltage waveform. Theoretical efficiency: 90.7%. Inverse Class F: open at 2nd, short at 3rd. Square current waveform. Class J: specific reactive harmonic terminations. Achieves high efficiency over wider bandwidth than Class F. Without harmonic load pull, the designer cannot determine these optimal harmonic impedances experimentally. Harmonic load pull uses additional tuners or active injection at 2f0 and 3f0 while independently controlling the fundamental impedance. This requires sophisticated multi-port tuner systems.

PA Design

Load Pull

/lohd pull/

Load pull varies the impedance presented to a transistor and maps power, efficiency, and linearity contours on the Smith chart. Finds Z_opt for each target: P_max, PAE_max, ACPR_min (usually different impedances). Passive tuners (Maury, Focus): handle 100W+. Active: synthesizes any impedance including |Γ|>1. Harmonic load pull: critical for Class F/J waveform engineering.

Output: Smith chart contours

Passive: |Γ|<0.9

Active: Full Smith chart

Understanding Load Pull

Load pull is how PA designers bridge the gap between transistor data sheets and real-world matching networks. Data sheets provide S-parameters at a single operating point, but PA performance depends critically on the impedance at the fundamental, second harmonic, and third harmonic frequencies. Load pull measurements reveal the complete picture: where maximum power occurs, where maximum efficiency occurs (usually at a different impedance), and where the best linearity occurs (at yet another impedance).

Load Pull Concepts

Load-pull contours:
Constant P_out = f(Γ_L) on Smith chart
Constant PAE = g(Γ_L)

Optimal load:
Z_opt = R_opt + jX_opt
R_opt ≈ (V_DD−V_knee)²/(2P_out)

Contour spacing:
∼1 dB circles, 5% PAE circles

Load Pull System Comparison

System	\|Γ\| Range	Power	Speed	Harmonic	Application
Passive mechanical	0-0.9	100W+	Slow (min/pt)	Separate tuners	High-power GaN
Active open-loop	0-1.0+	Limited	Fast	Each harmonic	Full Smith chart
Active closed-loop	0-1.0+	Moderate	Moderate	Real-time	Modulated signals
Hybrid passive+active	0-1.0+	High	Moderate	Fundamental+harmonic	Doherty PA design
Simulated (X-params)	Full	Any	Fastest	All harmonics	Initial exploration

Key Equations

Decibel conversion:
Power: dB = 10log(P₂/P₁)
Voltage: dB = 20log(V₂/V₁)

dBm to watts:
P(W) = 10^{(dBm−30)/10}
0 dBm = 1 mW, +30 dBm = 1 W

Wavelength:
λ = c/f = 300/f(MHz) meters

Comparison

Metric	Typical	High-Power	Unit	Notes
P_out @opt	+20 to +30	+40 to +50	dBm	At Z_opt
PAE @opt	40–55%	55–75%	%	GaN advantage
Gain @opt	10–15	8–12	dB	Compressed
VSWR contour	1.5–3:1	2–5:1	ratio	−1dB contour
Source pull	NF_min circles	—	dB	Noise opt

Common Questions

Frequently Asked Questions

What does it produce?

Smith chart contours for power, PAE, ACPR, EVM. Each parameter has different optimum impedance. PA designer chooses compromise Z meeting all specs. Essential for Doherty design where impedance modulates with power.

System types?

Passive tuner: handles 100W+, limited |Gamma|<0.9. Active: synthesizes any Z including negative resistance. Hybrid: passive fundamental + active harmonic. Modern: real-time with 5G NR modulated waveforms.

Harmonic importance?

Harmonic terminations shape waveforms and set PA class. Class F: 2f0 short, 3f0 open = 90.7% efficiency. Without harmonic load pull, cannot determine these impedances experimentally. Requires multi-port tuner systems.

Related Terms

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