A transistor datasheet gives you one or two "optimum load" points measured on somebody else's fixture. Load-pull is how you find out what your device does at your frequency, bias and drive level. It is also easy to misread.

1. What load-pull measures
The fundamental load impedance seen by the device is swept across the Smith chart with a tuner while output power, gain, efficiency (PAE or drain efficiency) and sometimes ACLR or EVM are recorded. The result is a set of contours: the Pout optimum and the PAE optimum are almost never the same point, and the design load is a compromise between them.

2. Contours depend on drive
Contours taken at small signal, at P1dB and at P3dB are different. Always note the compression level used. A design matched at the P1dB optimum may be poor at the back-off level where a modulated signal spends most of its time. For OFDM or 5G signals, run the sweep with the real modulated signal or at least at the intended back-off.

3. Tuner reach and pre-matching
Passive mechanical tuners lose reach as the target impedance drops. Large GaN or LDMOS devices often want only a few ohms at the package plane, and fixture loss plus tuner loss make the edge of the chart unreachable. Solutions:
- A pre-matching transformer in the fixture to bring the region of interest closer to 50 ohm
- Active or hybrid-active load-pull, which injects a signal to synthesise high reflection
- Careful de-embedding so the reported impedance really is at the device reference plane

4. Harmonics matter
Second and third harmonic terminations shape the voltage and current waveforms. Class F, inverse F and class J behaviour all come from controlling 2f0 (and 3f0). With harmonic tuners you will often see the PAE optimum at the fundamental shift and improve by several points once 2f0 is set correctly. Without harmonic control, the fundamental contours describe only the harmonic loads your fixture happens to present.

5. Source-pull and stability
- Source impedance affects gain and sometimes linearity; for high-gain devices, do at least a coarse source-pull.
- Watch for oscillation near the chart edge, especially at low frequencies where the device has very high gain. Add out-of-band stabilisation in the fixture and monitor the spectrum during the sweep.

6. Turning contours into a matching network
Pick a load that balances Pout, PAE and linearity, then design the output network to present it across the band (the optimum moves with frequency, so plot it per frequency). Re-check with harmonic impedances of the actual network in simulation before layout.

How do you choose between the Pout and PAE optimum for a modulated-signal PA, and what back-off do you use for the sweep?