This invention relates to high power (nonlinear) testing of RF transistors (DUT) in the frequency and time domain (see ref. 1) using Load Pull. Load pull is the method by which the load impedance presented to the DUT at a given frequency is changed systematically and the DUT performance is registered, with the objective to find an optimum tuning condition for certain overall design objectives. This objective may be maximum power, maximum efficiency, linearity or else or a combination thereof. The same is valid for the source side of the DUT. Passive (slide screw) tuners are used to create the various impedances presented to the DUT (see ref. 2) for frequencies above 100 MHz. A typical test setup is shown in FIG. 1: a signal source (1) injects RF power into a DUT (3) via an input tuner (2) which generates the source impedance presented to the DUT. An output tuner (4) generates the load impedance. The outcoming power from tuner (4) is detected by the power meter (5). The whole is controlled by a PC (6) via digital communication with the test instruments (9) and the tuners (7), (8). Data are collected by the PC as a function of the impedances generated by the tuners and saved in load pull measurement files.
Impedance tuners are used to create the test impedances used in load and source pull testing. Some tuners use the slide-screw principle, (FIGS. 2 to 4), see ref. 2, others are using electronic components (PIN diodes, FIG. 6), see ref. 3. The basic design of a slide screw tuner comprises a low loss slotted airline (slab-line) (24, 44), 32) in which a metallic (reflective) probe (22, 41) is inserted and capacitively coupled with the center conductor (23, 34, 43). The proximity of the probe with the center conductor allows controlling the amount of reflected RF power and thus the reflection factor (which is the RF impedance generated by the tuner). Moving the probe along the axis of the slab-line (24, 45) allows controlling the phase of the reflection factor. If the horizontal movement reaches one half of the wavelength at the selected frequency, then a full circle on the Smith chart is covered and by that the whole spectrum of real (R) and imaginary (X) part of (complex) Impedances Z=R+jX can be synthesized. The only limitations of such a tuner are (a) the maximum reflection factor due to losses and limit of coupling proximity of the probe with the center conductor (galvanic contact must be avoided to avoid shorting the tuner and not being able to control the impedance) and (b) the tuning speed, since the carriage (28) which carries the probe (22) must be moved horizontally (217) and the axis (21) controlling the probe must be moved vertically (216); both those mechanical movements take time, and this slows down the tuning operations. Slide screw tuners have no further limitations, beyond the fact that they are bulky and heavy, but so is most auxiliary RF laboratory test equipment. FIG. 5 shows well distributed tuning points (50) generated using slide screw tuners over the whole Smith chart (51).
Speed and size/weight can be improved using electronic tuners (see FIG. 1 in ref. 3, replicated here as FIG. 6). In this case the adjustable reflective probe (22 in FIG. 2) is replaced by an array of electronic switches (PIN diodes, 1 to 15 in FIG. 6), distributed along a micro-strip line. By switching the diodes ON and OFF individual reflections are created and, when the diodes are placed strategically spaced along the transmission line in FIG. 6 between RF IN and RF OUT, the combination of the individual reflection factors can generate tuning patterns as shown in FIG. 7 (see ref. 3). It is obvious that the behavior of the tuning pattern of electronic tuners (FIG. 7) is inferior to the tuning pattern of slide screw tuners (FIG. 5) both in regularity of the tuned points and maximum reflection factor. The regularity and density of electronic tuner points depends on the number of diodes used, whereas the limitation in maximum reflection is due to losses associated with the micro-strip structures used in such prior art tuners (see ref. 3) and PIN diode losses.