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Active Tuners

Characterisation of active devices is a challenge for test engineering because the impedance of the DUT is usually very low. Accurate impedance matching is therefore the goal of the load-pull method. With passive tuners, a satisfactory result can be achieved in some applications, but in many cases even this method is limited because of the losses that the set-up still causes. This is where active tuners come in.

Forward and return wave of a 4-port DUT

The so-called tuning range is defined as the range in which the reflection coefficient is Γ = <a> / <b>, where <a> is the wave returning to the DUT port and <b> is the wave leaving the DUT, can vary. This value, as known from high frequency technology, is between 0 and 1.

When load-pull is active, the returning wave is amplified before it reaches the reference plane of the DUT. In that case you get a | Γ | >= 1, which manages to compensate for the losses of the set-up. Naturally, with a | Γ | > 1 impedance points will be outside the Smith chart, RE{Zload} < 0 which can be avoided by algorithms. | Γ | = 1 is however called RDUT = 1 Ω, which means that any DUT can be matched.

Feedback of the signal <a2> with active load-pull at the fundamental frequency

Analogous to load-pull, Focus also provides source pull using the Gamma Boosting Unit (GBU).

Active load-pull demands high power feedback amplifiers, because high power must be delivered due to the impedance difference with the DUT. This problem can be solved by using a hybrid configuration with the broadband passive tuners CCMT (fundamental) or MPT (harmonics). This can reduce the impedance difference or mismatch and also the required gain factor.

The digital tuner RAPID manufactured by subsidiary MESURO is the best candidate for such applications with high-speed load-pull (50 points/second), 100 MHz modulated signals for hybrid and harmonic tuning.

 

Rapid Hardware by MESURO
Active load-pull measurement using the MESURO rapid active tuner.