Innovate through Specialized RF Measurements

To further explore circuits' performance, IVCAD Measurement Specialized complements the standard measurement workflow with more advanced test features.

Passive impedance tuners may not be sufficient to cover the entire Smith Chart with the desired load impedances due to inherent losses between the tuner and the device. Active and hybrid load pulls combine active and reflected power waves toward the component to leverage the capabilities of load pull benches.

Specific measurement methodologies have been developed to reveal the low-frequency memory effects of power amplifiers caused by the non-constant envelope of modulation signals, such as those in 5G NR. These methods involve scanning the tone spacing between the probe and carrier frequencies, allowing for accurate modeling of RF circuit behavior under realistic operating conditions.

In addition, test setups made of high-frequency waveguide test accessories may require a specific calibration process and measurement routines. This module guides users step-by-step for accurate device characterization.

IVCAD Suite Measurement Specialized - IVZ

Pre-RF Pulse IV Measurements
Active and Time-Domain Load Pull
2-Tone Phase Coherent Tests
Control of Waveguide Setups

Reveal Actual Pulse IV Curves as a Function of Trap Level on GaN Transistors

GaN transistor models are typically extracted from pulsed IV and S-parameter measurements for different quiescent bias points and temperatures. A transistor tested with these short pulsed IV signals and low-power RF measurements will not behave the same if this one is driven by a power signal, as it will experience different impedances under nonlinear conditions.

Therefore, extracting a single model that accurately represents these different test conditions is not straightforward.

To streamline this process and avoid time-consuming and painful modeling, a specialized measurement methodology has been developed. This method involves measuring the pulsed IV curves on a transistor immediately after it is briefly illuminated by a power RF signal under controlled saturation and load impedance conditions.

This approach provides a set of IV curves that reflect the transistor's realistic behavior when amplifying an RF signal under realistic test conditions at the frequency of interest.

Synchronize Pre-RF Pulse and Pulse IV Measurements

Under fixed load impedance conditions, when the component is biased to its nominal quiescent value, a pre-RF signal is provided to trigger potential trapping effects in the GaN transistor, which impacts the IV pulse measurement performed immediately after this illumination.

Using a VNA based on IVCAD's load pull, an equivalent conversion of the a and b waves measured during the RF pulse is performed to calculate the equivalent voltage waves. These waves, discretized over a period of the periodic RF signal, generate a table of input and output voltage values reused by the IV pulse measurement test instrument immediately after RF illumination.

These equivalent input and output voltages measured by the AMCAD Pulse IV system thus provide a snapshot of the transistor's IV pulse curves when loaded by a load impedance, for different saturation levels.

These IV pulse measurements can then be easily integrated into the intrinsic reference plane of the transistor’s current source for accurate and accelerated modeling flow using IVCAD Suite Modeling (IVM) tool.

Change the Impedance Using Active Load Pull Source

Active Load Pull is a technique using active power injection to synthesize a reflection coefficient at the DUT reference plane. The power injection can be supplied using signal generators and power amplifiers.

Active Load Pull module allows control of the reflection coefficient at the load side (ΓL) as the ratio between the reflected (a2) and forward (b2) traveling waves at the output of the DUT. A generalized form of the formula can be written as ΓL = (a2/b2). The wave b2 is taken as the wave coming from the device, while a2 is the reflected wave seen by the device under test, coming either from a passive circuit, an active circuit or a combination of both for hybrid load pull.

Active injection load pull relies on external sources (with phase and amplitude control enabled) to inject a signal toward the DUT, thereby creating the appropriate a2. Because a2 is no longer limited to a fraction of the original reflected signal, external amplifiers may be used to increase the amplitude of a2 so that ΓL can even achieve unity.

Unlike harmonic passive tuners, perfect isolation can be achieved between fundamental and harmonic load tuning, which is important when the load at the fundamental frequency mainly drives performances.

Combine Passive & Active Loads

Unlike passive harmonic tuners, active load pull provides perfect isolation between the fundamental and harmonic load tuning, which is important when the load at the fundamental frequency primarily influences performance. However, active load pull requires power amplifiers to amplify the reflected signal to the device under test, which can be costly.

Combining the two approaches (active and passive) reduces the required RF power while removing the barriers of passive tuners.

Hybrid active load pull is a technique that uses both passive tuners and active injection. This method limits the need for a high-power amplifier for active injection by pre-matching the device using the passive tuner. This technique is a good compromise when high gamma is required at the reference plane of the device under test, but the active injected power is limited.

Furthermore, the combination of reflected signals sent by the passive tuner and active RF sources can be tuned for either fundamental or harmonic frequencies. This technique is often used to transform a load pull to control load impedances at fundamental frequencies only into a harmonic load control solution.

Shift from Frequency to Time Domain

Nonlinear time-domain signal analysis (LSA) is performed by waveform reconstruction, based on the inverse Fourier transform, which recombines the a and b waves to reproduce the original RF signal as a function of time.

This configuration requires a calibrated harmonic comb generator, such as the AMCAD-SAS HPR727A Harmonic Phase Reference Module, to calibrate the phase relationship between the different harmonic frequencies. Therefore, the LSA functionality allows plotting voltage and current waveforms and displaying dynamic load lines for each measurement state (impedance/power/bias). All measurements are de-embedded within the device's reference plane.

Used in conjunction with the linear transistor model description provided by IVCAD SUITE MODELING (IVM), these waveforms can be de-embedded further beyond the measurement plane, in order to observe in real time the technical impact of the waveform obtained by controlling the harmonic load impedances in the reference plane of the transistor current source, as a designer would do in simulation.

Such measurements can also be used to create an EPHD behavioral model of the DUT provided by IVCAD (IVM) dedicated to harmonic balance simulations in circuit simulators.