Methods for measuring device saturation power and gain

The complexity of power measurements on RF transistors and RF integrated circuits is increasing day by day. The most important thing in measuring the performance of high-power devices is to measure the saturation power, which is usually tested in the pulsed state because it is difficult to evaluate the parameter with CW techniques. The method presented in this article eliminates some major shortcomings of the classical methods for measurement. The method does not require an external PC, only uses some SMIQ signal generators from Rohde & Schwarz and exploits some little-known features of the FSP signal analyzer that works like a high dynamic range peak meter.

By using trace math and markers, gain and power can be read directly at any compression level up to the device saturation power level. Measurements on a high-gain LDMOS power RF integrated circuit from Freescale Semiconductor designed for the UMTS band (model MW4IC2230MB) show the benefits of this approach.

Saturation power is an important device or amplifier characteristic because digital predistortion systems are often used to linearize multi-carrier cellular base station power amplifiers. Saturation power is usually considered to be the maximum possible output power of a predistorted power amplifier. Even though LDMOS devices are more robust than bipolar transistors, it is still difficult to measure high CW power levels. In fact, self-heating devices are almost impossible to produce accurate and repeatable measurements. As a result, saturation power measurements are usually done with pulsed signals. Typically, a signal generator with a pulsed input and a peak power meter with two sensors are used. The input power to the device is then increased and the ratio of the partial output power to the input power is found with the help of a PC.

However, the accuracy of this method is limited. Dual-channel peak RF power meters require that both sensors operate within a given dynamic range to achieve better accuracy. This condition is easily achieved if the test bench is properly designed. However, if the device under test (DUT) has high gain, such as a multi-stage RF integrated circuit, another source of error occurs: the sensors do not operate within the same dynamic range during calibration (when the DUT is replaced by a penetrating reference) and measurement. Therefore, there is a mutual dependence between the measurement results and the power level at which the bench is calibrated.

Test bench

The test bench (Figure 1) uses a pulse generator connected to the "pulse" input of the SMIQ RF signal generator. In order to use the SMIQ in power sweep mode, the power sweep must be synchronized with the time base sweep in the signal analyzer. Fortunately, this type of signal generator has the feature to be used as a scalar network analyzer (SNA) when associated with a diode detector and a display like an XY mode filter. On the rear panel of the SMIQ, there are several BNC connectors with power sweep ramps to drive the X axis of the filter, and markers to calibrate the X axis of the display. In this case, the "marker" output is used as a trigger signal for the signal analyzer.

The "marker" output of the SMIQ is connected to a BNC cable to the "external trigger" output of the FSP. "Marker 1" is set to the "sweep start" value and the RF output of the SMIQ is connected to a variable attenuator. In this way, the power level at the DUT input can be adjusted without changing the "start" and "stop" values ​​of the sweep process in the signal generator.

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We tend to recommend this method because if the "start" and "end" values ​​are modified while sweeping, while the position of "marker 1" is unchanged, the synchronization of the spectrum analyzer will be irregular, and even out of synchronization once the marker is outside the power sweep range. A high power amplifier is used to drive the DUT to ensure that the driver amplifier does not saturate before the DUT. Input and output couplers allow sampling of the portion of the signal that is sent to the spectrum analyzer. A calibration attenuator is used as a load in order to obtain an accurate power parameter that can be measured as a deviation with a standard power meter after entering the load attenuation.

Before measurements can be made, the input and output paths of the spectrum analyzer must be calibrated. Typically, the DUT is replaced by a penetration reference and the signal generator is operated in CW mode. The power meter reads the power level through the penetration reference, while the spectrum analyzer reads the absolute power on the coupling path of the input or output coupler in "zero-range" mode. This makes it possible to determine the attenuation on the input and output paths leading to the spectrum analyzer. Call these values ​​"IN_OFFSET" and "OUT_OFFSET" for future parameters.

All parameters are determined so that the current consumption of the DUT does not deviate from the quiescent current, thus ensuring a stable thermal response. The signal generator is switched to pulse mode by selecting the Pulse option on the Analog Modulation list. In the Sweep list, the Power Sweep mode is selected. The start level is set to -20 dBm and the stop level to 0 dBm. 101 measurement points are possible with a step size of 0.2 dB. The dwell time must be chosen carefully. If too small a value is chosen, transients that may occur during the power sweep can cause the current consumed by the DUT to deviate from the quiescent current. A dwell time of 200 ms can have a negligible effect while maintaining a suitably short sweep time of 20 s. On the same list, marker 1 is set to the start value of the sweep, i.e. -20 dBm, activated by selecting the "On" state. Figure 2 shows the detailed configuration sequence.
As already mentioned, the spectrum analyzer is used in the "Zero Scale" mode. Both the resolution bandwidth and the video bandwidth are set to 10 MHz, since the spectrum analyzer is used to measure peak power. For the same reason, the detector must be set in the "Max Peak" mode. A sweep time of 25 seconds is chosen in order to obtain a full sweep of the screen. The option of external trigger is selected. It is also wise to use the "trigger deviation" feature to center the trace on the screen. -2 seconds is suitable. Figure 3 shows the detailed configuration sequence.

The pulse length and duty cycle must be chosen so as not to disturb the thermal state of the device under test and must also be consistent with the response time of the spectrum analyzer. A pulse length of 1 µs and a cycle period of 1 ms give good results.

The results presented in this section are based on measurements of a LDMOS power RF integrated circuit designed by Freescale Semiconductor for the UMTS band (MW4IC2230MB). It has a low-signal gain of approximately 30dB and a saturated power of well over +47dBm. Due to its high gain, it is a perfect example of the advantages of this approach.

The input variable attenuator is initially set to its maximum value. The DUT is connected and the spectrum analyzer is connected to the coupled path of the output coupler. When in "clear/write" mode, the input power ramp is depicted as an asymmetrical sawtooth on the analyzer screen. The variable input attenuator is then disconnected and the effects of DUT saturation will begin to appear (the top of the ramp begins to bend). The attenuation is continuously reduced until the top of the sawtooth is cut off, ensuring that saturation is achieved.

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The functional sequence for the spectrum analyzer is shown in Figure 4. At this moment, the spectrum analyzer is connected to the input and the input power is acquired in the second trace (Trace 2) and kept in "Observation" mode. This is the black trace in Figure 5. After the analyzer is reconnected to the output, the output power is obtained from the third trace (Trace 3), also kept in "Observation" mode. By setting the parameter Level Deviation to the "Output Deviation" value, which was set to 43.7dB during the calibration phase, the absolute output power in dBm can be read directly from "Trace 3", which is the green line in Figure 5. By placing a marker in the flat area at the end of the sawtooth, the saturated power can be read as shown in the green line in Figure 5. The MW4IC2230MB shows a saturated power of +47.23dBm.

Function sequence

The function sequence for the spectrum analyzer is shown in Figure 6. Trace 1 is set up to calculate Trace 1 minus Trace 2, which contains the input power. The newly acquired output power will then provide a gain plot in dB. The Trace Position feature is used as a bias to obtain a direct readout of the absolute gain in dB. The Trace Deviation is set to Output Deviation minus Input Deviation. In this case, the Input Deviation was determined to be 30.7 dB during the calibration phase, resulting in a deviation value of 13 dB. This value cannot be entered directly as a deviation value in dBm, as the Trace Position list only accepts input as a percentage of the Y scale. The rotary knob is used to obtain the correct percentage corresponding to the 13 dB deviation value. This is the result in the blue line in Figure 7.

Gain (blue line) and output power (red line) are plotted simultaneously, and each plot is "calibrated", that is, the marker readings are absolute values. Marker 1 is set at the low signal gain on the blue line, and the measured value is 29.8dB. This marker is used as a parameter for the "variable incremental marker". Still in incremental curves, one marker is used in the "variable incremental marker" mode to determine the 1dB compression level (marker 2) and the other to determine the 3dB compression level (marker 3). When the power trace is selected, the fourth marker (marker 4 on the green line) is set to the same horizontal axis as marker 2 or marker 3 to directly read the output power at 1 or 3dB compression level. In this example, a 1-dB compression level of +45.74dBm and a 3dB compression level of +46.69dBm are obtained. The method of performing power measurements under pulsed conditions allows it to be quickly and easily performed in high-power RF transistors and RF integrated circuits, eliminating the limitations of previous methods.