Models Help Designers Specify New High-Power Devices
By Beatrice Branger, Application Engineer; Jean-Marc Coupat and Sandra Gendraud,
Design Engineers; and Nelsy Monsauret, CAD and Modeling Engineer; Freescale Semiconductor

The multicarrier power amplifiers in wireless base station transceivers must accommodate signals that employ highly-complex modulation schemes. To do this effectively, devices such as the MRF7S18170H/HS, MRF7S19170H/HS, and the MRF7S21170H/HS from Freescale Semiconductor must deliver high CW power levels (about 170 W) in bands between 1.8 and 2.2 GHz while maintaining high efficiency and linearity. Dissipating heat at such high RF power levels is also a major consideration. To help designers integrate these devices in their RF subsystems, Freescale offers validated models that can be downloaded from the company’s Web site. These models can reduce design time and make it easier to select the best operating conditions for a specific application and accurately predict device behavior. The models have demonstrated their ability to create first-pass design success at the circuit board level.

The 170-W Devices and Models
The RF power transistors were developed in Freescale’s seventh-generation (HV7) LDMOS technology, which when compared to its sixth-generation predecessor delivers about 1.5 dB higher gain and 2% higher efficiency. The devices (Figure 1) operate from 1805 to 1880 MHz, 1930 to 1990 MHz, and 2110 to 2170 MHz respectively, with output power greater than 170 W CW. The devices operate at +28 VDC, have internal input and output matching, and are housed in low-thermal-resistance, metal-ceramic packages.

Freescale has developed non-linear electro-thermal models of the three devices, each one validated against small-signal and large-signal measurements. They include the effects of the active die, package, and linear matching networks. Although the simulations described in this article were completed using Agilent Technologies’ Advanced Design System (ADS) software, models can be used with APLAC Analog Design Tool and AWR Microwave Office. They are available at www.freescale.com/rf/models.

Model Verification
Under small-signal conditions, modeled and measured devices can be compared using the same quiescent current and drain voltage. Performance of the MRF7S19170H has been evaluated from 1.8 to 2.4 GHz with the TRL calibration method used to calculate the S-parameters in the package reference planes.(1) The small-signal parameters of the MRF7S19170H model agreed well with those of actual measurements over a large bandwidth.

Under the quiescent current specified in the product datasheets, the modeled and measured products were loaded at the input with optimum impedances for best input return loss. Pulsed load-pull measurements were then performed at the 1-dB gain compression point. The impedances for best power-added efficiency and best output power at 2.11 GHz are shown in Figure 2 at 1-dB gain compression. In Figure 2a, the curves are power contour and Zopt for optimum output power, and in Figure 2b they are efficiency contour and Zopt for maximum power-added efficiency.

Good correlation was obtained for the optimum impedance locations at 2.11 GHz. Device output was verified at 1-dB gain compression and the results are shown in Table 1. Simulated and measured RF performance is compared in Figures 3a and 3b at 1.88 GHz versus back-off from the P1dB output (to obtain optimum efficiency) and shows good agreement between the two. Similar results have been obtained by loading the model and the device with the optimum output power impedances.

Performance Evaluation
Freescale has implemented a new peak-to-average ratio (PAR) test based on customer saturated power requirements. From the average output power (Pout_avg) under W-CDMA test conditions and the PAR at the output of the device, the saturated output power (Psat) can be calculated:

Psat (dBm) = PAR (dB)+Pout_avg (dBm)

This formula can characterize a device operating in compression to reflect its behavior when modulated signal peaks appear during data transmission. The optimization is performed with the goal of obtaining manageable impedances at the gate and drain lead reference plane, so the device is easy to integrate into a customer application.

Load-pull measurements were made with a single-tone CW signal at the highest frequency in the band (2170 MHz) and were performed on one die that was internally matched to select the transistor periphery and to more effectively size the device. Matching was then optimized for each frequency band. To evaluate the behavior of the devices in conditions approximating those of their intended applications, the test-signal employed in production is a single-carrier W-CDMA signal that is compliant with the 3GPP specification. This complex signal has a source PAR equal to 7.5 dB at 0.01% probability on a Complementary Cumulative Distribution Function (CCDF) curve.

The main characteristics of each device (such as gain and input return loss), are measured with RF power meters, and Adjacent Channel Power (ACP) is measured with a spectrum analyzer that also measures the PAR at the device output. A peak-power meter can also be used to measure PAR. The test bench is calibrated to determine input and output power, input return loss, gain, efficiency, PAR, and ACP in the transistor reference planes.

The production test fixture for the MRF7S19170H is tuned with chip capacitors across the RF transmission line in order to present the best impedances to the transistor and have 50 ohms at the connector planes. The test fixtures for each device have been tuned to achieve the best combination of PAR, efficiency, gain, ACP, input return loss, and flatness over the frequency range.

Conclusion
In order to ensure that Freescale’s new HV7 high-power RF transistors can perform in actual service conditions, the company subjected them not only to the conditions specified for W-CDMA by 3GPP, but to more demanding tests as well. The models created to predict this performance have been verified many times by several organizations and have shown themselves to be in very good agreement with actual device measurements. These models are available to all designers wishing to employ the MRF7S18170H/HS, MRF7S19170H/HS, and the MRF7S21170H/HS.

Reference
1. “Impedance measurement for high-power RF transistor using TRL method”, Jean-Jacques Bouny, Microwave Engineering Europe, April 1999, p. 118.

Freescale Semiconductor
www.freescale.com
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