Broadband Power Amplifier Digital Predistortion Test Solution

Summary:

As the communications industry carries out 5G equipment research projects, the demand for broadband digital pre-distortion testing of 5G power amplifiers is gradually increasing. In response to this, Rohde & Schwarz (hereinafter referred to as R&S) has proposed a digital pre-distortion test solution based on the signal source SMW200A and the spectrum analyzer FSW. This set of instruments can directly generate and analyze signals with a bandwidth of up to 2GHz, which is suitable for broadband digital pre-distortion testing.

Keywords: digital predistortion; power amplifier; DPD; SMW200A; FSW

1 Introduction

Currently, 5G research projects have gradually become a hot topic in the communications industry. Compared with 4G LTE signals, the carrier frequency of 5G signals has added millimeter wave bands in addition to the frequency bands below 6GHz, and the single-carrier signal bandwidth of 5G signals has also expanded from 20MHz in the 4G era to 100MHz or even 400MHz.

In response to the changes brought about by 5G signals, RF device manufacturers have proposed various RF devices that can correspond to 5G devices. Among various RF devices, power amplifier devices are more complex, and as active devices, their RF test solutions are also more complex than other devices. In particular, the digital pre-distortion test of power amplifiers is a key project in the performance evaluation of power amplifiers. In response to this, Rohde & Schwarz has proposed a corresponding test solution for broadband digital pre-distortion testing of power amplifiers.

2. Broadband power amplifier digital pre-distortion test solution

2.1 Introduction to the construction and test process of the power amplifier pre-distortion test system

For amplifier testing, in addition to providing traditional test tools such as network analyzers, R&S can also provide a test solution based on the signal source SMW200A and the spectrum analyzer FSW. With this solution, the signal source and the spectrum analyzer can communicate through the network cable, which can easily realize the closed-loop test of the digital pre-distortion of the power amplifier, and can realize multiple iteration tests. This solution can simultaneously test various important indicators before and after the digital pre-distortion of the power amplifier, such as AM/AM, AM/PM, error vector magnitude (EVM), adjacent channel leakage ratio (ACLR), 1dB compression point, gain, etc. The schematic diagram of the instrument connection is shown in Figure 1.

Figure 1 Schematic diagram of power amplifier test instrument connection

For digital pre-distortion testing, the signal source SMW200A provides an excitation signal through the RF output port. After the signal is amplified by the power amplifier, it is distorted and output. The distorted signal is received by the spectrum analyzer FSW, which is called the "measurement" signal. At the same time, SMW200A will also send the original IQ data directly to FSW through the network cable. This signal is called the "reference" signal. The power amplifier measurement option FSW-k18 of the spectrum analyzer will perform synchronous calculations on the "measurement" signal and the "reference" signal to obtain the difference between the two, thereby obtaining indicators such as signal EVM. The system flow diagram is shown in Figure 2.

Figure 2 System flow chart

2.2 Introduction to the functions of the power amplifier pre-distortion test system

The digital pre-distortion test system composed of SMW200A and FSW can support the current mainstream lookup table model and polynomial model, as shown in Figure 3. FSW uses the network cable to directly update the AM/AM lookup table and AM/PM lookup table to the SMW200A. SMW200A uses the obtained lookup table to generate a signal with digital pre-distortion, which passes through the device under test and is then tested by FSW.

Figure 3 SMW200A and FSW digital pre-distortion test configuration interface

In addition to conventional lookup tables and polynomial models, the R&S spectrum analyzer FSW also provides a unique "direct" digital pre-distortion (Direct DPD) test method. This test method is different from conventional methods. It does not use any digital pre-distortion model, but uses the method of directly compensating the signal time domain sampling points point by point within the FSW to achieve digital pre-distortion. After the digital pre-distortion compensation is performed, the data will be transmitted back to the SMW200A through the network cable and transmitted again. After the signal is distorted by the power amplifier under test, it will be captured again by the FSW for direct compensation point by point. This process can be repeated many times. In short, this "direct" digital pre-distortion test method does not rely on the model, but directly compensates for each sampling point, and can achieve multiple iterations to optimize the digital pre-distortion step by step. Since this method does not rely on the existing pre-distortion model, it cannot output the digital pre-distortion model parameters, but it allows users to understand the "ideal" pre-distortion effect of the power amplifier under test in the simplest and fastest way, and this method supports memory effect and frequency response correction functions. The schematic diagram of point-by-point compensation and iteration of this “direct” digital pre-distortion is shown in Figure 4.

Figure 4: Schematic diagram of point-by-point compensation and iteration of the “direct” DPD method

Whether it is "direct" digital pre-distortion or model-based digital pre-distortion, FSW can provide multiple indicators such as AM/AM, AM/PM, error magnitude (EVM), adjacent channel leakage ratio (ACLR), etc. The three figures in Figure 5 are the test results obtained by using the "direct" digital pre-distortion method, among which Figure 5a is the measurement result before digital pre-distortion, Figure 5b is the measurement result after one digital pre-distortion, and Figure 5c is the measurement result after the second digital pre-distortion iteration. From the changing trends of the three figures in Figure 5, we can see that the error vector magnitude (Raw EVM) is continuously optimized from 4% to 0.9% and then to 0.7%, and the adjacent channel leakage ratio (ACLR) is optimized from -35dBc to -47dBc and then to -50dBc. From the test curve diagram, the linear region of the gain compression curve (Gain Compression vs Input Power) gradually extends to the high power region as the number of digital pre-distortion iterations increases. The EVM curve (EVM vs Input Power) also achieves EVM optimization at high power as the iteration coefficient increases. The AM/PM curve (Phase Deviation vs Input Power) also becomes more linear as the iteration coefficient increases.

Figure 5a: PA measurement results before digital pre-distortion

Figure 5b: Power amplifier measurement results after a digital pre-distortion operation

Figure 5c: Power amplifier measurement results after two iterations of digital predistortion calculations

2.3 R&S Digital Pre-Distortion Test Instrument Performance

This test solution mainly consists of the signal source SMW200A and the spectrum analyzer FSW, which can help customers quickly build a set of amplifier digital pre-distortion test platform, support different digital pre-distortion models (lookup table, polynomial), and support R&S's unique "direct" digital pre-distortion test method, helping users quickly confirm the "ideal" pre-distortion effect of the amplifier. In addition, for amplifier testing, FSW with SMW200A also supports envelope tracking amplifier testing, further expanding the scope of application of this solution.

The signal source SMW200A used in this solution can support a carrier frequency of up to 40 GHz and a signal bandwidth of 2 GHz. The signal source has a built-in broadband signal quality optimization mode, which ensures the flatness of the broadband signal generated by the signal source. As can be seen in Figure 6, the 2 GHz broadband multi-tone signal output by SMW200A has an amplitude flatness of about 0.4 dB within 2 GHz measured by the spectrum analyzer FSW, which ensures the signal quality of the input signal of the power amplifier.

Figure 6 SMW200A 2GHz broadband multi-tone signal amplitude flatness test chart

The spectrum analyzer FSW used in this solution can support carrier frequencies up to 90GHz, with a built-in 2GHz analysis bandwidth, and the built-in amplifier measurement option of FSW can directly remotely control the signal source SMW200A through a network cable, further simplifying the complexity of the test. In this way, the test solution of SMW200A+FSW can support 2GHz bandwidth, ensuring that users can test broadband high-frequency amplifiers. According to the convention that the digital pre-distortion sampling bandwidth is 5 times the signal bandwidth, this solution can support ideal digital pre-distortion testing of signals with a bandwidth of 400MHz.

After having the ideal test equipment, we also need to consider that the amplifier test system may also include various accessories such as RF cables, preamplifiers, attenuators, filters, etc. The amplitude frequency response and phase frequency response of these accessories themselves will also affect the amplifier test. For this scenario, R&S's signal source SMW200A and spectrum analyzer FSW both provide corresponding k544 options, which allow users to import the S parameter file (SnP file) of external accessories into the signal source and spectrum analyzer. The instrument will make real-time corrections to the frequency response of the external accessories in the digital domain, thereby greatly reducing the impact of the frequency response of the external accessories on the amplifier test. See Figure 7 for a schematic diagram. As for the S parameters of external accessories, they can be measured in advance using network analyzers, power meters, spectrum analyzers and other equipment.

Figure 7 Schematic diagram of real-time correction method for external frequency response of signal source and spectrum analyzer

3. Summary

This article introduces the test solution based on the R&S signal source SMW200A and spectrum analyzer FSW for digital pre-distortion testing. This test solution can support lookup tables, polynomials and other modes, and also supports the unique "direct" digital pre-distortion test solution, allowing customers to easily and conveniently obtain the "ideal" pre-distortion effect of the amplifier. Finally, a brief performance introduction of the test instrument is given. A single SMW200A instrument can support the generation of 2GHz bandwidth signals within a frequency of 40GHz, and a single FSW instrument can analyze 2GHz signals within 90GHz. The broadband characteristics of these two instruments are very suitable for broadband digital pre-distortion testing.

Note: The article was published in the 2nd issue of Telecommunications Network Technology in 2018