Envelope Tracking/DPD Designs Need Good Test & Analysis Tools

While at DesignCon in January, I stopped by the National Instruments booth and was amazed when I saw its PXI RF modular instrumentation setup looking at an RF power amplifier (PA) with an envelope tracking and DPD design. This modular system, along with NI LabView Modulation Toolkit and NI RF Software Measurement Suites, enables an RF signal generator to output custom or standard modulation formats as LTE, GPS, GSM, WCDMA, MIMO, and many more.

I remember the days when it took a rack of equipment to adequately test such an ET and DPD performance on a PA. Now the compact modules fit into a system the size of most bench-top power supplies or the old RF power meters I used at General Microwave in the 80s (Figure 1).

Figure 1: NI's PXI RF Modular Instrumentation array of plug-ins at DesignCon to test DPD and ET on a PA.

Figure 1: NI’s PXI RF Modular Instrumentation array of plug-ins at DesignCon to test DPD and ET on a PA.

Envelope tracking is a clever way to help correct the peak-to-average power ratio (PAPR), which causes inefficiency in the PA. Digital pre-distortion (DPD) is an additional way to increase the efficiency of those power-hungry PAs.

Both of these enhancements to increasing the PA efficiency were demonstrated at DesignCon in a live demo that analyzed the efficiency of the design and showed the percent improvement of each technique by itself and then the combined overall efficiency of the duo (Figures 2 and 3).

Figure 2: PA optimization is shown before and after applying ET and DPD.

Figure 2: PA optimization is shown before and after applying ET and DPD.

The most difficult ET test challenge is ensuring instrument synchronization between the RF signal generator and the arbitrary waveform generator (AWG). Since maximizing the power-added efficiency (PAE) of the PA occurs when we choose an optimal Vcc value based on the input power, poor synchronization between these instruments will result in having a Vcc value that is either too high or too low for any given output power. The RF signal generator and AWG must not only be synchronized, but the synchronization should also be repeatable. NI equipment demonstrated this capability.

Figure 3 shows the almost 8 dB improvement in the PA with DPD.

Figure 3 shows the almost 8 dB improvement in the PA with DPD.

There are considerable challenges to designers in the design and testing of PA efficiency enhancement techniques. I suggest viewing the NI whitepaper, which discusses ET fundamentals and robust test solutions and challenges.

10 comments on “Envelope Tracking/DPD Designs Need Good Test & Analysis Tools

  1. Netcrawl
    February 6, 2014

    @Steve great post, thanks for that. Envelope tracking is now essential in achieveing PA efficiency, currently we have several techniques that can be used to improve the PAE of amplifier like DPD, ET is quickly gaining attention amongst vendors, base stations have been employing ET not only to increase efficiency but also to reduce cooling requirements due to wated energy dissipated as heat. 



  2. etnapowers
    February 7, 2014

    Steve, very nice blog, I worked with an application engineer of National Instruments to develop an automated testing equipment powered with the dedicated labview software. I found very interesting the software interface that allows the user to drive the power suppliers easily in real time. The power amplifier worked very well, having a really high efficiency.

  3. etnapowers
    February 7, 2014

    Many times the NI solutions feature a calibration checking hardware and software, to verify the effectiveness of the PA , for example, by referencing to a reference tool that ensures the correct working of the PXI system over time.

  4. etnapowers
    February 7, 2014

    “The RF signal generator and AWG must not only be synchronized, but the synchronization should also be repeatable. ” The repeatability is really important not only for the RF PA but in every testing equipment that has to verify periodically the efficiency of the system in a repetitive mode

  5. Steve Taranovich
    February 7, 2014

    Thanks for your excellent comments and observations @etnapowers, you are correct—all of the equipment associated with the particular test being done needs to be very repeatable. National Instruments has some great tutorials on this and other aspects of test tools for Analog designers to make sure that their tests are as accurate as can be.

  6. Netcrawl
    February 9, 2014

    @etnapowers you're right about NI, they're pretty good in test technology, all of thesed evelopment can lead to less power consumption and efficiency. NI is really good in identyfing possible trends in automated tests. They're getting serious, I think NI is aiming for something big, its trying to build a software-centric network with its LabVIEW lines.  

  7. Victor Lorenzo
    February 9, 2014

    I agree with you about NI @Netcrawl in that “they're pretty good in test technology “, I've been following their LabWindows/CVI, LabView and data acquisition boards from those old MS-DOS/Windows 3.1 times. NI has a very wide range of control, instrumentation and test boards support.

    In my previous occupation I used several PCI motion control and digital I/O boards from NI for 3D (3 coordinated axes) machine control and automation with outstanding results. I decided not to use LabView for productivity and cost (it is too expensive) reasons.

  8. Netcrawl
    February 9, 2014

    Its quite expensive but it a very impressive piece of technology ideal for any messurement and control system. LabVIEW is the heart of NI design technology, NI is putting a lot of effort integrating all the fucntionality and tools that engineers need to build a wirde rang of applications. Its capable of reducing test system development time and offers unprecendeted integration with existing legacy software, overall I think its still “a must have tools”. 

  9. etnapowers
    February 10, 2014

    @Netcrawl, yes you're right, NI is getting real serious in automated testing solutions, moreover, during a presentation held by a technical marketer of National Instruments I knew that NI is proposing a complete automated test (both hardware and software) solution for mass production purposes, this solution having the same functionalities of a traditional setup with a cost reduction of more than 40%

  10. etnapowers
    February 10, 2014

    Victor, I'm happy for your appreciation of NI, because it is a really good company that is developing its know how in automated testing solutions and recently the costs of NI solutions are decreasing compared to the gain related to the mass production volumes that the NI solutions can guarantee.

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