Analog Devices ADRV9001 User Manual page 225

Reference Manual
DIGITAL PREDISTORTION (DPD)
Once the setup is ready, further configure the TES and available external components properly, which includes the following major steps:
Select the desired profile.
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Perform board configuration to indicate that external loopback path with external PA is available.
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Enter the peak power of the loopback signal (ideally, adjust it to -18 dBm ± 5 dB. Do this by tuning the external step attenuator).
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Measure the external loopback delay and provide it through TES. Do this through API commands, which are discussed at the end of this
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section.
Configure other initialization parameters such as RF frequency, LO source, and so on, as desired. Also, enable DPD for the transmitter and
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configure the model tap polynomial terms. It is recommended to start with the default model tap. The next section discusses the method to
tune the model tap order.
Turn on the DPD tracking calibration and all other available tracking calibrations, and start with the default DPD post calibration parameter
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settings provided in the TES.
After programming, load, and play the provided sample transmit input file.
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Properly tune the transmitter attenuation and/or the step attenuator to make sure that the ACPR performance at the device transmitter output
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is satisfactory before passing to the PA. In addition, make sure that the transmit peak signal is around the P1 dB compression region for
optimal DPD performance.
Compare the ACPR performance through a spectrum analyzer with and without using the integrated DPD. Significant ACPR performance
improvement with the integrated DPD should be observed even with internal LO sources. For TETRA waveforms, the ACPR after the second
iteration of DPD is between −70 dB and −60 dB at an amplifier compression of P1 dB. For LTE waveforms, the ACPR after the second iteration
of DPD is between −55 dB and −50 dB at an amplifier compression of P1 dB.
Tuning the Model Tap Order
DPD can be considered an adaptive filter modeled according to the behavior of the PA. The ADRV9001 default model (model 4) consists of four
taps. Each tap consists of a series of polynomial terms to fit the nonlinear behavior due to compression at higher output power. The order of
polynomial terms is determined by intermodulation falling closer to the carrier spectrum. In DPD, the orders of intermodulations to consider are
usually the third, fifth, and seventh orders, with decreasing magnitude, respectively. An nth-order intermodulation expands the signal bandwidth
'n' times. By inspecting the bandwidth expansion factor on a spectrum analyzer, estimate how many orders of intermodulations to include in the
polynomial terms, in order to suppress the spectral regrowth down to the required ACPR. It is important not to include higher order power terms
than needed, which might cause the DPD to be unstable.
DPD model 4 consists of four taps, as shown in the example tap arrangement diagram in
categories:
The main tap: The main tap of the DPD adaptive filter suppresses most of the spectral regrowth due to intermodulation. Hence, it has the
greatest number of polynomial terms. It is labeled TAP
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Figure 201. An Example Setup for Testing the Integrated DPD in Narrowband Applications
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ADRV9001
Figure 202. The four taps can be classified into three
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