DPD negative feedback circuit design considerations
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DPD negative feedback circuit design considerations

Posted Date: 2024-01-24

DPD has been discussed in many papers. The main focus is on the construction of the nonlinear model of the DPD power amplifier. There are also some analysis and simulations on this aspect in the previous article. You can look through it.

Today I will mainly talk about the negative feedback circuit of DPD

Digital predistortion (DPD) is one of the most basic building blocks in current wireless communication systems. It is used to increase the efficiency of power amplifiers. By reducing the distortion produced when the power amplifier operates in its nonlinear region, the efficiency of the power amplifier can be greatly improved.

As can be seen from the above figure, DPD mainly consists of a power amplifier circuit, a pre-distortion model, and a feedback channel.

The function of the feedback circuit is to extract the nonlinear component, that is, the distortion component.

Therefore, the feedback circuit is a key step in whether DPD can work (don’t underestimate this feedback circuit)

So what are the precautions for feedback circuits?

1. Bandwidth

Generally, DPD is used to improve the adjacent channel suppression ratio. The adjacent channel power suppression ratio is generally at least 2, and more is three. Therefore, in order to accurately extract nonlinear distortion, the bandwidth of the feedback channel is generally 5 times that of the forward circuit. ~7 times, that is, at least 5 intermodulation components are extracted.

2. ADC sampling rate

The bandwidth of the feedback channel is 5 to 7 times that of the forward channel, which means that the feedback channel requires a higher sampling rate.

3. The feedback channel cannot produce new nonlinearities

The function of DPD is to offset the nonlinearity of the forward circuit, so the feedback circuit cannot introduce new nonlinearity, which will greatly reduce the effect of DPD. Because the forward power is very high, active RF devices are generally not introduced into the feedback circuit.

4. The collected feedback power is constant and stable

In order to reduce the complexity of data calculation, generally speaking, the power sent to the ADC for collection is stable, so the feedback circuit usually contains a numerically controlled attenuation circuit (don't ask why it is not amplification)

5. The ADC must have enough sampling bits

SNR=6.02N+1.76

The ADC must not only have a high sampling rate, but also have a large dynamic range, because it needs to collect five or even seven intermodulations of the signal. In order to prevent the signal from being distorted and to collect the desired signal at the bottom, the SFDR of the ADC must be sufficient. big. For example, some base stations require the adjacent channel index to reach 70dBC, so during the convergence process, the dynamic range collected through feedback must at least meet this requirement.

Summarize

Compared with the forward circuit, for RF engineers, there are more things that need to be considered and paid attention to when designing the feedback circuit.

Note: The article is reproduced from the[Radio Frequency Communication Chain]public account.

Review Editor: Tang Zihong


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