DAC Phase Noise Sources
For high-speed DACs, phase noise mainly comes from the following aspects: clock noise, power supply noise, and internal noise and interface noise.
Figure 1: Sources of DAC phase noise (Image credit: ADI)
Two of the most important sources are clock noise and power supply noise. This article will focus on the impact of clock noise on DAC phase noise.
Generation of Clock Phase Noise
The DAC clock is the number one cause of phase noise in a DAC. The clock determines when the next sample is sent, so any noise in the phase (or timing) directly affects the phase noise of the output.
Figure 2: Correlation of clock and phase noise (Image credit: ADI)
As shown in the figure above, the effect of the clock on phase noise can be viewed as the multiplication of successive discrete values by a rectangular function, the timing of which is defined by the clock.
Figure 3 Phase noise convolution (picture from ADI)
In the frequency domain, multiplication is converted to a convolution operation, as shown in the figure above. As a result, the desired spectrum is corrupted by clock phase noise.
Signal frequency and phase noise
The ratio of the signal frequency to the clock frequency, which is amplified or reduced relative to the noise of the carrier, for every half of the signal frequency, the noise improves by 6 dB. To demonstrate this, the figure below shows a modulated clock signal (precisely controlled) with a slight 100kHz phase offset at different frequencies (5GHz, 1GHz, 500MHz) to simulate phase noise. Let’s see how the signal frequency compares with phase noise relationship.
Figure 4: Clock output phase noise with 100kHz phase modulation (Image credit: ADI)
We can see that a 20dB drop is observed from the 5GHz clock to the 500MHz DAC output, and a 6dB increase is observed from the 500MHz output to the 1GHz output.
Reduce DAC Phase Noise
Choosing a crystal oscillator with good performance can achieve twice the result with half the effort in dealing with phase noise.
In the Digi-Key Chinese technical forum, the technical resource library required by the majority of Electronic circle engineers is gathered, and there are also many posts related to crystal oscillator selection and DAC phase noise reduction. For details, please visit – electronic components Selection Basics – Crystal Oscillator.
Crystal oscillators can be divided into active crystal oscillators and passive crystal oscillators. The following table compares the two crystal oscillators:
For the comparison of the characteristics and typical applications of different types of crystal oscillators, a detailed analysis is made in the following table:
If you want to know how to quickly select a low phase noise crystal through the Digi-Key website, you can refer to the following article – How to choose a low phase noise crystal?
Summary of this article
To sum up, only by understanding the cause of noise can we prescribe the right medicine. Clock noise has a great impact on DAC phase noise, so choosing a high-precision crystal may be the easiest and most feasible solution.
Source: DigiKey, Author: Alan Yang
The Links: ESM4016 NL8060BC26-35D