How to use the spread spectrum clock generator to reduce electromagnetic interference EMI
Electromagnetic interference (EMI) is an energy that can affect the performance of an electrical/electronic device by causing an unexpected response or full operational effectiveness.
EMI is generated by radiated electromagnetic fields or induced voltages and currents. High clock frequencies and short edge rates in current high speed digital systems can also cause EMI problems.
An important source of conduction and emission EMI is electrical equipment that connects AC power lines, such as computers and switching power supplies, as well as electrical equipment with electric motors, such as refrigerators, air conditioners, and motorcycles.
Once the EMI of the electrical device is conducted into a circuit, the cable inside will “broadcast” the conducted EMI into the entire circuit in the form of RFI (radio interference) like an antenna.
EMI may have little or no catastrophic failure, so effective control of EMI is very important. Electromagnetic compatibility (EMC) refers to the ability of a system to operate in a specified environment without transmitting or transmitting excessive battery interference.
EMI standards and related costs
The purpose of the EMC standard is to ensure that electronic equipment does not affect the operation of other electronic equipment or even cause equipment failure.
The requirements for EMI shielding for consumer electronics devices such as "TV, radio, portable entertainment devices, video games and Internet devices" vary.
So far, various organizations have issued EMI specifications. In the US, the FCC released for A? Part 15 and Class B electronic equipment Part 15 J specification. A? Class and A specifications are for industrial equipment, while Class B and B specifications are for consumer electronics. EMI rules reduce interference between electronic devices and address health and safety issues.
How to control EMI generally consider the following factors:
1) PCB design - "Isolation of sensitive components, power supplies and ground layers"
2) Circuit current - "EMI radiation will increase with increasing current"
3) Frequency, including slew rate - "EMI radiation causes an increase in frequency"
5) Circuit loop area - "Keep in minimum"
6) Shielding/Filtering - "Combining reasonable design, filtering, shielding and other techniques to control EMI to the required level at the lowest cost"
7) Spread spectrum clock - "appropriate spread frequency and modulation frequency"
8) Jitter the center frequency of the clock in the application system to spread the radiant energy into multiple frequency bands instead of letting all energy radiate to one frequency.
Control and reduce EMI methods
There are two basic ways to control and reduce EMI: inhibition and absorption. The most common methods of noise reduction include proper device circuit design, shielding, grounding, filtering, isolation, separation and orientation, circuit impedance level control, cable design, and noise cancellation.
These methods require the use of passive and active components such as filters, chokes, ferrite beads, foils and? ? Pieces, combined with PCB design rules and Spread Spectrum Clock Generator (SSCG).
Solve EMI problems at the source
A fundamental principle of EMC design is to reduce EMI at the source of the PCB. The spread spectrum method is intended to extend the radiant energy generated in a particular bandwidth to the frequency domain to produce a signal with a larger bandwidth. The Exhibit Clock Generator (SSCG) can perform this function.
When choosing a spread spectrum clock to reduce EMI in consumer electronics, developers must ensure the following:
1) The system must pass the EMI type test. A good frequency profile and modulation frequency are the most important. The high-quality Hershey Kiss frequency profile is the best at reducing EMI; in contrast, the triangular frequency profile requires a larger amount of expansion to reduce EMI to the same level (see Figures 1 through 3). The higher the modulation frequency, the lower the EMI (Figure 4).
2) Maintain system performance even if the spread spectrum has side effects. First, the PLL must operate in an ideal state, such as higher PFD and VCO frequencies and appropriate bandwidth. Second, the amount of frequency spread must be as small as possible to maintain high system timing margin and low inter-cycle jitter. With a smaller amount of frequency expansion, the average frequency of the system will not be reduced too much, so the system will not run as slow.
3) Minimize the impact on the total cost of the system. In consumer electronics, the price of spread spectrum clock chips has always been a major price issue. However, while consumer electronics is becoming more complex in recent years, developers must carefully consider development costs and risks.
For example, even if only one requirement is not met in suppressing EMI and jitter, the system clock of consumer electronics needs to be more likely to be adjusted. The flexibility of the programmable EMI suppression method can significantly reduce development costs and risks, ensuring that all requirements are met.
Spread spectrum clock generator
Spread Spectrum Clock Generator (SSCG) can be divided into programmable and non-programmable, or it can be classified according to whether it has Hershey Kiss frequency or triangular spread spectrum. The spread spectrum clocks of different consumer electronics products have different requirements for frequency, center or downward spread, spread amount, modulation frequency, Hershey Kiss or triangle spread spectrum.
Since the non-programmable spread spectrum clock chip is customized for specific applications, there are only a few fixed options for frequency range and expansion. It is very difficult to meet the optimal spread spectrum requirements while maximizing cost/performance. .
Most fixed-function clock chips on the market have multiple fixed selectable input frequency ranges (eg 20-40MHz, 40-80MHz and 80-160MHz) and expansion rates (eg 0.5%, 1%, 2% and 3%). ). To achieve optimization, two sets of PLL parameters are required, one for EMI suppression and the other for PLL performance.
When the actual configuration deviates from these ideal settings, various side effects occur. For example, if the input frequency is not in the middle of the selected range, the VCO and modulation frequency will be adjusted linearly.
If the PLL bandwidth is too low, the frequency profile will be distorted, affecting EMI performance.
When the input frequency is lowest, the result is the worst: because the PDF and VCO frequencies are very low, the inter-cycle jitter is greatly increased, and the frequency profile may be distorted due to the low modulation frequency, and the EMI suppression performance is greatly reduced.
When the choice of the amount of expansion is limited, the developer must choose a larger amount of expansion beyond what is needed. This often increases inter-cycle jitter and reduces system timing budget.
If there isn't a scaling rate that meets the system requirements, developers must ask the clock vendor to make changes to the design and provide a new chip. In the process, even if it's just a matter of changing a metal layer, it takes at least a few weeks. Time, and the cost is generally very high.
In contrast, a programmable spread-spectrum clock generator provides a universal clock that supports field-programmability, combined with on-chip non-volatile memory for dynamic spread-spectrum parameter reset, eliminating the need for manufacturers to spend A lot of time and cost have changed the chip.
Programmability also allows the performance of the spread spectrum clock to be optimized for the required specifications. For example, developers can specify an exact rate of expansion of 2.1% (instead of 3% of a fixed selection) or optimize the modulation mode to achieve the desired frequency setting.
How to use a 4PLL type clock chip with 2 spread spectrum PLLs to easily reduce EMI by 3 to 4 dB by modulation frequency optimization. These extended PLLs are available in two independent expansion modes.
Most developers prefer to use the Hershey Kiss spread-spectrum clock for better EMI suppression, but many clock vendors only offer linear spread-spectrum clocks. Ideally, an SSCG must provide both Hershey Kiss and a linear spread spectrum clock. Demonstrating the Hershey Kiss spread spectrum clock, the one-time EMI reduced by 1.67dB under the 4PLL clock chip test conditions shown above.
In addition, important clock parameters, such as PLL charge pump current, VCO gain, and output drive strength, must be programmable. This flexibility can greatly improve system performance, reduce system development time, minimize changes and reduce risk.