Reduce EMI interference using a programmable spread spectrum clock generator

Electromagnetic interference (EMI) is a type of energy that can affect the performance of electrical/electronic equipment by causing unexpected responses or complete failure of operation.

EMI is generated by radiated electromagnetic fields or induced voltages and currents. The high clock frequencies and short edge rates in today's high-speed digital systems can also cause EMI problems.

An important source of conducted and radiated EMI is electrical equipment connected to the AC power line, such as computers and switching power supplies, and electrical equipment with electric motors, such as refrigerators, air conditioners, and motorcycles.

Once the EMI from electrical equipment is conducted into a circuit, the cables inside will act like an antenna, "broadcasting" the conducted EMI to the entire circuit in the form of RFI (radio interference).

Figure 1: Overview of Hershey Kiss spread spectrum clock frequencies in the clock domain.

EMI can have a minor effect or cause 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 conducting or emitting excessive battery interference.

EMI Standards and Related Costs

The purpose of EMC standards is to ensure that electronic equipment does not affect the operation of other electronic equipment or even cause equipment failure.

Countries have different requirements for EMI shielding for consumer electronic devices such as “TVs, radios, portable entertainment devices, electronic games and Internet devices”.

Various organizations have published EMI regulations so far. In the United States, the FCC has published Part 15 Subchapter J regulations for Class A and Class B electronic devices. Class A and Tier A regulations are for industrial equipment, while Class B and Tier B regulations apply to consumer electronics. EMI rules reduce interference between electronic devices and address health and safety issues.

Figure 2: Overview of spread spectrum clock frequencies in the frequency domain.

How to control EMI, generally consider the following factors:

1) PCB design - "Isolation of sensitive components, power supply and ground layer"

2) Circuit Current - "EMI radiation increases as current increases"

3) Frequency, including slew rate – “EMI radiation will cause the frequency to increase”

4) Bandwidth

5) Circuit loop area - "keep it to a minimum"

6) Shielding/Filtering - "Combining proper design, filtering, shielding and other techniques to control EMI to the required level in the lowest cost way"

7) Spread spectrum clock - "appropriate spread spectrum amount and modulation frequency"

8) Dither the center frequency of the clock in the application system to spread the radiated energy over multiple frequency bands instead of having all the energy radiated to one frequency.

Methods to control and reduce EMI

There are two basic methods to control and reduce EMI: suppression and absorption. The most commonly used noise reduction methods include reasonable equipment circuit design, shielding, grounding, filtering, isolation, separation and orientation, circuit impedance level control, cable design and noise elimination.

These methods require the use of passive and active components such as filters, chokes, ferrite beads, foils and ?? components, combined with PCB design rules and spread spectrum clock generators (SSCG).

Figure 3: Overview of the advantages of Hershey Kiss spread spectrum.

Solving EMI Problems at the Source

A basic principle of EMC design is to reduce EMI at the source of the PCB. Spread spectrum refers to intentionally spreading the radiated energy generated in a particular bandwidth to the frequency domain to generate a signal with a wider bandwidth. The spread spectrum clock generator (SSCG) can perform this function.

When selecting spread spectrum clocking to mitigate EMI in consumer electronics, developers must ensure the following:

1) The system must pass EMI type testing. A good frequency profile and modulation frequency are the most important. A high-quality Hershey Kiss frequency profile performs best in reducing EMI; a triangular frequency profile requires a larger spread to reduce EMI to the same level (see Figures 1 to 3). The higher the modulation frequency, the lower the EMI can be reduced (see Figure 4).

2) Even if the frequency spreading has side effects, the system performance must be maintained. First, the PLL must operate in an ideal state, such as high PFD and VCO frequency and appropriate bandwidth. Second, the frequency spreading amount must be as small as possible to maintain high system timing margin and low cycle-to-cycle jitter. With a smaller frequency spreading amount, the average frequency of the system will not drop too much, so the system will not run as slowly.

3) Minimize the impact on the total system cost. In consumer electronics, the price of spread spectrum clock chips has always been a major price issue. However, as consumer electronics products have become increasingly complex in recent years, developers must also carefully consider development costs and risks.

For example, if even one of the requirements for EMI and jitter suppression is not met, there is a greater chance that the system clock of a consumer electronic product will need to be adjusted. The flexibility of programmable EMI suppression methods can greatly reduce development costs and risks, ensuring that all requirements are met.

Figure 4: EMI reduction by modulating frequency.

Spread Spectrum Clock Generator

Spread spectrum clock generators (SSCG) can be divided into two types: programmable and non-programmable. They can also be classified according to whether they have Hershey Kiss frequency or triangular spread spectrum. The requirements for spread spectrum clocks of different consumer electronic products are different in terms of frequency, center or downward spread, spread amount, modulation frequency, Hershey Kiss or triangular spread spectrum, etc.

Since non-programmable spread spectrum clock chips are customized for special applications, there are only a few fixed options for frequency range and spread amount, which makes it 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 (such as 20-40MHz, 40-80MHz and 80-160MHz) and spreads (such as 0.5%, 1%, 2% and 3%). To achieve optimization, two sets of PLL parameters are required, one for EMI suppression performance and the other for PLL performance.

Figure 5: Frequency adjustment in the GP SSCG buffer chip.

When the actual configuration deviates from these ideal settings, various side effects can occur. For example, if the input frequency is not exactly in the middle of the selected range, the VCO and modulation frequencies will be adjusted linearly (Figure 6 below).

If the PLL bandwidth is too low (typically due to control cycle-to-cycle jitter, as shown in Figure 6), the frequency profile will be distorted, affecting EMI performance.

The results are worst when the input frequency is lowest: because the PDF and VCO frequency are low, the cycle-to-cycle jitter increases significantly, and the frequency profile may be distorted due to the low modulation frequency, and the EMI suppression performance is greatly reduced.

Figure 6: Frequency scaling compared to ideal schematic.

When the choice of spread is limited, developers are forced to choose a larger spread than necessary. This often increases cycle-to-cycle jitter and reduces the system timing budget.

If no scaling rate meets system requirements, developers must ask the clock supplier to change the design and provide a new chip, a process that takes at least several weeks and is generally very costly, even if it is as simple as changing a metal layer.

In contrast, a programmable spread spectrum clock generator can provide a universal clock that supports field programmability and, combined with on-chip non-volatile memory, can achieve dynamic spread spectrum parameter reset, eliminating the need for manufacturers to spend a lot of time and cost to modify the chip.

Programmability also allows spread spectrum clock performance to be optimized to the required specification. For example, developers can specify an exact spreading rate of 2.1% (rather than a fixed selection of 3%), or optimize the modulation pattern to achieve the desired frequency setting.

Figure 4 above shows how to use a 4PLL clock chip with 2 spread spectrum PLLs to easily reduce EMI by 3 to 4 dB through modulation frequency optimization. These spread spectrum PLLs have two independent spread modes to choose from.

Most developers prefer to use Hershey Kiss spread spectrum clocks to achieve better EMI suppression performance, but many clock vendors only provide linear spread spectrum clocks. Ideally, an SSCG must provide both Hershey Kiss and linear spread spectrum clocks. Figure 3 shows that Hershey Kiss spread spectrum clocks reduce EMI by 1.67dB in the test conditions of the 4PLL clock chip shown above.

In addition, important clock parameters, such as PLL charge pump current, VCO gain and output drive strength, must be programmable. Such flexibility can greatly improve system performance, reduce system development time, minimize changes and reduce risks.