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The biggest problem with maintaining impedance control is the signal path discontinuities, including return path on ground plane: The ideal signal path has a continuous copper plane immediately underneath. In such a case, impedance control is confined to proper terminations, usually at the load end. For slower signals, where EMI control is the predominant issue, source termination is often an appropriate choice, as it also limits the emission currents from leaving the driver chip.
Source termination does slow the signal, which may not be acceptable for highest speeds. Unfortunately, this is not a fully acceptable solution at high frequencies, but will be reasonably good for lower frequency signal paths. Return current path is discontinuous when switching reference planes 3. A lesser discontinuity occurs if the signal is transitioning from one ground plane to another. Here, the return path from plane to plane must be made continuous and impedance control effected.
Typically, this is handled by inserting ground to ground vias around the perimeter of the signal via, and controlling the keepout, pad size and via size and length in order to match impedances.
WHAT IS EMI/EMC? « Electronic Environment
For highest speed signal integrity, you will need to minimize the impedance discontinuity by controlling the via size and length, and the diameter of the keepout. Return current past discontinuity is minimized when keeping same reference plane 5.
The return path has to go around the gap in the plane, raising the characteristic impedance at the gap, and energizing the opening as a slot antenna.
This can occur when a portion of the plane is stolen to accommodate another trace, at a split plane boundary, or at a connector cutout. Signal return path is disrupted by cut in ground plane 6.
Signal path at mandatory discontinuities. This assumes that impedance control needs to be maintained across the boundary. In such a case, an impedance matching network needs to be placed at the boundary. This is handled by controlling the copper parameters at the boundary. Larger cutouts increase inductance while leaving more copper at the boundary increases capacitance. In both cases, most of the problems lie with a very few of the traces.
But the critical traces are typically different for signal integrity and EMI. For signal integrity, the problem is limited to the relatively few high speed signal traces. For EMI, the problem concentrates on those lines entering or leaving the circuit board.
The primary emitters are those that carry high speed clock and data lines, along with the parasitic coupling to slower lines, power lines and especially ground lines. The primary receptors are low level analog input lines for RFI and digital lines for transients.
Once these lines are identified, you can place the chips on board to facilitate good routing. The simpler the path for critical traces, the easier it is to maintain signal integrity and EMI control. Decoupling Starting with the supply voltages, the voltage tolerances are basically a signal integrity issue. This does not show up at the EMC level except to the extent that external interference corrupts voltage at the power supply or on-board regulators.
The big difference lies with the demand for decoupling. So decoupling demands for EMI are a thousand times more demanding than for signal integrity. The chip manufacturer recommends decoupling capacitors as needed for Vcc droop. This means that the target frequencies for signal decoupling are at the clock frequency and below, while the frequencies for emissions are at the clock harmonics, typically ten times the clock frequency or even higher.
Thus, the demands for decoupling for emissions are substantially higher than with signal integrity. At modern computer speeds, your high frequency harmonics are inevitably operating above the series resonant frequency of the typical decoupling capacitor.
Just add one to two nanohenry of lead length in each decap and you will find that the impedance is too high for effective filtering. If the impedance is above one ohm, you should look for better filtering, or more decaps in parallel. The good news is that at higher frequencies, the interlayer capacitance of multilayer boards becomes the dominant factor above a couple hundred MHz.
Crosstalk Crosstalk can be an issue for both signal integrity and EMI. Crosstalk is unintended coupling to adjacent metallic members, usually to an adjacent signal, power or ground path. Crosstalk includes field coupling from one line to an adjacent line.
It is a major issue with cables that will usually need to be addressed, but may also be a problem with adjacent trace coupling at the circuit board level. Any coupling from very high speed signal lines can degrade signal quality we see signal speeds well into the GHz range, and we hear GHz is just around the cornerwhether to an adjacent trace or any other metallic element on the circuit board. Often, this problem can be eliminated by separating these lines.
The spacing in between need not be wasted, but can be used for less critical lines. In both cases, increased spacing is beneficial, as coupling falls off with the square of the distance. Other Signal Path Issues In addition to crosstalk, other losses may come into play, with series resistance and shunt dielectric loses being the major issue.
Signal path losses would include series resistance in the conductive path and shunt conductance in the dielectric. For the most part, these losses are not a problem at the circuit board level, unless you are using a high resistance signal path, such as conductive epoxy which is rarely used.
These losses become much more of a problem at the cable level, especially with signal integrity, where losses track directly with eye diagram shrinkage, to the point of signal failure. For EMI, the problem is a bit less noticeable. But obviously, if the signal strength is weakened, it takes less external interference to create data errors. Imbalance is an extension of crosstalk, becoming increasingly significant for differential signals as serial data speeds increase.
Balance loss will occur with unequal coupling paths, as mentioned above, and will also show up due to unequal propagation times from driver to receiver. This is much more of an issue with signal integrity than with EMI.
Although rare, such incidents do happen, and it takes a well experienced pilot to recognize the anomaly and take over the flight controls. It states that an EMI problem can be viewed as a theater act staging 3 players: These three actors are needed on the stage for the performance to exist. If only one is missing, there will be no playing.
Then eliminating an interference problem can be accomplished by acting on one, or several of the three actors, whichever is accessible to changes, at a reasonable cost. Acting on the source left-hand column in figure 3.
This is of course unfeasible for natural events. It is also hardly feasible with intentional Radio Frequency transmitters, radars etc since these systems are authorized devices, operating at allocated frequencies and designed and installed for performing specific services. Yet, even an authorized transmitter may generate spurious harmonics or other side effects that can be reduced.
So, in general, only non-intentional, fortuitous RF sources can be modified so as to reduce their undesired RF emissions. Most common examples are digital circuits, switchmode power converters, motors etc … Making the victim circuit less vulnerable. This carries the constraint that the essential characteristics of this circuit like detection level, bandwidth or time constant etc must not be, or only slightly, affected by the change, since they are necessary to the functional performance.
Therefore this option has often a very limited range. Acting on the source-to-victim coupling path. This is probably where the designer has the largest choice of solutions: An other interesting side of this concept is that it is perfectly reversible.Introduction to ElectroMagnetic Interference and Compatibility
The mechanisms that could causes the circuit to be a victim are the same that could make it a potential source of interference. In an interference case, your equipment can be the victim, or the source. In some cases, it may even aggravates as frequency squared. A quick example can give a measure of this frequency aspect. Assume a long piece of wire carrying a constant value AC currrent of 1A. At 10 cm distance, this 1A current will cause, by magnetic coupling: Proper EMC tests will verify: This is accomplished by measuring the amplitude of high frequency noise that is emitted by conduction and by radiation, with specific instrumentation.