# High Frequency PCB Layout

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"High Frequency PCB Layout"

```					                High Frequency PCB Layout

24 March, 2008

Kyle Swartz
Chris Koliba
Namruth Nalla
Matt Cloutier
Sean Hatch

Michigan State University Team 05—ESD Gun Design. 3/24/08.        1
Objective
When dealing with high frequency circuits, it becomes important to
analyze the circuit from a electromagnetic standpoint—basic circuit
begins to break down.

Electromagnetic effects and interactions are especially important
when a high frequency circuit is put on a PCB. Component proximity
and orientation, and trace characteristics will effect the characteristics
of the circuit.

This presentation will give an overview of electromagnetics topics
relevant to laying out PCBs, and apply these topics to a few specific
examples.

Michigan State University Team 05—ESD Gun Design. 3/24/08.                2
Presentation Overview
Electromagnetics Review:
- Lumped Vs. Distributed Circuits
- Transmission Lines
PCB Layout Issues:
-Component Orientation
-Circuit Geometry
-Trace Length
-Trace Geometry
Selected Examples:
-Transmission Lines
-Traces Acting as Antennas
-MOSFET System Arrangement
-Clock Signal Traces
-Trace Corners

Michigan State University Team 05—ESD Gun Design. 2/11/08.   2
Lumped Vs. Distributed Circuits
When is my circuit “high frequency”?:
-Depends on tr, the rise time of the signal
-Also depends on td, the travel time between two points
-need to know l, length, and v, propagation velocity of trace
-Electromagnetic effects must be considered when tr/td < 2.5
-Example:
CMOS technology can be used to create signals with rise
times of 0.5 ns.

Common PCB traces have a propagation velocity of about
0.47*c

 Circuit becomes high frequency when distance falls
below 2.82 cm

 The traces on the PCB must be taken as transmission
lines, not nodes.

Michigan State University Team 05—ESD Gun Design. 3/24/08.                               4
Transmission Lines
Important Aspects:
-Characteristic Impedance, Z0:
- depends on width of trace
- depends on material (permittivity)
- depends on thickness of board
- depends on thickness of trace

-Reflections:
- Voltage waves can bounce off of load!
- ΓL= R L – Z0 / R L + Z0
- Wasted power
- Damaged components
- Attenuated signals

Michigan State University Team 05—ESD Gun Design. 2/11/08.      2
Transmission Lines
Other Aspects:
-Propagation time
-Realize that node voltages can no longer be assumed
-Distort Signals

Solutions:
- Impedance matching can fix these problems
- Computer programs can do this for you
- Smith Charts

Michigan State University Team 05—ESD Gun Design. 2/11/08.          2
- High frequency signals in traces cause EM radiation
- Traces act as antennas (antennas receive and transmit)
- Time varying fields can induce currents and voltages on nearby
conductors and system components, especially inductors
- See Maxwell’s equations for more specific details on how these
problems are caused

Crosstalk/EMI:
- The induction of voltages and currents on nearby traces from
radiated EM fields is referred to as crosstalk or EMI (electromagnetic
interference
- These currents and voltages can falsely trigger switches and logic in
a circuit
- PCB traces in loops (or similar shapes) make especially good
antennas

Michigan State University Team 05—ESD Gun Design. 3/24/08.                         7
Some sources of EMI:
- Switching; MOSFETS, for example
- Oscillators
- Motors
- Reflection interfaces
- Outside sources too, but shielding can fix this

How to Prevent EMI on a PCB:
- Shielding
- Short traces
- No parallel inductors
- Package types act as shields (PGA and BGA)
- EMI Filtering:
- Near output to prevent noise
- Near EMI source to prevent Crosstalk

Michigan State University Team 05—ESD Gun Design. 3/24/08.        8
Ex: MOSFET System Arrangement
Schematic:

Description:
- Gate of MOSFET driven with high frequency signal
- Driver should be able to handle high frequency signal

Michigan State University Team 05—ESD Gun Design. 3/24/08.             9
Ex: MOSFET System Arrangement (cont’d)
High Frequency Considerations:
-Distance between driver output and gate  Time Delay
-Distance between driver output and gate  Inductance
-When combined with gate capacitanceRinging!

Gate Voltage: Fairchild BSS123, L(trace) = 200n, Z0 = 10 ohms, RL = 1k

-Voltage spikes can damage transistor.
-Transistor can bounce
-Current travels into driver

Michigan State University Team 05—ESD Gun Design. 3/24/08.                            10
Ex: MOSFET System Arrangement (cont’d)
High Frequency Considerations (cont’d):
-Characteristic Impedance  Reflections
-Different voltage at gate (could be more, or less)
-Could damage driver

Solutions:
-Ringing
-Shorten distance between driver and gate to reduce inductance
-Narrow trace to increase characteristic impedance (dampen).
-Comes at the cost of increased power dissipation.

Gate Voltage: Fairchild BSS123, L = 20n, Z0 = 50 ohms, RL = 1k.
Michigan State University Team 05—ESD Gun Design. 3/24/08.                                11
Ex: MOSFET System Arrangement (cont’d)
Solutions (cont’d):
-Impedance mismatches
-Adjust trace width to match selected impedance
-Compromise with ringing.

Other Notes:
-Same problems exist between logic signal generator and driver IC.
-The solutions can be applied to many similar problems, but as
proximity increases:
-Capacitance increases
-Crosstalk increases.

Michigan State University Team 05—ESD Gun Design. 3/24/08.                        12
Ex. Clock Signal Traces
Description:
-Two clock signal traces travelling parallel to each other, the two clock
signals are 180 degrees out of phase:

High Frequency Considerations:
- The radiated fields from the two lines will nearly cancel each other.
-decreases EMI throughout board.

Michigan State University Team 05—ESD Gun Design. 3/24/08.                             13
Ex. Clock Signal Traces (cont’d)
Images:

Left: Two current sources of equal magnitude placed near each other, 180 degrees
out of phase. Right: Two current sources placed relatively further away from each
other, 90 degrees out of phase.

Michigan State University Team 05—ESD Gun Design. 3/24/08.                                   14
Ex. Trace Corners
Description:

High Frequency Considerations:
- Remember that the impedance of the trace depends on its width
- The width of a trace at its corner is greater than it is elsewhere
- This causes reflections at the corner of the trace
- EMI is also prevalent near these reflection centers

Solution:
- Round the corners (constant width, less reflection)

Michigan State University Team 05—ESD Gun Design. 3/24/08.                          15
Summary
Some High Frequency PCB Effects, & Solutions:
-Increased trace length greater inductance waveform deformation
-Solution: shorter traces
-Increased trace density capacitance waveform deformation
-Solution: reduced board density
-Increased board density crosstalk false triggering, noise
-Solution: reduced board density, orthogonal positioning, phase
cancellation (clock signals), filtering
-Characteristic Impedance reflections circuit damage or
attenuation
-Solution: impedance matching

Remember: At high frequencies, board layout is governed more by
electromagnetic interactions, and less by electronic circuit theory. MOSFET
arrangement, trace corners, and clock signal positioning are just a few
applications of this idea.
Michigan State University Team 05—ESD Gun Design. 3/24/08.                                 16
References
Texas Instruments. “PCB Layout Guidelines for Power Controllers.”
http://focus.ti.com/lit/ml/slua366/slua366.pdf

Baker, Bonnie. Microchip Technology Inc. “Circuit Layout Techniques And Tips.”
http://www.analogzone.com/acqt0729.pdf

Olney, Barry. “EMC Design for High Speed PCBs.”
http://www.icd.com.au/articles/emc.html

Pawson, James. “Leakage Inductance.”
http://thedatastream.4hv.org/gdt_leakage.htm

Inan; Inan. Engineering Electromagnetics. 1998.