Grounding and Shielding Computer Controlled Audio Devices

Grounding and Shielding Computer Controlled Audio Devices
Considerations in Grounding and Shielding
Computer-Controlled Audio Devices
Stephen R. Macatee
Rane Corporation, Mukilteo. WA 98275-3098. USA
Adding computer control to audio devices raises design issues, such as electromagnetic emissions tests, digital
power and grounding systems, shielding and filtering schemes. This paper describes the emissions test process and
reviews product design methods, such as proper grounding, shielding and filtering, which are shown to improve
product and system performance both in emissions testing and in the field.
Computer controlled audio is new ground for many
designers and users. New territories such as computer
platforms and the good and bad realities of graphical
user interfaces are one hurdle to overcome,
electromagnetic interference (EMI) issues are another.
Many audio equipment designers have little high
frequency (megahertz and above) design experience.
Adding computer control to any audio device involves
using microprocessors to control the device’s
parameters. Inherent in microprocessor systems is the
use of a system clock, the heartbeat of all
microprocessors. U.S. government regulations require
any device with a clock above 9 kHz to pass
electromagnetic compliance (EMC) tests. Other
countries have similar requirements. The purpose of
these tests is to control electromagnetic pollution by
setting standards for unlicensed electronic equipment [1].
In order to pass these government regulations, designers
are faced with new challenges over and above the
already daunting task of keeping computer controlled
audio quiet.
Most countries require safety and electromagnetic
compatibility tests on electronic equipment. In the
United States, Underwriter’s Laboratories (UL) 813
requires professional audio equipment operating above
42.4 volts peak to pass its safety tests. Safety
organizations in other countries require similar tests.
CSA in Canada and the document IEC 65 in the
European Union (EU) countries cover similar safety
issues. This paper discusses another arena of device
testing covering electromagnetic compatibility testing.
Similar to safety agency approval, the Federal
Communications Commission (FCC) in the US, requires
tests on products that contain an oscillator clock
operating above 9 kilohertz. The FCC segregates devices
into three categories: intentional radiators, unintentional
radiators and incidental radiators. Intentional radiators
emit radio frequency (RF) energy by design, through
“radiation or induction.” An unintentional radiator is
defined as a device that “intentionally generate(s) radio
frequency energy for use within the device, or that
send(s) radio frequency signals by conduction to
associated equipment... but which is not intended to emit
RF energy by radiation or induction.” Incidental
radiators include DC motors or mechanical switches that
generate RF during operation, although they are not
intentionally designed to do so.
Part 15 section B of the FCC rules applies to most
computer controlled audio equipment which usually fall
under devices considered “unintentional radiators.” The
approval tests are grouped into two classes or
compliance levels. Class A approval covers devices
designed for use exclusively in business or industrial
environments. Less stringent Class B approval covers
devices intended for use in homes. Note that devices
designed for use in a business or a home are considered
Class B devices. Special product labeling and user
information requirements are also dictated in Part 15.
The FCC requires testing both conducted and radiated
emissions. Conducted emissions through the line cord
and I/O cables are tested separately than the radiated
emissions that travel through the air.
Independent testing facilities are located throughout the
U.S. For those new to FCC testing, shopping for a
reliable testing facility combined with staff with clear
communication abilities is invaluable. Finding a test
facility that encourages participation in the testing
process can save hundreds of troubleshooting hours and
provides hands-on education with specific product
problems. The average cost for testing facilities is
approximately $150 per hour. An “average” device may
take a full day to fully test, though as few as 4 hours can
be expected for devices that require no alterations to pass
the required tests. If the device fails, the tests can take
considerably longer.
Part 68 of the FCC rules requires registration of
equipment connecting to the telephone communications
network. Equipment which contains a modem or a
telephone input or output may fall into this category.
For imported equipment, the FCC holds the importer
responsible for compliance with its requirements. This is
also true in the European Union countries.
Other countries have similar EMC requirements.
Industry Canada covers Canada. The EU has similar
requirements contained in its EMC Directive,
89/336/EEC. [The European Union countries include
Belgium, Denmark, France, Germany, Great Britain,
Greece, Ireland, Italy, Luxembourg, Netherlands,
Portugal and Spain. Others countries considering joining
the EU include Austria, Czech Republic, Finland,
Hungary, Norway, Poland, Slovakia, Sweden and
A not-so-subtle difference between FCC and EU testing
involves the EU requirement to also undergo immunity
tests. These tests classify equipment into various
performance degradation levels with regard to the
devices ability to reject external EMI and still continue
to operate as designed. Many EU standards are still being
refined and are expected to be continually updated as
technologies change. The EU’s EMC Directive is
designed to allow manufacturers to self test their
equipment. Equipment must be marked with the “CE
marking” and a Declaration of Conformity must be
An excellent source for further information on all of
these compliance issues is Compliance Engineering
magazine (See the Bibliography).
Design Methods to Achieve Compliance
There are at least two methods that reduce the radiation
from a clocked device. The first is shielding. Surround
the device and its cables with a conductive shield.
(Where and how to properly connect this shield is
covered later.) The second method is to reduce the
radiation at its source. Removing the clock is the most
obvious way to reduce emissions. Doing so during
emissions testing is more educational than one might
think, Reducing the source of the emission can reduce or
eliminate the need for shielding. More important,
however, is the fact that reducing the radiation source
also improves a device’s noise and immunity
Let’s examine how radiated emissions occur in a printed
circuit board (PCB). The clock is a significant source of
the radiation. Every transition of the clock, from high to
low and vice versa, creates sharp transients in the power
supply rails, in both the power and the ground. The
inductance in these power and ground traces along with
the power and ground transients cause radio frequency
potentials to develop. The traces literally act as antennas.
Since the traces act as antennas and the input and output
(I/O) cables can be connected to them, the I/O cables
also act as antennas. Similar to removing the clock,
removing the I/O cables is also quite educational during
emissions testing.
The example in Figure 1 shows the problem [2]. Here,
two inverters hooked to the same supply rails are shown.
The first inverter drives an internal circuit while the
second drives an output cable. The first inverter might be
the clock buffer, the second might be a computer control
cable. A typical PCB trace inductance is approximately
20nH per inch. At l00 MHz, an inch long trace between
the two inverters would be 12.5 ohms.
Ζ = 2 π f L (ohms)
[Eq 1]
Even if the second inverter is not being toggled, the
voltage introduced by the first inverter causes the output
rails, and therefore the output signal to modulate with the
clocked signal. This common mode energy on the supply
rails is independent of the signal driving the second
inverter. The common mode voltage can be 200
millivolts and can produce radiated signals many times
the acceptable FCC limits.
It is seen from Equation 1 that two methods exist for
reducing this common mode voltage build-up on the
power supply rails. Either reduce the frequency (or
amplitude) of the radiation source (f) or reduce the
inductance (L) of the circuit by keeping the traces short.
Note that reducing the circuit’s inductance also entails
keeping the loop area of both supply rails, as well as the
rest of the circuit, small.
Simply using wider traces does little to reduce a trace’s
inductance. Much more effective is to use a grid pattern
which provides many parallel conductors between
points. A ground or power plane is the ultimate version
of a gridded trace pattern. Figure 2 [3] clearly illustrates
the relationship between loop area and trace inductance.
The PCB in figure 2a shows a common power and
ground layout scheme. This configuration suffers from
large current loops (and therefore large loops areas), high
power and ground inductance, and high noise. Adding
bypass capacitors next to each IC (Figure 2b) reduces the
loops area and inductance. Running the traces beneath
the ICs (Figure 2c) reduces the loop area, inductance and
noise even more. Adding cross-ties (Figure 2d) on
another layer creates a gridded trace pattern providing
small loop area and very low inductance and noise. The
power and ground scheme of Figure 2d can be shown to
have one-sixth the inductance, and therefore one-sixth
the noise, of the scheme in Figure 2a.
Another subtle practice involves making smooth
transitions when bending traces. Sharp comers (90”)
change the inductance of the trace. Limiting bends to 45°
or using curved traces keeps the trace inductance nearly
constant from DC to several gigahertz.
As was seen in the above example, careful power supply
bypassing reduces the loop area and inductance of a
circuit. When a logic gate toggles, current flows between
the supply rails and the load. Placing a decoupling
capacitor close to the IC allows the high frequency
currents to flow in the shorter loop area through the
capacitor, thus reducing the high frequency emission
(See Figure 3).
Selecting the proper capacitor for the bypassing function
is also important. The capacitors used should have low
impedance and inductance at the frequencies of interest.
The construction of electrolytic and most mylar
capacitors is unsuitable for bypassing since their rolled
structure affords them high impedance. Ceramic disk
capacitors have much more desirable decoupling
characteristics. The capacitor leads have more inductance
than the actual capacitor. Keeping the leads short helps
keep the inductance low.
Reducing Clock Emissions
Do not over-design your clock. Clocks are usually
designed to provide fast rise and fall times which
improves system synchronization accuracy.
Unfortunately fast rise and fall times provide added EM1
radiation. Prepackaged oscillators are designed for fast
transition times and high fan-out. Unfortunately, these
convenient packages can come with more EM1 than
necessary. Changing to a different supplier can
drastically change the EM1 characteristics of these
packages. Often a simple inverter with a discrete crystal
based oscillator circuit provides the designer with a
better solution for EM1 control.
Applying filters on specific high frequency outputs can
reduce emissions. Damping resistors (20 to 50 ohms),
ferrite beads and small inductors placed in series with
and close to the source of high frequency lines help
control emissions. Ferrite beads have very small DC
resistance and act as resistors at high frequencies
providing smooth high frequency roll off. Beads come in
many shapes and sizes. Multiple “turn” beads can be
chosen for desired cutoff frequencies. Inductors should
be carefully chosen to provide roll off of high
frequencies, without compromising the desired
frequencies needed for proper circuit operation.
Loading clock signals with bypass capacitors can help
reduce high frequency ringing that may be present in
clock, thus reducing emissions. Care should be taken to
ensure that this practice does not increase the emissions
at frequencies beyond the bandwidth of the oscilloscope
one may use to verify reduced clock overshoot. This
practice works better on high source impedance
components, thus forming a desirable RC network.
Cable Emissions
Due to the high frequency common mode signals present
on the power supply rails (as seen above Figure l), the
I/O cables and the line cord act as RF antennas. The
antenna’s resonant frequency can be changed simply by
moving or twisting the cable. This is quite a new concept
for audio designers. Simply moving a cable changes the
entire frequency response of the system. Just as above
with PCB design, the radiation of the cables can be
controlled through shielding, filtering and bypassing.
Bypassing the I/O conductors to ground shunts the high
frequency energy and reduces emissions. The important
issue is which ground do you bypass to? Bypassing to
the PCB’s digital ground may make the emission worse.
Remember, the digital power and ground are the source
of the emissions in the first place. The best (and only)
ground that reduces emissions is earth ground. In audio
products with a three pin line cord, it is no coincidence
that the unit’s chassis is required to connect to the
“earthed” green wire. The proper ground to bypass I/O
cables to is chassis ground. However, even a few inches
of trace length from the I/O to chassis ground will render
our bypass capacitor useless since the trace inductance
quickly swamps the capacitor’s effectiveness. Usually
the connection to earth involves a long wire whose
inductance may not permit reduced emissions to
acceptable levels.
Another alternative is to create a “local earth.” This is
done by either introducing a large metal surface capable
of absorbing the charge without changing potential
significantly or by using a shield around the cable.
If reducing the source of the emissions does not provide
adequate EM1 reduction, shielding becomes the next
alternative. Shields simply block electric fields and are
only as good as the “local earth’ ground they are
connected to. Just like bypassing I/O, shielding I/O
requires effective and short connections to chassis
ground to keep emissions under control. Again, even a
trace of a few inches may render the RF shielding
inadequate. This requirement for short connections from
I/O cables to the chassis is a new problem for some
audio manufacturers. (See [4] for further information on
effective shield grounding) Typically metal chassis act as
a “local earth” and as the case shield due to their large
charge capacity.
Careful attention to these PCB design issues early in the
product design stage saves a great deal of time and
money on EM1 control later. High frequency traces and
the components driving them need special attention in
the PCB design process. Power supplies for these
components should be local bypassed, gridded and high
frequency outputs should be damped or filtered as
needed. All high frequency traces should be kept as short
as possible and should be clustered together. Keep the
cluster well away from the I/O area.
A review of the electromagnetic interference testing
process was covered. Product design methods to achieve
compliance with these required tests were also covered.
Early understanding of these new requirements for
equipment design and distribution are key elements in
the evolving computer controlled audio field.
l. Dash, Glen, and Straus, Isidor, Inside Part 15: Digital
Device Approval, Compliance Engineering, 1994
Reference Guide.
2. Straus, Isidor, Designing for Compliance, Part 1:
Design of the PC Board, Compliance Engineering, 1994
Reference Guide.
3. Barnes, John R., Electronic System Design:
Interference and Noise Control Techniques, PrenticeHall, Englewood Cliffs, 1987.
4. Macatee, Stephen R., Considerations in Grounding
and Shielding Audio Devices, AES Vol. 43, No. 6,
97th AES Convention, 1994.
1. Compliance Engineering magazine, 629 Massachusetts
Ave., Boxborough, MA 01719, USA.
2. Barnes, John R., Electronic System Design:
Interference and Noise Control Techniques, PrenticeHall, Englewood Cliffs, 1987.
3. Handbook of EU EMC Compatibility, Compliance
Design Inc., Boxborough, MA 01719, USA, 1994.
4. Morrison, Ralph, Grounding and Shielding Techniques
in Instrumentation (John Wiley and Sons, Inc., NY,
5. Ott, Henry W., Noise Reduction Techniques in
Electronic Systems (John Wiley and Sons, Inc., NY,
Common made voltage build up in supply rail
inductances cause radiation in I/O cables.
Figure 2
Power and ground bussing on two-sided boards
Figure 3
Bypass capacitor should be close to the ICS they
serve to reduce the loop area and inductance.
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