AN-5059 Datasheet, PDF (1/6 Pages) Fairchild Semiconductor – LVDS Technology Solves Typical EMI Problems Associated

AN-5059
Fairchild Semiconductor
Application Note
May 2005
Revised May 2005
LVDS Technology Solves Typical EMI Problems
Associated with Cell Phone Cameras and Displays
Differential technologies such as Low Voltage Differential
Signaling (LVDS) will be explained and compared to legacy
single ended LVTTL. Through specific application exam-
ples, this article demonstrates the improved spectral con-
tent and advantages offered by LVDS technology.
Today’s Cell Phones
Today’s cell phones, that are becoming continuously
smaller and lighter, have an increasing possibility to
adversely affect surrounding devices. This is because of
the increased shear number and opportunity for close prox-
imity to other ultra-portable electronic devices. Twenty
years ago, a cell phone would not be in close proximity to
an implanted defibrillator. Today it can be a common occur-
rence.
Typical EMI Problems
EMI (Electromagnetic Interference) problems with cell
phones usually fall into one of three categories:
1. Blatant EMI radiation that exceeds regulatory emis-
sions limits during product qualification (FCC,
and ETS/EN testing, etc.).
2. EMI that although meets regulatory requirements, con-
tinues to adversely affect devices in close proximity.
3. EMI that adversely affects the cell phone itself through
harmonics and other spurious signals.
Products on the Edge
Cell phones, although usually designed by a single sup-
plier, can be marketed in many countries. Unfortunately,
regulatory requirements vary from country to country, and
often one country does not recognize the standards or test
results of another country. For example, a cell phone must
undergo EMI testing for the requirements of each respec-
tive country. A device that “squeaks by” regulatory testing
in one country, may barely fail in another country. Com-
monly, the same product design may be marketed under
separate model numbers to reflect the different EMI testing,
and can include minor circuit changes to allow regulatory
compliance for a specific locale.
Harmonics and Why They are Bad
in Cell Phone Applications
Harmonics are exact multiples of a fundamental frequency.
As an example, a square wave clock operating at 100MHz
in a cell phone can have visible harmonics on a spectrum
analyzer at 300MHz, 500MHz, and 700MHz. Additional
peaks are often seen with the spectrum analyzer but may
represent additional spurious signals as a result of a local
parasitic oscillation or signal reflection. Unfortunately, in
this example of a 100MHz clock in a cell phone, a harmonic
may exist at 700MHz and at 900MHz. According to the
FCC Frequency Allocation Table dated April 13, 2004,
900MHz is the frequency utilized by fixed land mobile sta-
tions and 700MHz is allocated to fixed mobile broadcast-
ing. In this case, a cell phone with a noisy clock will likely
interfere with these mobile stations and nothing else.
The realistic application of this cell phone clock may not be
a perfect 100MHz. It may be off slightly, say 97MHz. In this
case, the 9^th harmonic winds up at 873MHz. The FCC fre-
quency allocation table identifies 873MHz as nearly the
middle of the cell phone frequency band. This means that
the clock frequency of this cell phone can dramatically
reduce the sensitivity of the cell phone receiver itself,
potentially rendering it inoperative.
LVDS Technology
LVDS technology is a comparatively new technology that is
rapidly replacing legacy TTL or LVTTL technologies. LVDS
is a standards-based technology that utilizes two conductor
paths rather than one as with TTL or LVTTL. At first glance,
it may seem inefficient to utilize two conductors rather than
one, however this two conductor system has the distinct
advantage of operating at much higher speeds than its pre-
decessor. It should be noted that with either TTL or LVTTL,
a second conductor exists that is actually the power
ground. The architecture of the LVDS technology is such
that the two wires will utilize opposing polarities that will
change at the same time based on a change with the data
input. This means that the two wires (or other medium such
as flex circuit wires, twisted pair of wires, etc.) will have
opposing currents during the polarity change. The oppos-
ing currents in effect cancel each other so that the net cur-
rent change is comparatively quite small. It is this
advantage, combined with the fact that the voltage swing is
typically 350mV, rather than 3.3V or 5.0V with TTL, that
results in significantly reduced overall current change, ulti-
mately resulting in less EMI.
Compare and Contrast Typical
Application Emissions with
LVTTL to LVDS
As a means of directly comparing technologies, a test was
designed to compare only the interface technologies. The
remaining parameters, equipment, and test environment
remain the same. In this case the parameters were a single
bit, 100MHz, repetitive square wave, and the transmission
medium was a 10cm flex circuit. 10cm represents a typical
length found in many cell phones available today. Identical
circuit boards were fabricated with the exception being 1
set utilized LVDS devices and the second set utilized
LVTTL devices. Figure 1 shows the test set-up.
© 2005 Fairchild Semiconductor Corporation AN500908
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