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Texas Instruments Comparison of Electromagnetic Interference Potential of Integrated Logic Circuit Application notes
Application Report
SCEA018 - February 2001
Comparison of Electromagnetic Interference Potential of
Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
Johannes Huchzermeier
Standard Linear & Logic
ABSTRACT
Although ideal digital signals are square waves, in practice, these signals have a trapezoidal
shape that, with period duration (frequency) of the recurrent signal, determines their
electromagnetic-interference potential. Effective electromagnetic radiation depends to
signal characteristics and circuit-board design.
This application report compares different technologies for data transmission with respect on
their electromagnetic-interference behavior. Fourier-series development is used to compare
spectral content of output signals and harmonics from AVC, GTLP, BLT, and LVDS
technologies. Amplitude spectra of four representative devices, measured using a
transverse electromagnetic mode measurement cell, are presented.
Contents
Fundamental Structure of Periodic Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Signal Form Determines Electromagnetic Radiation That Influences the Layout . . . . . . . . . . . . . . . 4
Fourier-Series Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Measurement of the Harmonic Contents of a Digital Signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Measurement of the Amplitude Spectrum in the TEM Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Measurement Boards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Measurement Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
AVC16244 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
FB2033A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
GTLPH1655 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
LVDS31 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
1
SCEA018
List of Figures
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
2
Example of an Antenna Loop (Basic Loop) on a Circuit Board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Time Curve for a Trapezoidal Signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Spectrum for a Triangular Signal, Signal and Components in Time Domain . . . . . . . . . . . . . . . . . . . 9
Spectrum for a Triangular Signal (Signal From Figure 3 in the Frequency Domain) . . . . . . . . . . . . 9
Spectrum for a Trapezoidal Signal, Signal and Components in Time Domain,
Same Signal in Frequency Domain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Spectrum for a Trapezoidal Signal (Signal From Figure 5 in Frequency Domain) . . . . . . . . . . . . . 10
Comparison of Various Technologies’ Harmonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Measurement of the Electromagnetic Field Using a TEM Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Layer Structure of Measurement Boards for the TEM Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Input and Output Signals of AVC16244, VCC = 3.3 V, f = 10 MHz,
Input Signal LVTTL tr,f = 10 ns; Output Signal LVTTL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Input and Output Signals of AVC16244, VCC = 3.3 V, f = 10 MHz,
Input Signal LVTTL tr,f = 2 ns; Output Signal LVTTL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Spectrum Measured in the TEM Cell, AVC16244, VCC = 3.3 V, f = 10 MHz . . . . . . . . . . . . . . . . . . 17
Input and Output Signals of AVC16244, VCC = 1.2 V, f = 10 MHz;
Input Signal 1.2 V, tr,f = 10 ns; Output Signal 1.2-V CMOS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Input and Output Signals of AVC16244, VCC = 3.3 V, f = 10 MHz,
Input Signal 1.2 V, tr,f = 2 ns; Output Signal 1.2-V CMOS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Spectrum Measured in the TEM Cell, AVC16244, VCC = 1.2 V, f = 10 MHz . . . . . . . . . . . . . . . . . . 18
Input and Output Signals of FB2033A, f = 10 MHz,
Input Signal LVTTL, tr,f = 2 ns; Output Signal BTL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Input and Output Signals of FB2033A, f = 10 MHz,
Input Signal LVTTL, tr,f = 10 ns; Output Signal BTL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Spectrum Measured in the TEM Cell, FB2033A, f = 10 MHz,
Input Signal BTL; Output Signal LVTTL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Input and Output Signals of FB2033A, f = 10 MHz;
Input Signal BTL, tr,f = 2 ns; Output Signal LVTTL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Input and Output Signals of FB2033A, f = 10 MHz,
Input Signal BTL, tr,f = 10 ns; Output Signal LVTTL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Spectrum Measured in the TEM Cell FB2033A, f = 10 MHz,
Input Signal LVTTL; Output Signal BTL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Input and Output Signals of GTLPH1655, f = 10 MHz,
Input Signal LVTTL, tr,f = 2 ns; Output Signal GTL+ With Slow Edge Rate . . . . . . . . . . . . . . . . . . . 22
Input and Output Signals of GTLPH1655, f = 10 MHz,
Input Signal LVTTL, tr,f = 10 ns, Output Signal GTL+ With Slow Edge Rate . . . . . . . . . . . . . . . . . . 23
Spectrum Measured in the TEM Cell, GTLPH1655 f = 10 MHz,
Input Signal LVTTL; Output Signal GTL+ With Slow Edge Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Input and Output Signal of GTLPH1655, f = 10 MHz,
Input Signal LVTTL, tr,f = 2 ns; Output Signal GTL+ With Fast Edge Rate . . . . . . . . . . . . . . . . . . . 24
Input and Output Signals of GTLPH1655, f = 10 MHz,
Input Signal LVTTL, tr,f = 10 ns; Output Signal GTL+ With Fast Edge Rate . . . . . . . . . . . . . . . . . . 24
Spectrum Measured in the TEM Cell for GTLPH1655, f = 10 MHz,
Input Signal LVTTL; Output Signal GTL+ With Fast Edge Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Input and Output Signals of GTLPH1655, f = 10 MHz,
Input Signal GTL+, tr,f = 10 ns; Output Signal GTL+ LVTTL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Input and Output Signal of GTLPH1655, f = 10 MHz,
Input Signal GTL+, tr,f = 2 ns; Output Signal LVTTL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
SCEA018
30
31
32
33
Spectrum Measured in the TEM Cell for GTLPH1655, f = 10 MHz,
Input Signal GTL+; Output Signal LVTTL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Input and Output Signals of LVDS31, f = 10 MHz,
TTL Input Signal, tr,f = 10 ns; Output Signal LVDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Input and Output Signals of LVDS31, f = 10 MHz,
Input Signal TTL, tr,f = 2 ns; Output Signal LVDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Spectrum Measured in the TEM Cell for LVDS31, f = 10 MHz,
Input Signal TTL; Output Signal LVDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
List of Tables
1
2
Comparison of Signal Levels Among Various Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Calculated Amplitudes of Harmonics for Various Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
3
SCEA018
Fundamental Structure of Periodic Signals
Digital data transmission ideally is based on square-wave signals. However, in practice, signals
have a limited slope because a certain time is required for the transition from one logic state to
another. Therefore, it is realistic to study trapezoidal signal patterns with different signal edge
rates that are due to various load conditions at the driver output.
When referring to the speed of a digital system, for example, that of a personal computer (PC),
normally we mean its clock frequency. Clock frequency is the highest-frequency signal of an
asynchronous system. In a synchronous system, all data or control signals relate to a consistent
system clock. The higher the selected frequency for the system clock, the faster the system is,
but, also, the higher is the clocked signal’s electromagnetic interference potential for the same
signal rise and the same signal form.
A periodic signal can be displayed both in the time range and in the frequency range.
When plotting the current or voltage curve over time, we obtain function f(t) = u(t), i(t),
respectively, in the time range. The signal can be displayed in the time range using an
oscilloscope.
In the frequency range, we plot voltage or current components over frequency. Here, sine-wave
signals of various frequencies and amplitudes are plotted and their total produces the signal.
Amplitudes also are designated as spectrum lines that can arise as multiples of the base
frequency. By measurement, a signal’s frequency spectrum can be detected using a spectrum
analyzer.
It is possible to comment on frequency proportions and, thus, on electromagnetic interference
potential, if these prerequisites are known:
•
Signal form, i.e., in the case of digital signals, the signal’s slope and amplitude
•
Frequency, i.e., the period duration of the recurrent signal
Each periodic signal form can be generated by superimpositions of sine-wave functions of
differing frequencies and amplitudes. Fourier-series development provides the basis for
calculating a signal’s spectrum components.
Signal Form Determines Electromagnetic Radiation That Influences the Layout
Effective electromagnetic radiation is not determined by signal form and frequency alone. The
signal determines only the interference potential. The three components of the interference
signal source, the interference channel, and the interference signal drain are inseparably
involved in electromagnetic radiation.
Interference signal sources within a system are diverse and can, for example, result from line
reflections due to mismatching, crosstalk between two adjacent signal lines, current peaks
arising at the moment of switching due to totem-pole output stages, etc.
4
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
SCEA018
Circuit layout exerts a decisive influence on how effectively the generated faults can be
irradiated or propagated. The benefit arising from data transport between a driver output and the
input of the subsequent stage can be offset, if circuit design is poor, by interference
characteristics with the effective-antenna effect in the cable connection.
In addition to cable connection length, the enclosed area between the data line (between the
transmitter and the receiver) and the signal return path via the GND connection (between the
receiver and the transmitter) can have a decisive influence on the level of propagation or
radiation of electromagnetic waves. This problem is illustrated in Figure 1.
Internal Radiation Loop
Shielding
Bypassing Loop
Connector Loops
V
µP
Buffer
Signal loop
Ambient-Field Loops
Effective Antenna Area Between µP and Buffer
Figure 1. Example of an Antenna Loop (Basic Loop) on a Circuit Board
In this context, particular emphasis is placed on the signal loop that is set up between the output
of the microprocessor (µP) through the buffer module input, through the ground connection of
the buffer module to the system ground, and through the system ground back to the
microprocessor ground. Figure 1 also contains further examples of basic loops that have a
possible antenna effect.
The highest level of effectiveness for radiation and propagation occurs when an interference
source is transmitting on a frequency whose quarter wavelength corresponds to the line length
in a given layout. For example, for a loop length of 10 cm, an antenna’s transmission frequency
is 749.5 MHz.
The objective is not simply to utilize a technology that possesses the lowest possible
interference potential. Developers also should ensure that the antenna effect of circuit-board
layout is minimized by using appropriate line lengths and, for example, by additional screening in
the form of ground lines or ground planes, to ensure the lowest-impedance path possible for
interference or faults.
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
5
SCEA018
Fourier-Series Development
Using the example of the trapezoidal signal in digital systems, the signal is broken down into its
spectral components, using Fourier-series development.
The conclusion of theoretical considerations is represented by the comparison of the spectra of
a triangular signal and of a typical digital signal, both having equal amplitude and fundamental
frequency. Various technologies for data transmission at 5 V, 3.3 V, 2.1 V, and 1.5 V are
compared regarding their spectral contents. For this purpose, the Fourier-series development
was applied to the nominal-output signals of each technology. The typical edge rate and signal
rise time for the corresponding technology has been calculated for 10 MHz.
The prerequisite for Fourier-series development is that it should relate to an unequivocal
continuous-section periodic function whose base period is T0 in the interval of <0, T0>.
The basic formula for Fourier-series development is:
f (t)
+ A20 )
ȍƪ
R
ǒ Ǔ)
A n cos nw 0t
n–1
Where:
Radian frequency, w 0
ǒ Ǔƫ
B nsin nw 0t
(1)
+ 2Tp
0
T0 = signal period duration
Equation 1 shows that the complete time signal is made up of an identical proportion and an
increment of the sine and cosine functions of multiples of the base frequency. The purpose of
Fourier-series development is to determine coefficients An and Bn.
ŕ
ŕ
ŕ
By definition, the general form of the coefficients is:
A0
+
t0
2
T0
)T
0
(2)
f (t)dt
t0
An
+ T2
0
t0
)T
0
ǒ Ǔ
(3)
ǒ Ǔ
(4)
f (t) cos nw 0t dt
t0
Bn
+ T2
0
t0
)T
0
f (t) sin nw 0t dt
t0
The study can be reduced to the primary period where t0 = 0. Figure 2 illustrates the signal path
of a trapezoidal signal that changes in the borderline cases of t1 = 0 to a square-wave signal,
and, where t1 = T/4, the signal changes into a triangular signal.
6
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
SCEA018
A Fourier series is developed in a general form for the function u(t) as follows:
u(t)
T0
2
*t
1
T0
2
)t
1
T0 ± t1
û
t1
T0
2
T0
t
Figure 2. Time Curve for a Trapezoidal Signal
Initially, the signal is defined by equations 5 through 9 in the five function sections of the basic
period:
+ u2 ) 2tu
^
u(t)
^
(5)
t
1
Where:
0
u(t)
ttvt
+u
(6)
^
Where:
t1
u(t)
0
+ u – 2tu
^
^
t –
Where:
T0
– t1
2
+0
1
T0
– t1
2
t t v T2 ) t
(7)
0
1
(8)
Where:
T0
2
+ 2tu
^
u(t)
ƪ ǒ Ǔƫ
t t v T2 * t
1
u(t)
1
1
)t ttvT
1
ƪ ǒ
0
Ǔƫ
– t1
t – T0 – t1
Where:
T0 – t1
ttvT
(9)
0
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
7
SCEA018
Next, the functions of equations 5 through 9 for f(t) are substituted in equations 1 through 3 and
partially integrated. For the factor A0, this gives the value û/2, which corresponds to the dc
content of the signal.
Components An do not appear. The absence of cosine components, An, is typical for so-called
odd functions, which are discrete symmetrical to the point of origin, reduced by, or added with,
any dc component.
Ť ǒ ǓŤ
Components Bn result in:
Bn
+
sin np
2
4u^
pö
1
n2
(10)
sin (nö)
Where:
ö
+ 2p Tt
1
ƪ
0
ƫ
The following total is obtained as the Fourier series for the trapezoidal signal in Figure 2:
4u
+ pö
^
u(t)
ǒ Ǔ ) 31
1
12
sin(ö) sin w 0t
2
ǒ Ǔ ) 51
sin(3ö) sin 3w 0t
2
ǒ Ǔ )@@@
sin(5ö) sin 5w 0t
(11)
In the special case where t1= T0/4 (see Figure 3), a triangular curve results. The Fourier series
then results in:
+ p8uö
^
u(t)
ƪ
ǒ Ǔ
1
12
ǒ Ǔ ) 51
sin w 0t – 12 sin 3w 0t
3
2
ǒ Ǔ @@@)@@@
sin 5w 0t –
ƫ
(12)
From equations 11 and 12, the amplitudes of the spectrum lines of a triangular signal decrease
far more rapidly than those of a trapezoidal signal.
Figures 4 and 6 show the frequency spectra for a trapezoidal signal and for a triangular signal
whose base frequency is fo = 10 MHz, as calculated with equations 11 and 12. Amplitudes are
stated in µVdB.
While the spectrum for the triangular signal illustrated on the left decreases constantly at 40 dB
per decade, the calculated spectrum for the trapezoidal signal shows two ranges:
1. Up to a cutoff frequency, spectrum components descend by 20 dB per decade.
2. Above the cutoff frequency, components decay at 40 dB per decade.
The cutoff frequency is determined by the signal’s edge rate and can be calculated using
equation 13.
f cutoff
+p
1
t r,f
Where:
tr = rising edge
tf = falling edge
8
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
(13)
SCEA018
Complete signal
DC part
Base
Sum of 1 through 30 harmonic
0
10
20
30
40
50
60
70
80
90
3.0
2.0
1.0
0.0
–1.0
–2.0
–3.0
–4.0
–5.0
100
Volts
Volts
Triangular Signal, 50-ns Edge Rate at 10 MHz
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Time – ns
Figure 3. Spectrum for a Triangular Signal, Signal and Components in Time Domain
Volts – dBµ V
Amplitude Spectrum of Harmonics in dBµV
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
fcutoff = 6.37 MHz
Spectral lines, used to display time domain
20-dB line
40-dB line
Spectral components up to 1 GHz
10
100
Frequency – MHz
1000
Figure 4. Spectrum for a Triangular Signal (Signal From Figure 3 in the Frequency Domain)
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
9
Trapezoidal Signal, 1.5-ns Edge Rate at 10 MHz
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Complete signal
DC part
Base
Sum of 1 through 30 harmonic
0
10
20
30
40
50
Time – ns
60
70
80
90
3.0
2.0
1.0
0.0
–1.0
–2.0
–3.0
–4.0
–5.0
100
Volts
Volts
SCEA018
Figure 5. Spectrum for a Trapezoidal Signal, Signal and Components in Time Domain,
Same Signal in Frequency Domain
Volts – dBµ V
Amplitude Spectrum of Harmonics in dBµV
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
fcutoff = 212 MHz
Spectral lines, used to display time domain
2. cutoff frequency
20-dB line
40-dB line
Spectral components up to 1 GHz
10
100
Frequency – MHz
1000
Figure 6. Spectrum for a Trapezoidal Signal (Signal From Figure 5 in Frequency Domain)
10
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
SCEA018
From these considerations, it directly follows that, in the case of a steeper slope, the
higher-frequency-spectrum components are contained with higher amplitudes. This
consideration is confirmed by Fourier-series development. In the case of the triangular signal,
the 20-dB range is not even recognizable because the transition from 20 dB to 40 dB per
decade occurs with tr,f = 50 ns at 6.37 MHz, according to equation 13. Figure 4 shows that the
first harmonic of the triangular signal at 20 MHz is on the 40-dB attenuation line and, therefore,
is clearly attenuated.
In the comparison above, signal amplitude is set as 5 V in both cases. This amplitude
corresponds to typical 5-V CMOS drivers. However, with modern technologies, lower-level
definitions are employed. The reasons are power saving, more reliable signal analysis of voltage
levels, and lower interference potential. Table 1 shows typical parameters for various
technologies for data transmission. In addition to the switching thresholds and voltage swings,
the typical values for signal slopes of output stages and maximum cycle frequency also are
given.
Table 1. Comparison of Signal Levels Among Various Technologies
PARAMETER
Supply voltage
5-V CMOS
LVTTL
BTL
GTL
LVDS
5V
3.3 V
5V
3.3 V od. 5 V
3.3 V
VIL
VIH
1.5 V
0.8 V
1.47 V
0.8 V
3.5 V
2.0 V
1.62 V
VREF + 50 mV
VREF – 50 mV
VOL
0.1 V
0.1 V
0.75 V
0.55 V
247 mV to 454 mV
Typ 340 mV†
VOH
Rise/decay time
4.9 V
3.2 V
2.1 V
1.2 V/1.5 V
†
~2.5 ns
~2 ns
≥2 ns
~2 ns
500 ps
Maximum frequency
90 MHz
150 MHz
150 MHz
160 MHz
† For differential technologies, there are no static output levels referred to ground.
2.0 V
400 MHz
The following statements can be made:
•
The greater the signal voltage swing, the higher the amplitudes of the spectrum components.
•
The smaller the period duration of the signal, the higher the frequency range in which
spectrum lines arise.
•
The shorter the rise/decay time, the slower the spectrum components of the signal decay.
Thus, the values shown in Table 2 for the first harmonic are calculated, taking account of the
parameters of voltage rise and fall times inherent in this technology for a periodic signal with
10-MHz frequency.
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
11
SCEA018
Table 2. Calculated Amplitudes of Harmonics for Various Technologies
AMPLITUDE OF
1ST HARMONIC
(dBµV)
CUTOFF
FREQUENCY
(MHz)
SPECTRUM COMPONENT
AT 490 MHz
(25th HARMONIC)
(dBµV)
120.08
63.66
80.8
LVTTL
116.31
79.58
58.8
GTLP
107.33
79.58
49.3
GTL
104.57
79.58
46.5
BTL
106.48
79.58
67.4
LVDS
92.55
318.3
48.5
TECHNOLOGY
5-V CMOS
The technology with the smallest voltage swing also is the technology that has the lowest
amplitude of the first harmonic.
It also is worth noting the curve of the spectrum lines. The 40-dB-per-decade cutoff frequency
for a signal with longer rise and fall times is lower than that of a signal with a steeper slope.
The cutoff frequency is calculated according to equation 13 for LVDS devices at 318 MHz, while,
for the other technologies, the cutoff frequency is 60 MHz to 80 MHz. This is due to the very
short rise/decay times for the LVDS drivers, stated as 500 ps in the data sheets.
Figure 7 illustrates the complete curve of the spectrum components of a 10-MHz signal for
5-V CMOS, 3.3-V LVTTL, BTL, GTL, and LVDS signals for 10 MHz to 1 GHz. In this context, we
have taken into account the varying rise/fall times.
Comparison of Spectral Components of Various Technologies
140 dB
120 dB
Amplitude –dBµ V
100 dB
80 dB
60 dB
40 dB
5V
3.3 V
BTL
GTL
LVDS
20 dB
10
100
Frequency – MHz
1000
Figure 7. Comparison of Various Technologies’ Harmonics
12
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
SCEA018
Figure 7 shows ranges where the harmonics of a signal with a higher switching level have a
lower value than a signal with a lower switching level. Thus, the spectrum components of the
LVDS signal, despite the lower voltage rise, reside in the range of 400 MHz to 600 MHz higher in
this theoretical study than GTL and BTL signals. Even the 5-V CMOS signal includes ranges
with lower values for the harmonic than the LVDS technology.
Measurement of the Harmonic Contents of a Digital Signal
It is possible to measure electromagnetic interference potential using various methods. Two
common methods are the line-related method and measurement methods using a transverse
electromagnetic mode (TEM) measurement cell. While in the case of the line-related
measurement methods the interference potential is measured directly at the output of the device
under test (DUT), with the TEM-cell method, the electromagnetic field within the measurement
cell, which is generated by the test device, is measured.
Texas Instruments has released an application report concerning the line-related measurement
method. Results reported indicate good correlation with the theoretical considerations set out in
the first section of this application report.
Measurement of the Amplitude Spectrum in the TEM Cell
Using a TEM measurement cell, the entire electromagnetic field generated by a module can be
measured. Figure 8 shows the test setup for the measurement.
A DUT in the measurement cell is actuated using the power-supply voltage specified for it and
the signal levels applicable to it. One output is operated at a time. Switching frequency is
10 MHz at a duty cycle of 50%.
Amplifier
+29 dB
TEM Cell
DUT
Termination Resistor
Spectrum Analyzer
Figure 8. Measurement of the Electromagnetic Field Using a TEM Cell
The septum of the TEM cell detects the electromagnetic field generated by the DUT.
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
13
SCEA018
An amplifier (Advantest R 14601) provides preamplification, by 29 dB, in the range from 9 kHz to
1 GHz. The cascaded spectrum analyzer (Rhode & Schwartz FSEA 20/30) then examines the
amplified signal for its spectral components. The spectrum analyzer provides the result in dBm.
The dBm value can be converted into dBµV, using equation 14. The formula accounts for the
50-Ω environment, preamplifier, and distance from the DUT to the septum.
xǒdBmVńmeterǓ + 20 log
ȡȧ Ǹ
Ȣ
50W
10 –3 W
10 –6 V
10
x(dBm) – 29 (dBm)
10
0.043 m
ȣȧ
Ȥ
(14)
During measurement with TEM cells, all frequencies within the measurement range that are
generated or irradiated by the integrated switching circuit in the form of electromagnetic waves
are detected.
In the process of measurement, no distinction was drawn between electromagnetic radiation
generated from the power supply currents and electromagnetic radiation generated by the
output signal.
The actuation signal at the DUT input also controls a component of the spectrum. Although it is
possible to examine exclusively the output signal using competitive measurements, in practice,
there is no means to fade out the input interference potential.
Measurement Boards
Four-layer printed circuit boards (PCBs) were produced for the measurement. The layer
structure is illustrated in Figure 9. On the side of the PCB facing the TEM cell, there is only the
module to be examined (DUT); the other side is filled in as the grounding plane. All necessary
actuation signals and feed lines to the measurement panel are on the outside of the
circuit-board-actuation side of the TEM measurement cell.
ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ
Control/ Signal Layer
35 µ m
130 µ m
Ground and Signal Layer
~1.5 mm
Ground Layer
DUT
35 µm
Figure 9. Layer Structure of Measurement Boards for the TEM Cell
14
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
SCEA018
In the design of the measurement board, all through contact points from the control layer to the
DUT side were routed in the contact surfaces of the DUT pins. Investigations have shown that,
by this means, it is possible to minimize inhomogeneities in the track guidance and, thus,
minimize additional sources for radiation.
Measurement Results
The spectra in Figures 10–33 consist of the output signal generated by the DUT and the
actuation signal, which is at the input of the probe. The devices’ input signal was 10 MHz for all
measurements.
In order to measure the influence of the actuated signal, two different rise and fall times were
selected for measurement: tr,f = 2 ns and 10 ns.
The following modules were examined for comparison:
•
SN74AVC16244, as an example of CMOS technology, operated at 3.3 V and 1.2 V
•
SN74FB2033A, as an example of backplane transceiver logic
•
SN74GTLPH1655, as an example of Gunning transceiver logic
•
SN65LVDS31, as an example of low-voltage differential signaling
Measurement results illustrate both the oscilloscope signal and the frequency spectrum
recorded by the spectrum analyzer.
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
15
SCEA018
AVC16244
AVC16244 Input/Output Signal in Time Domain
4
3.5
3
Voltage – V
2.5
2
1.5
1
0.5
0
Output
3.3-V input (tr,f = 10 ns)
–0.5
0
20
40
60
Time – ns
80
100
120
Figure 10. Input and Output Signals of AVC16244, VCC = 3.3 V, f = 10 MHz,
Input Signal LVTTL tr,f = 10 ns; Output Signal LVTTL
AVC16244 Input/Output Signal in Time Domain
4
3.5
3
Voltage – V
2.5
2
1.5
1
0.5
0
–0.5
Output
TTL input (tr,f = 2 ns)
–1
0
20
40
60
Time – ns
80
100
120
Figure 11. Input and Output Signals of AVC16244, VCC = 3.3 V, f = 10 MHz,
Input Signal LVTTL tr,f = 2 ns; Output Signal LVTTL
16
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
SCEA018
60
40
20
dBµ V/m
0
60
40
20
0
Start 9 MHz
Stop 1.5 GHz
Figure 12. Spectrum Measured in the TEM Cell, AVC16244, VCC = 3.3 V, f = 10 MHz
AVC16244 Input/Output Signal in Time Domain
1.4
1.2
1
Voltage – V
0.8
0.6
0.4
0.2
0
1.2-V input (tr,f = 10 ns)
Output
–0.2
0
20
40
60
Time – ns
80
100
120
Figure 13. Input and Output Signals of AVC16244, VCC = 1.2 V, f = 10 MHz;
Input Signal 1.2 V, tr,f = 10 ns; Output Signal 1.2-V CMOS
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
17
SCEA018
AVC16244 Input/Output Signal in Time Domain
1.6
1.4
1.2
Voltage – V
1
0.8
0.6
0.4
0.2
0
Output
1.2-V input (tr,f = 2 ns)
–0.2
0
20
40
60
80
100
120
Time – ns
Figure 14. Input and Output Signals of AVC16244, VCC = 3.3 V, f = 10 MHz,
Input Signal 1.2 V, tr,f = 2 ns; Output Signal 1.2-V CMOS
60
40
20
dBµ V/m
0
60
40
20
0
Start 9 MHz
Stop 1.5 GHz
Figure 15. Spectrum Measured in the TEM Cell, AVC16244, VCC = 1.2 V, f = 10 MHz
18
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
SCEA018
FB2033A
FB2033A Input/Output Signal in Time Domain
4
3.5
3
Voltage – V
2.5
2
1.5
1
0.5
0
TTL input (tr,f = 2 ns)
–0.5
0
20
40
60
Time – ns
80
BTL output
100
120
Figure 16. Input and Output Signals of FB2033A, f = 10 MHz,
Input Signal LVTTL, tr,f = 2 ns; Output Signal BTL
FB2033A Input/Output Signal in Time Domain
4
3.5
3
Voltage – V
2.5
2
1.5
1
0.5
0
TTL input (tr,f = 10 ns)
BTL output
–0.5
0
20
40
60
Time – ns
80
100
120
Figure 17. Input and Output Signals of FB2033A, f = 10 MHz,
Input Signal LVTTL, tr,f = 10 ns; Output Signal BTL
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
19
SCEA018
60
40
20
dBµ V/m
0
60
40
20
0
Start 9 MHz
Stop 1.5 GHz
Figure 18. Spectrum Measured in the TEM Cell, FB2033A, f = 10 MHz,
Input Signal BTL; Output Signal LVTTL
FB2033A Input/Output Signal in Time Domain
4.5
BTL input (tr,f) = 2 ns
4
TTL output
3.5
Voltage – V
3
2.5
2
1.5
1
0.5
0
–0.5
0
20
40
60
80
100
120
Time – ns
Figure 19. Input and Output Signals of FB2033A, f = 10 MHz;
Input Signal BTL, tr,f = 2 ns; Output Signal LVTTL
20
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
SCEA018
FB2033A Input/Output Signal in Time Domain
4.5
BTL input (tr,f) = 10 ns
4
TTL output
3.5
Voltage – V
3
2.5
2
1.5
1
0.5
0
–0.5
0
20
40
60
Time – ns
80
100
120
Figure 20. Input and Output Signals of FB2033A, f = 10 MHz,
Input Signal BTL, tr,f = 10 ns; Output Signal LVTTL
60
40
20
dBµ V/m
0
60
40
20
0
Start 9 MHz
Stop 1.5 GHz
Figure 21. Spectrum Measured in the TEM Cell FB2033A, f = 10 MHz,
Input Signal LVTTL; Output Signal BTL
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
21
SCEA018
GTLPH1655
A particular feature of the GTLPH1655 is the option to vary the GTL driver’s output rise and fall
times between slow and fast. This enables optimum dynamic adaptation to load and speed in
the application.
GTLPH1655 Input/Output Signal in Time Domain
4
GTL+ output (Verc = VCC)
TTL input (tr,f) = 2 ns
3.5
3
Voltage – V
2.5
2
1.5
1
0.5
0
–0.5
0
20
40
60
Time – ns
80
100
120
Figure 22. Input and Output Signals of GTLPH1655, f = 10 MHz,
Input Signal LVTTL, tr,f = 2 ns; Output Signal GTL+ With Slow Edge Rate
22
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
SCEA018
GTLPH1655 Input/Output Signal in Time Domain
4
TTL input (tr,f) = 10 ns
3.5
GTL+ output (Verc = VCC)
3
Voltage – V
2.5
2
1.5
1
0.5
0
–0.5
0
20
40
60
Time – ns
80
100
120
Figure 23. Input and Output Signals of GTLPH1655, f = 10 MHz,
Input Signal LVTTL, tr,f = 10 ns, Output Signal GTL+ With Slow Edge Rate
60
40
20
dBµ V/m
0
60
40
20
0
Start 9 MHz
Stop 1.5 GHz
Figure 24. Spectrum Measured in the TEM Cell, GTLPH1655 f = 10 MHz,
Input Signal LVTTL; Output Signal GTL+ With Slow Edge Rate
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
23
SCEA018
GTLPH1655 Input/Output Signal in Time Domain
4
GTL+ output (Verc = fast)
TTL input (tr,f) = 2 ns
3.5
3
Voltage – V
2.5
2
1.5
1
0.5
0
–0.5
0
20
40
60
Time – ns
80
100
120
Figure 25. Input and Output Signal of GTLPH1655, f = 10 MHz,
Input Signal LVTTL, tr,f = 2 ns; Output Signal GTL+ With Fast Edge Rate
GTLPH1655 Input/Output Signal in Time Domain
4
TTL input (tr,f = 10 ns)
3.5
GTL+ output (Verc = GND)
3
Voltage – V
2.5
2
1.5
1
0.5
0
–0.5
0
20
40
60
80
100
120
Time – ns
Figure 26. Input and Output Signals of GTLPH1655, f = 10 MHz,
Input Signal LVTTL, tr,f = 10 ns; Output Signal GTL+ With Fast Edge Rate
24
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
SCEA018
60
40
20
dBµ V/m
0
60
40
20
0
Start 9 MHz
Stop 1.5 GHz
Figure 27. Spectrum Measured in the TEM Cell for GTLPH1655, f = 10 MHz,
Input Signal LVTTL; Output Signal GTL+ With Fast Edge Rate
GTLPH1655 Input/Output Signal in Time Domain
3
2.5
Voltage – V
2
1.5
1
0.5
0
–0.5
GTL input (tr,f = 10 ns)
TTL output
–1
0
20
40
60
Time – ns
80
100
120
Figure 28. Input and Output Signals of GTLPH1655, f = 10 MHz,
Input Signal GTL+, tr,f = 10 ns; Output Signal GTL+ LVTTL
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
25
SCEA018
GTLPH1655 Input/Output Signal in Time Domain
3.5
GTL input (tr,f = 2 ns)
3
TTL output
2.5
Voltage – V
2
1.5
1
0.5
0
–0.5
–1
0
20
40
60
Time – ns
80
100
120
Figure 29. Input and Output Signal of GTLPH1655, f = 10 MHz,
Input Signal GTL+, tr,f = 2 ns; Output Signal LVTTL
60
40
20
dBµ V/m
0
60
40
20
0
Start 9 MHz
Stop 1.5 GHz
Figure 30. Spectrum Measured in the TEM Cell for GTLPH1655, f = 10 MHz,
Input Signal GTL+; Output Signal LVTTL
26
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
SCEA018
LVDS31
The LVDS31 module is the only representative of differential technology in this comparison.
Consequently, measurements with the oscilloscope at the module output were performed using
a differential probe.
LVDS31 Input/Output Signal in Time Domain
4.0
3.3-V input (tr,f = 10 ns)
3.5
Output
3.0
Voltage – V
2.5
2.0
1.5
1.0
0.5
0.0
–0.5
0
20
40
60
80
100
120
Time – ns
Figure 31. Input and Output Signals of LVDS31, f = 10 MHz,
TTL Input Signal, tr,f = 10 ns; Output Signal LVDS
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
27
SCEA018
LVDS31 Input/Output Signal in Time Domain
4
3.5
3
Voltage – V
2.5
2
1.5
1
0.5
0
–0.5
Output
TTL input (tr,f = 2 ns)
–1
0
20
40
60
Time – ns
80
100
120
Figure 32. Input and Output Signals of LVDS31, f = 10 MHz,
Input Signal TTL, tr,f = 2 ns; Output Signal LVDS
60
40
20
dBµ V/m
0
60
40
20
0
Start 9 MHz
Stop 1.5 GHz
Figure 33. Spectrum Measured in the TEM Cell for LVDS31, f = 10 MHz,
Input Signal TTL; Output Signal LVDS
28
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
SCEA018
Summary
The trend toward development of new systems for data transmission is continuing, unchanged,
toward lower supply voltages. The advantages of lower supply voltages not only are lower power
consumption and the option to create smaller structures, but also, as illustrated most clearly in
the context of the example of the AVC module, lower electromagnetic radiation.
All other technologies investigated in this context (BTL, GTL, and LVDS) operate as translators
between the respectively specified logic levels at one port side and LVTTL levels at the other
port connection. With regard to electromagnetic radiation, however, these modules produce poor
characteristics in comparative measurements using the TEM cell. However, this result can be
explained by the principal measurement setup. By contrast with the line-related measurement
method, in the TEM cell the signal at the input to the DUT also is what is detected. On the basis
of actuation by the LVTTL signal, a much higher spectrum is obtained than in the case of
actuation by a 1.5-V CMOS signal.
To account for all spectrum components, the DUT should have rotated through 360 degrees
during measurement, and each frequency’s maximum spectrum components determined by that
means, whereas, this is not possible using the Fischer TEM cell that was employed. However,
with regard to the illustrated measurement results, it is important to note that measurements
were performed only in the most unfavorable position of the measurement cell (of four possible
positions), thus producing the highest measured harmonic content.
Use of the technology with the lowest electromagnetic potential does not, by itself, ensure
interference-free operation.
However, awareness of the possible antenna effect of the layout and using suitable remedies,
such as additional lines, shorter lines, and correct line termination to minimize electromagnetic
propagation and radiation, provide very good prospects for a successful design that is free of
electromagnetic interference.
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
29
SCEA018
Glossary
AVC
Advanced very-low-voltage CMOS
BTL
Backplane transceiver logic
CMOS
Complementary symmetry metal-oxide semiconductor
DUT
Device under test
dB
A common unit of measurement or reference. The basic unit of measurement is the
logarithmic ratio between two products.
dBm
10 log (power/mW)
dBµV
20 log (volt/1 µV)
EMC
Electromagnetic compatibility
EMI
Electromagnetic interference
FB
Future bus, device identifier for backplane transceiver logic devices
GND
Ground
GTLP
Gunning transceiver logic plus
I/O
Input/output
LVTTL
Low-voltage transistor-transistor logic supplied with 3.3-V, compatible TTL
LVDS
Low-voltage differential signaling
PCB
Printed circuit board
Slew rate The slew rate is derived using the following equation:
slew rate = ∆V/∆t = (0.8 VOH – VOL)/tr,f
30
TEM cell
Transverse electromagnetic mode measurement cell
TTL
Transistor-transistor logic
tpd
Propagation delay time
tf
Time to transit from a logical high to logical low, measured between the 90% and
10% values of the steady logical-high level
tr
Time to transit from a logical low to logical high, measured between the 10% and
90% values of the steady logical high level
VCC
Supply voltage
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
SCEA018
Bibliography
Donald R. J. White, EMI Control in the Design of Printed Circuit Boards and Backplanes,
Interference Control Technologies Inc.
Mark I. Montrose, Printed Circuit Board Design Techniques for EMC Compliance, IEEE Press
Series on Electronics Technology.
Mark I. Montrose, EMC and the Printed Circuit Board, IEEE Press Series on Electronics
Technology.
Howard Johnson, Martin Graham, High-Speed Digital Design – a Handbook of Black Magic,
Prentice-Hall.
Texas Instruments, G. Becke, E. Haseloff, Das TTL- Kochbuch, (The TTL Cookbook), literature
number SDYZG17.
Texas Instruments, Automotive PCB Design for Reduced EMI, May 1998, literature number
SDYA017.
Texas Instruments, AVC Logic Family Technology and Applications, 1998, literature number
SCEA006A.
Texas Instruments, ABT Logic Advanced BICMOS Technology Data Book, 1997, literature
number SCBD002C.
Texas Instruments, AVC Advanced Very Low Voltage CMOS Logic Data Book, March 2000,
literature number SCAD008B.
Texas Instruments, Logic Selection Guide and Data Book CD-ROM, April 1998, literature
number SCBC001B.
Texas Instruments, CBT (5-V) and CBTLV (3.3-V) Bus Switches Data Book, 1998, literature
number SCDD001B.
Texas Instruments, AHC/AHCT Logic Advanced High-Speed CMOS Data Book, 2000, literature
number SCLD003B.
Texas Instruments, Design Considerations for Logic Products, 1997, literature number
SDYA002.
Texas Instruments, Digital Design Seminar – Reference Manual, 1998, literature number
SDYDE01B.
Texas Instruments, What a Designer Should Know, March 1997, literature number SDYA009B.
Texas Instruments, Electromagnetic Emission from Logic Circuits, March 1998, literature number
SDZAE17.
Texas Instruments, The Bergeron Method, October 1996, literature number SDYA014.
Texas Instruments, Bus-Interface Devices With Output-Damping Resistors or Reduced-Drive
Outputs, August 1997, literature number SCBA012A.
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
31
SCEA018
Texas Instruments, Live Insertion, November 1995, literature number SDYA012.
Texas Instruments, Thin Very Small Outline Package (TVSOP), March 1997, literature number
SCBA009C.
Texas Instruments, Low-Voltage Logic Families, April 1997, literature number SCVAE01A.
Texas Instruments, Input and Output Characteristics of Digital Integrated Circuits at VCC = 5-V
Supply Voltage, literature number SCYA002.
32
Comparison of Electromagnetic Interference Potential of Integrated Logic Circuits AVC, GTLP, BTL, and LVDS
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