Radiated Emissions Example; LVDS - Low Voltage Differential Signaling
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EMI Analyst™ EMI ANALYST ™ Software Suite Intuitive Accurate Effective adiated emissions example S – Low Voltage Differential Signaling
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Transcript of Radiated Emissions Example; LVDS - Low Voltage Differential Signaling
EMI Analyst™
EMI ANALYST™ Software SuiteI n t u i ti v e A c c u r a t e E ff e c ti v e
Radiated emissions exampleLVDS – Low Voltage Differential Signaling
EMI Analyst™
Radiated Emissions Test SetupLVDS DRIVER LVDS RECEIVER
EMI Analyst™
LVDS Circuit and Equivalent Model
LVDS Circuit Model
Equivalent Circuit from IBIS Model and Datasheet
DO+
DO-
RI+
RI-
100
Ω
50m1.05193n
0.16890p
58
50m1.05308n
0.16861p
58
DO+
DO-
RI+
RI-
100
Ω
50m 1.05193n
0.16890p 240k
50m 1.64135n
0.37335p 240k
EMI Analyst™
0V
VDO-
VDO+
VDM = VDO+ - VDO-
tRISE tFALL
0V80%
20%
LVDS Circuit Characteristics• Voltage: 360 mV, differential• Current: 6 mA• Rise/fall time: 700/800 psec, 20%-80%• Skew: 400 psec typ.• Char Z: 100
Falling edge and rising edge waveforms
EMI Analyst™
Radiated Emissions ComparisonsFive conditions evaluated • Three cable configurations
• Unshielded Twisted Pair• Shielded Twisted Pair, Braid Shield• Shielded Twisted Pair, Conductive
Fabric Tape Shield
• Two common mode noise sources• CM voltage induced by other
circuits• CM voltage induced by LDVS signal
skew
# Cable Shield CM Noise Skew
1 None (TPUJ) N N
2 None (TPUJ) Y N
3 Braid (TPSJ) Y N
4 Braid (TPSJ) Y Y
5 Conductive Fabric Tape (TPTJ)
Y Y
EMI Analyst™
• Conditions• 10 MHz data rate• No skew• No CM voltage• Ideal ground conditions
LVDS Condition 1Unshielded Cable, No Skew, No CM noise
0V
VDO-
VDO+
VDM = VDO+ - VDO-
VCM = VDO+ + VDO-
tRISE tFALL
0V80%
20%
DO+
DO-
RI+
RI-
100
Ω
EMI Analyst™
RE Results for Condition 1Unshielded Cable, No Skew, No CM noise
EMI Analyst™
• Conditions• 10 MHz data rate• No skew• 10 mV, 1 MHz CM flat spectrum
LVDS Condition 2Unshielded Cable, No Skew, 10 mV CM Noise
DO+
DO-
RI+
RI-
100
Ω
VCM
(noise)
0V
VDO-
VDO+
VDM = VDO+ - VDO-
VCM = 10 mV
tRISE tFALL
0V80%
20%
EMI Analyst™
RE Results for Condition 2Unshielded Cable, No Skew, 10 mV CM Noise
EMI Analyst™
• Conditions• 10 MHz data rate• No skew• 10 mV, 1 MHz system CM noise
LVDS Condition 3Braid Shielded Cable, No Skew, 10 mV CM Noise
DO+
DO-
RI+
RI-
100
Ω
VCM
(noise)
0V
VDO-
VDO+
VDM = VDO+ - VDO-
VCM = 10 mV
tRISE tFALL
0V80%
20%
EMI Analyst™
RE Results for Condition 3Braid Shielded Cable, No Skew, 10 mV CM Noise
EMI Analyst™
LVDS Condition 4Braid Shielded Cable, 200 ps Skew, 10 mV CM Noise
• Conditions• 10 MHz data rate• 0.2 nsec signal skew induces
70 mV CM pulses• 10 mV, 1 MHz system CM noise
DO+
DO-
RI+
RI-
100
Ω
VCM
(noise)
0V
VDO-
VDO+
VDM = VDO+ - VDO-
Signal VCM = VDO+ + VDO- = 70 mV
tRISE tFALL
0V80%
20%
System VCM = 10 mV
EMI Analyst™
RE Results for Condition 4Braid Shielded Cable, 200 ps Skew, 10 mV CM Noise
EMI Analyst™
LVDS Condition 5Laird Tape Shielded Cable, 200 ps Skew, 10 mV CM Noise
• Conditions • 10 MHz data rate• Signal skew• No CM circuit noise• Ideal grounding
DO+
DO-
RI+
RI-
100
Ω
VCM
(noise)
Z
0V
VDO-
VDO+
VDM = VDO+ - VDO-
Signal VCM = VDO+ + VDO- = 70 mV
tRISE tFALL
0V80%
20%
System VCM = 10 mV
EMI Analyst™
RE Results for Condition 5Laird Tape Shielded Cable, 200 ps Skew, 10 mV CM Noise
EMI Analyst™
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