Introduction

A pair of traces (wires) between the driver and receiver, one trace carries the

positive signal and the other carries a negative signal that is both equal to, and

the opposite polarity from. This is just so-called differential signal [1].

The real PCB differential trace :

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Advantage

One advantage of differential signal is with less EMI radiation. EMI radiation is

fundamentally caused by any electrical transitions with sharp edges, which

produces electromagnetic radiation. In digital systems, periodic clock signals are

the major cause of EMI [2].

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But, with balanced differential devices signal lines, the fields around the two

electrical paths oppose each other, and the concentric magnetic fields tend to

react with one another and cancel each other, and then results in lower

emissions [2-3].

This is the reason why usually high speed digital signals are differential form.

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The second major benefit of differential signaling is that it is highly immune to

outside electromagnetic interference(EMI) and crosstalk from nearby signal

conductors. An example of how it works is shown in following figure [4] :

B and C are differential pair, and A is the nearby interference. In terms of

S-parameter, the interference that A couples to B is SBA, and the interference that

A couples to C is SCA. If both B and C are close to each other, SBA and SCA will be

roughly equal. Besides, B and C have opposite direction on signal propagating.

That is to say, SBA and SCA will be roughly equal and opposite, and then cancel

each other. It’s the reason why that differential signaling is highly immune to

outside interference.

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In RF circuits, compared with Tx signal, Rx signal is much weaker. Therefore, in

order to be highly immune to interference and avoid degrading the sensitivity,

the Rx signal usually adopt differential form.

In addition, I/Q signal does affect the modulation and demodulation accuracy.

Therefore, both Tx I/Q and Rx I/Q signals adopt differential form to avoid

being interfered by outside interference, and then degrading the modulation and

demodulation accuracy.

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Length

From previous analysis, we know that the differential signal has two major

benefits :

1. Produce less interference

2. Be highly immune to interference

However, if the differential pair has different length in each other, perhaps the

benefits will disappear. Let’s take the following figure for example :

B and C are differential pair. If the length of B is not equal to which of C, there will

be an additional length, just “length 2”. At this moment, length2 is single-end

signal, which will interfere A, or be interfered by A.

If B and C are RF Rx signal differential pair, the length difference will introduce a

large impedance discontinuity (100 Ohm => 50 Ohm), which causes mismatch

loss and sensitivity degradation.

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As mentioned above, usually I/Q signals adopt differential form. That is to say,

there will be four I/Q signals : I+、I-、Q+

、Q-. And the phase relationship is as

following figure :

The length of the four I/Q signals must be all equal, or there will be IQ phase

Imbalance, which will produce the undesired sideband, so-called image.

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The amplitude difference between the desired signal and image is sideband

suppression. If the length of the four I/Q signals are all equal, in theory, there will

be no IQ phase Imbalance, just like the left side of above figure.

In contrast, in the right side of above figure, the more length 2 is, the more IQ

phase Imbalance and the less Sideband Suppression will be.

As illustrated in above figure, we know that the less Sideband Suppression is, the

more phase error will be, which will make EVM increase.

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According to [5], we know that SNR is inversely proportional to EVM :

That is to say, in terms of Rx I/Q differential signal, the more length difference is,

the more EVM, the less SNR, and the poorer sensitivity will be.

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Separation

The impedance of differential pair is related to the separation between each

other, as illustrated in following figure :

Consequently, for the high speed digital differential pair (e.g. MIPI, HDMI, and

USB, etc.), the separation should be constant, or there will be reflection due to

impedance discontinuity, and then EMI radiation interference will be stronger.

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In addition, the differential pair separation is not only related to impedance, but

also immunity to interference. Let’s take the following figure for example :

B and C are differential pair, and A is the nearby interference. In terms of

S-parameter, the interference that A couples to B is SBA, and the interference that

A couples to C is SCA. According to previous analysis, if both B and C are close to

each other, SBA and SCA will cancel each other. Nevertheless, if B and C are not

close to each other, i.e. S2 is large, SBA > SCA. In that case, B will be interfered by

A. In light of this, the less separation between each other is, the better immunity

to interference will be. But, we already know that the separation affects

impedance, the less separation is, the less impedance will be. So, to be accurate,

the separation between each other should be as small as possible without

destroying target impedance(e.g. 100 Ohm or 150 Ohm) to posses better

immunity to interference.

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From another point of view, we regard A as the nearby victim. According to

previous statement, we know that with balanced differential devices signal lines,

the fields around the two electrical paths oppose each other, and the concentric

magnetic fields tend to react with one another and cancel each other, and then

results in lower emissions. Nevertheless, if S2 is large, the concentric magnetic

fields around B and C are not able to cancel each other completely, and then

emissions will become higher. Consequently, the separation between each other

should be as small as possible without destroying target impedance to posses

lower emissions.

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Bend

In real PCB, it’s impossible for the differential traces to be only straight. The bend

is inevitable, and then causes extra length and additional common-mode noise.

Consequently, the conventional method is to bend again to compensate for the

length difference.

But, as shown in the above figure, these bends are all bad. Because these bends

are all sharp, i.e. 90 degree, which causes impedance discontinuity and signal

reflection [6].

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Besides, there is no mode conversion in ideal differential signal. Nevertheless,

the sharp bends cause mode conversion (differential-mode to common-mode)

and additional common-mode noise.

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According to [6-7], the less the rising time is , the more the common-mode noise

due to sharp bend is.

And as shown in the following figure, the more length of L is, the more the

common-mode noise due to sharp bend is.

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In terms of eye diagram, because the sharp bend causes impedance discontinuity

and mode conversion, which cause loss. Besides, there will be jitter due to phase

imbalance. In other words, the sharp bend will narrow the eye height and width.

Thus, in PCB layout, we should avoid the sharp bend.

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Of course, as mentioned above, the bend is inevitable. But we are able to make

use of round or 45 degree corner, rather than 90 degree.

For example, for the dual-bend mentioned above, we can make use of the 45

degree, as illustrated in the right side of the following figure.

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According to [8], the ranking of phase imbalance due to bend :

90 degree > 45 degree > round corner

And the common-mode noise due to round corner is actually smaller than 90

degree.

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Consequently, among the three types of bend, no matther which one is adopted,

there must be loss, phase imbalance, and common-mode noise, the only

difference is how much they are. Now that the bend is inevitable, what we can do

is only to mitigate the influence by means of 45 degree and round corner.

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EMI Filter

For the high speed digital differential signal, we make use of termination resistor

to do impedance matching. Nevertheless,

mismatching is inevitable.

or because of via itself and connector.

As mentioned above, the impedance mismatching causes reflection, and then

enhances EMI noise. In other words

there must be noise emission

For the high speed digital differential signal, we make use of termination resistor

to do impedance matching. Nevertheless, in real PCB, the impedance

mismatching is inevitable. Perhaps it’s because of bypassing the via,

or because of via itself and connector.

the impedance mismatching causes reflection, and then

In other words, as long as there is high speed digital

noise emission.

For the high speed digital differential signal, we make use of termination resistor

the impedance

s because of bypassing the via,

the impedance mismatching causes reflection, and then

as long as there is high speed digital signal,

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For example, MDDI or MIPI may radiate noise to RF antenna, and then sensitivity

degrades [10].

In terms of time-domain, the noise will distort the signal waveform.

Thus, we need to suppress the noise due to high speed digital signal. The most

common method is to make use of EMI filter.

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For the noise due to differential signal, there are two kinds : common-mode and

differential-mode.

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Nevertheless, with differential-mode noise suppression, perhaps the signal is

attenuated as well. But with common-mode noise suppression, the signal is

hardly attenuated.

Besides, comparing with differential-mode noise, the common-mode noise is

stronger. Thus, for the differential signal, we usually focus on common-mode

noise suppression. And we often make use of common-mode choke, composed of

inductor and capacitor, to suppress common-mode noise. As shown in the

following figure :

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In terms of time-domain, with EMI filter, the distortion of signal waveform

reduces much.

In terms of frequency-domain, with EMI filter, the noise floor reduces much.

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In terms of RF, there will be sensitivity degradation issue while LCM is on. Thus,

we can add common-mode choke in MIPI signal to mitigate the noise to improve

the sensitivity.

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As mentioned above, for the differential signal, we usually focus on

common-mode noise suppression. Consequently, we should select the

common-mode choke with higher common-mode noise suppression.

From the following figure, we can see that there is still de-sense issue in some

channels with the original common-mode choke. But, there is no de-sense issue

in all channels with the new common-mode choke with higher common-mode

noise suppression.

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Besides, there is also de-sense issue when camera is on.

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We can also add EMI Filter in the camera differential signal to suppress the noise.

As illustrated in the following figure, the de-sense issue is mitigated with EMI

Filter.

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But, as mentioned above, for the high speed digital differential signal,we make

use of termination resistor to do impedance matching, and common-mode choke

is composed of inductor and capacitor. In other words, the internal resistance

within the inductor of common-mode choke may change the impedance, cause

the noise reduction is less than expected, even cause stronger noise emission

than the situation without common-mode choke. Therefore, when selecting the

common-mode choke, not only focus on common-mode noise suppression, but

also on the internal resistance within the inductor. The safest method is to

discuss with your vendor before adding the common-mode choke.

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Ground

According to [11], all currents return to their source. In other words, currents

flow in loops.

As illustrated in the following figure and formulas, we know that the larger the

loop area is, the larger the inductance is and vice versa.

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According to [12], current will always follows the path with least impedance. At

low frequency, the resistance of a path dominates the impedance, while at high

frequency, the inductance of a path dominates. This is best summarized as

follows:

– Low frequency current flows through the path with least resistance.

– High frequency current flows through the path with least inductance

As mentioned above, the smaller the loop area is, the smaller the inductance is.

That is to say, the return current of high frequency signal follows the path with

least loop area. Therefore, as illustrated in the above figure, although the return

path of high frequency is longer than which of low frequency, the loop area of

high frequency is smaller than which of low frequency. Consequently, we realize

that the return current of high frequency is underneath the signal path.

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And the simulation result supports the statement as well. At low frequency, the

return current concentrates on the path from load to source. But at high

frequency, the return current concentrates on the path underneath the signal

path.

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However, as illustrated in the following figure, the return current of differential

signal, i.e. GND current, is almost zero. Thus, we wonder that does the return

current of differential signal exist in adjacent trace, not in ground ?

According to [13], we know that differential pair not only couples to ground, but

also to adjacent trace in differential-mode.

As mentioned above, the return current of high frequency signal follows the path

with least loop area. That is to say, for differential signal, the path with least loop

area is just the return path.

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Take the following PCB stack-up for example, if we lay differential traces on top

layer, and we regard layer 2 as ground plane, the spacing between differential

traces and ground plane is only 2.8 mil. Even though we regard layer 4 as ground

plane, the spacing is only 8.4 mil at best.

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But, with the same PCB stack-up, the spacing between each other of the

differential pair on the top layer is about 10 mil with 100 ohm target impedance.

Thus, in theory, the return current has two paths : the adjacent trace and the

ground plane. But in real PCB, because the ground plane can make the

differential pair have least loop area, the return current almost concentrates on

the ground plane. The simulation result supports the statement as well.

.

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But, as mentioned above, the return current of differential signal, i.e. GND current,

is almost zero. Is there a contradiction ?

No! these two statements are both correct. One is transient state, and the other is

steady state. As illustrated in following figure, in transient state, the return

current is underneath the differential signal and on the ground plane. But in

steady state, because i1 and i2 will cancel each other, GND current is almost zero.

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Now that we realize that the return current almost concentrates on the ground

plane, the completeness of ground plane matters. As illustrated in following

figure, with solid ground plane, the return loss is at least -20 dB, and insertion

loss is small as well.

But, with a break in ground plane, the return loss becomes poor, and insertion

loss increases.

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As mentioned above, the spacing between the differential signal and ground

plane impacts impedance. In other words, with a break in ground plane, there

will be impedance discontinuity and reflection, which worsen return loss. And,

a break in ground plane is equivalent to a series inductor, which attenuates the

signal and increases insertion loss.

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According to [14], Ground Bounce distorts the signal waveform, affects the circuit

logic, and worsens the system stability.

As illustrated in following figure, with a break in ground plane, there will be

Ground Bounce.

Therefore, the ground plane should be as solid as possible.

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Although in steady state, the ground current is almost zero. Nevertheless, due to

crosstalk, there will still be an induced current flowing in a closed loop on the

ground plane under the differential pair, and produces noise emission [1].

According to [15], the 3W rule can further minimize 70% crosstalk. So, should

we increase the spacing between the differential pair and the ground plane to

reduce the induced current ?

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No! you can’t increase the spacing. Because in real PCB, the length of differential

pair is hardly equal. That is to say, more or less, there must be an extra length,

which is single-end signal with return path on the ground plane. Thus, the more

spacing is, the larger the loop area of single-end signal is. Undoubtedly, large

loop area causes strong noise emission.

According to[1], the loop area of induced current is the same as which of

differential pair. And as mentioned above, in real PCB, the return current almost

concentrates on the ground plane. In other words, instead of increasing the

spacing, you should decrease the spacing to minimize the loop area of induced

current and differential pair. Besides, for loop area reduction, you should shorten

the length of differential pair, and decrease the spacing between each other of the

differential pair (without affecting target impedance) as well.

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Conclusion

The differential signal has two major benefits :

1. Produce less interference

2. Be highly immune to interference

Three points that you should pay attention to :

1. Make the length of each other of the differential pair as equal as possible

2. Make the spacing between each other of the differential pair as constant as

possible

3. Make the spacing between each other of the differential pair as small as

possible(without affecting the target impedance)

For the bend, round corner is better than 45 degree, but sharp corner is

forbidden.

When selecting EMI filter, you should pay attention to :

1. Common-mode noise rejection

2. The internal resistance within the inductor

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Ground plane should be as solid as possible.

For loop area reduction, you should :

1. Shorten the length of differential pair

2. Minimize the the spacing between each other of the differential pair

(without affecting the target impedance)

3. Minimize the the spacing between the differential pair and the ground plane

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Reference

[1] Differential Signals:Rules to Live By

[2] LVDS Reduces EMI, FAIRCHILD

[3] Design For EMI, INTEL

[4] What is a Differential Signal? Lattice

[5] On the Extended Relationships Among EVM, BER and SNR as Performance

Metrics

[6] Signal Integrity Issues for High-Speed Serial Differential Interconnects

[7] INTERCONNECT SIGNAL INTEGRITY, SAMTEC

[8] Your layout is skewed

[9] Breaking Up A Pair

[10] EMI countermeasure for Smartphone, TAIYO YUDEN

[11] Emc & the Printed Circuit Board: Design, Theory, & Layout Made Simple

[12] Part 4 –PCB LAYOUT RULES FOR SIGNAL INTEGRITY

[13] The Truth about Differential Pairs in High Speed PCBs

[14] Understanding and Minimizing Ground Bounce, FAIRCHILD

[15] Designing for Board Level Electromagnetic Compatibility

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