© Semiconductor Components Industries, LLC, 2020
December, 2020 − Rev. 0
1 Publication Order Number:
AND90081/D
Active Cable usage with
ON Semiconductor Redrivers
Introduction
With the increasing requirement for high speed transfer
of larger bulk of data, better quality video and power
delivery, change over from analog to digital videos,
bi−directional transfers, it become essential to use a proper
cable that delivers the data correctly between external hard
drives and other systems or end products.
The use of Active cable becomes a necessity with increase
in cable lengths of more than 5 meters, number of ports
and data rates 10 Gbps and above. To match these using
passive cables will become expensive with increased weight
and thickness.
This application notes looks at need for active cables,
cable losses and solutions available with ON redrivers
for improving usage of long length cable with Type−C
and Type−A connectors.
Necessity for Active Cables
Generally users would prefer to use a longer and thinner
cable. With passive cables, this leads to signal loss
and the end device may fail to work or not receive
the expected input. The USB device will not function
and with no output. To make a passive cable of higher
quality with thicker or multiple copper wires, shielding
and better quality connectors will lead to more expense
increasing thickness and handling issues.
Factors like impedance, skew, cross−talk, attenuation,
differential impedance, bit error rate (BER), jitter
and insertion loss contribute to the cable length limit.
To overcome the above problems, use of active cables
with built−in electronics will help.
Advantages of using Active Cables
Following are the positives in using active cables
with redrivers:
• Can be of longer lengths than passive ones.
• The redriver compensates for signal loss due to skew,
jitter, EMI, cross talk and attenuation that affects signal
integrity.
• Redriver in active cables improves in boost of signal
with increased eye opening, extending the link length
and enabling pass of compliance test.
• Smaller wire gauge can be used providing more
flexibility supporting better bending of cables with
improved air flow.
• Less expensive due to use of low cost wire.
• Active cable supports connectivity of data, video,
security and power application over a single connector
as per USB Type−C standard that allow multiple
functions over the same connector.
Requirement and Issues with Long Cables:
In order to connect to different outputs (like printer,
scanner) that are at various distances from the main input
without loss of data would call for use of a hub in the center
that will act as a cable extension adding to hardware
and electrical point.
Hence the need arises for use of longer cables. But long
cables have losses that varies with frequency, length,
thickness and with different vendors.
To overcome the issue of physical size and flexibility
of usage and to maintain signal integrity calls for long cables
with thinner wire gauge that leads to use of active cables
with redrivers and control circuitry.
www.onsemi.com
APPLICATION NOTE
AND90081/D
AND90081/D
www.onsemi.com
2
Cable Losses:
In passive cables, the losses increases with increase
in frequency, length and with change in wire gauge.
Increasing the thickness of wire improves losses, but limited
by the skin effect over frequency and handling becomes an
issue due to increased cable thickness.
Figure 1 shows losses in cables for 1 meter and 5 meter
cables with respect to wire thickness AWG and frequency.
As can be seen, the variation is not linear when thickness
and frequency is varied. The cable attenuation increases
with increase in frequency. The same length of cable
with different gauge wire will have different attenuation
levels.
Figure 2 shows the loss allocation allowed by USB−IF
with the center portion showing losses allowed in a cable.
The system designers faces lot of issues in trying to stay
within the allotted −6 dB spec for the cable.
From the Data shown in Figure 1 and Figure 2, materials,
PCB, lengths are limiting factors to keep losses below
the −6 dB spec. All of the above data indicates a need
for a cable that compensates for these losses over longer
lengths. This leads to need for active cables, adding redrivers
allows them to use cheaper materials (PCB, copper wires),
which saves money while in parallel extending the length
of the cables.
For 28 AWG:
Frequency (MHz) 200 600 1250 2500 5000
Attenuation (dB/1m) 0.5 0.9 1.3 1.85 2.7
Attenuation (dB/5m) 2.5 4.5 6.5 9.25 13.5
For 30 AWG:
Frequency (MHz)
200
600
1250 2500
5000
Attenuation (dB/1m)
0.6
1
1.5
2.2 3.2
Attenuation (dB/5m) 3 5 7.5 11 16
For 32 AWG:
Frequency (MHz) 200 600 1250 2500 5000
Attenuation (dB/1m) 0.74 1.36 2 2.8 4
Attenuation (dB/5m) 3.7 6.8 10 14 20
Figure 1. Losses in Cables 1M and 5M Length with Respect to AWG and Frequency
The loss allocation as per USB
-IF is shown below
Figure 2. USB−IF Specified Loss Allocation for Cables
Loss budget as specified in USB−IF for Gen 2 5 Ghz: Host (left), Cable (center) and devices (right)
Loss budget as specified in USB−IF for Gen 1 2.5 Ghz: Host (left), Cable (center) and devices (right)