AN1484: Extending the KVM Video Range to 1.5k Ft

AN1484: Extending the KVM Video Range to 1.5k Ft
Application Note 1484
By Rudy Berneike and David Laing
Extending the KVM Video Range to 1.5k Ft
The present KVM extension solution, Intersil’s ‘Hermes’
board, support RGB video transmission down Cat 5 cable
to about 1,000ft. Yet, you can extend this solution to
1,500ft by simply adding a pre-compensation network
cable driver. This pre-comp driver is inserted between the
EL4543 line driver RJ45 output plug and the Cat 5 cable
to extend the range by 500ft (refer to Figure 3). We will
also offer an option which you can connect directly to the
computer’s VGA video output plug to extend the range of
the PC’s RGB output on Cat 5 cable.
Problem
Cat 5 is not an ideal transmission medium for video
yet, is very economical vs using RG-59 Coax. The
present KVM solution used the EL4543 which embeds
the HSYNC and VSYNC as a difference common mode
signal onto the RGB line pairs. The embedding of the
sync’s reduces the cable requirement to just three
twisted wire pairs in a Cat 5 cable bundle of four wire
pairs. The present KVM solution is good to about
1,000ft and uses the EL9111 or three EL9110s to
receive and extract the HSYNC and VSYNC from the RGB.
Yet, the longer the Cat 5 cable, the more the signal
losses of the higher frequencies. For a flat frequency
response needed for video, the cable will need
compensation. Cat 5 cable losses are about 3dB per
decade so simple RC passive circuits to boost the HF
components will not compensate for such losses. How
then is the range extended by 500ft?
FIGURE 2. HERMES’ BOARD COMPENSATION WITH
THE 5009’ DRIVER EXTENTION
Solution
This application note will address using active peaking
circuitry at the driving end of the Cat 5 cable to extend
the usable range an additional 500ft. The same
peaking circuit can be used to extend standard RGB
output from a PC up to 500ft as well. Key is insuring
the high frequency elements, such as the HSYNC, VSYNC
and color information are peaked in such a manner to
overcome Cat 5 cable losses. This design is done in the
analog domain to maximize the overall performance at
the lowest cost.
We will discuss one channel, the other 2 channels will
need to be replicated for a complete RGB system.
The Amplifier
The amplifier peaks the incoming RGBHV of the EL4543
and drives the long distance Cat 5 cable (See Figure 4).
To accomplish this, we will need to complete the
following steps:
1. Impedance match the incoming Cat 5 to the peaking
network.
2. Adjust the sync CMV level to within the input range
of the Amp to optimize its operation.
3. Add peaking circuitry to correct sync offset rising
and falling edges due to the CMV offset circuit.
FIGURE 1. RGB DOWN 1,500ft CAT 5 NO
COMPENSATION
December 11, 2009
AN1484.0
1
4. Peak the three critical portions of the signal, which
are the Flatness, High Frequency components and
the DC Gain level
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc.
Copyright Intersil Americas Inc. 2009. All Rights Reserved
All other trademarks mentioned are the property of their respective owners.
Application Note 1484
l
FIGURE 3. PLACEMENT OF THE DRIVER EXTENDER BOARD
Basic Overall Circuit
Description
recovered and repplied as a common mode voltage
(CMV) to the amplifier insuring the CMV range of the
amplifier is optimized.
The compensation circuitry needs a very high BW to
provide the gain for the high frequency elements of the
incoming video signal. We selected the EL5378 Triple
700MHz Differential Twisted-Pair Driver as the basis for
the incoming amplifier and peaking network. Key for its
selection is the wide bandwidth and unique differential
input structure and its external reference input. This
unique input structure isolates the input from the
feedback network, greatly reducing design problems. We
can set the Gain and Offset without being concerned
about how they would negatively impact the input video
signal.
EL4543
R G OR B
The Sync Offset circuit will also reduce the sync
amplitude by 1/3 due to the series 1k and 2k resistors,
thus requiring modification of the gain in order to peak
the syncs leading and trailing edge and recover the sync
amp.
Peaking Circuit
The Cat 5 cable signal losses are at a higher frequency.
For a flat frequency response needed for video, the cable
will need compensation. Cat 5 cable losses are about 3dB
per decade. Thus, simple RC passive circuits will not
work.
FEEDBAC K
IN P U T
M A T C H IN G
NETW O RK
+ IN
- IN
SYNC
OFFSET
C IR C U IT R Y
1 /3
-A M P
R E F E L IN
5378
+ FB
- FB
G A IN
FEEDB ACK
P E A K IN G
C IR C U IT R Y
Even though the video signal has gained up to recover
the termination loss, there is a need to selectively peak
the gain for certain bandwidths as the Cat 5 cable effects
different frequency bands differently. Thus, developing a
peaking/frequency boost network to compensate for this
effect. The peaking network is connected across the gain
resistor to adjust the gains at different frequencies.
P E A K IN G C IR C U IT
FIGURE 4. BASIC BLOCK DIAGRAM OF THE CIRCUIT
EL4543 RGB with Differential CMV Sync
Encoded
The EL4543 is normally used as a 0V to 5V video
amplifier with the output offset to half the supply voltage
of 2.5V. The EL5378 must be used at ±5V to allow for the
frequency peaking voltage output range. Thus, the
output of the EL4543 must be offset to match the 0V
input need for the EL5378. If the EL4543 is used at ±5V,
then the output has no offset and the offset circuit can be
set for 0V offset with a switch.
INPUT MATCHING NETWORK
(Refer to Figure 6) The incoming R, G or B video is a
differential signal. Once the differential signal is properly
matched and adjusted to the input of the EL5378 (Input
Matching Network), the signal will require a 2x gain to
compensate for the output cable termination loss of the
EL5378. The EL5378 is a differential amplifier and to set
the gain will require the classic two feedback resistors
from the outputs to the feedback pins and a gain resistor
between the feedback pins.
SYNC OFFSET CIRCUITRY
The peaking circuitry needs to be applied to the R, G or B
incoming signals separately. The HSYNC and VSYNC are
encoded on the R, G and B differential signal lines by the
EL4543. The encoded HSYNC and VSYNC will need to be
2
R FLT
C FLT
LF POLE
FOR
FLATNESS
C HFB
HF BOOST
R SPA
V ID E O
A M P L IT U D E
R HFL
HF BOOST
L IM IT E R
FIGURE 5. PEAKING CIRCUITRY
The peaking network selected, is a classic configuration
with separate R, C and RC elements to modify the gain
for Flatness, HF Boost and Video Amplitude. The single
capacitor, CHFB will peak the highest frequencies and the
network gain resistor controls the peak gain/sharpness.
The additional series RC network, RFLT and CFLT form
the low frequency pole to determine the flatness of Back
Porch. The last resistor to ground, RSPA, determines the
height of the low frequency video. We added RHFL to gain
limit the HF Boost for stability and reduce HF noise.
The Peaking circuit is parallel to the DC gain resistor
which modifies the overall gain of the amplifier to
generate a flat frequency response needed for video. You
can break down the video requirements into four
different parts (refer to Figure 5):
1. SYNC Flatness – The Back Porch flatness sets the
overall luminance reference for the horizontal scan.
AN1484.0
December 11, 2009
Application Note 1484
Using a simple low frequency pole to peak the gain
at low frequencies ensures the received back porch
at the other end of the cable will be reasonably flat.
A simple series RC forms the low frequency pole.
2. HF Boost – Since the Cat 5 cable has greater losses
at higher frequencies, we use a capacitor to peak
the gain at high frequencies.
3. Video Amplitude – A simple resistor can insure we
have correct gain. Why not just use one resistor to
set the DC gain? We will explain our reasoning in the
following sections.
4. Limit HF Boost – We use a small value resistor to
reduce the effect of the peaking circuit on the
summing node of the EL5378. This resistor will limit
the HF Boost, prevent the circuit from going into
oscillations, and help reduce the HF noise from
being amplified. A small series resistor is used to
reduce the effects of any capacitance you may have
on the summing node and their effects. More on this
topic is discussed later in this application note.
Detail Circuit Description
Input Matching Network (Figure 6)
The input network is set up to terminate the Cat 5 cable
impedance with two 51Ω resistors in series across the
cable for a 100Ω load match. We use two series resistor
to establish a mind point for Sync Extraction.
SYNC Extraction w/Offset Adjust
We can use the midpoint of the two resistor termination
and extract the common mode Syncs. We can also use
the same node to apply the required CMV adjustment.
The encoded HSYNC and VSYNC are recovered by the 1k
resistor parallel with the 0.1µF capacitor to the 51Ω mid
point. If the EL4543 is operating in single supply mode,
+5VDC, the CMV point will be at 2.5VDC. In turn, the
EL5378 operates with a dual supply of ±5V and its
common mode zero point is at 0VDC. Thus, we must
have the REF input and the DIFF output at 0V average.
To accomplish this, we need to offset the incoming Sync
by -2.5VDC to force Syncs CMV to match the EL5378s
CMV. We can connect a 1k resistor from the termination
mid point in series with a 2k resistor terminated at -5V,
to offset the Sync by -2.5VDC. If the source (EL4543) is
operated at ± 5V, then the offset is not needed and the
2k resistors should be connected to ground. A universal
option is given by using switch S1, see Figure 1, to
connect the 2k resistors to ground or -5V. The 1k and 2k
induce a 1/3 loss of the sync signal amplitude but the
0.1µF capacitor will give the edges a full level signal so
that no useful sync signal is lost.
Peaking Circuit
Even though the video signal has been gained up, certain
bandwidths need to be selectively peaked as the Cat 5
cable effects different bands differently. It is then
necessary to develop a peaking/frequency boost network
to compensate for the cable effect. The discussion on ng
how to design this circuit will not be discussed, however,
we will mention how it works. The key point to remember
is that each of the peaking networks is additive.
LIMITING THE HF BOOST
The EL5378 is not truly a 2x gain amp due to stray PCB
capacitance impacting the HF frequency gain. This small
stray capacitance will peak the gain at other higher
frequencies. It is near impossible to remove all the stray
capacitances but it is possible to reduce the stray
capacitance. However, this stray capacitance will cause
the gain to be peaked at other than the desired
frequencies. By placing a small resistor after the
selection switch (see Figure 6) close to the summing
node, will act as an isolation resistor to the stray
capacitance by reducing the overall Q of the parasitic
capacitors. The reasoning for being concerned about
stray capacitance is the more capacitance you have on
the summing node the more unstable the wide band
amplifier becomes. Make sure to place these resistors
very close to the summing node, to further reduce the
stray capacitance of the PCB trace between the resistor
and the node. Also, for the components that form the
summing node, keep the physical distance to a
minimum. As for the basic layout guidelines, use a good
ground plane and a ground plane moat under the
summing node and the network RCs to help reduce noise
and stray PCB capacitances.
DC/AC
DECOUPLING
NETWORK
51Ω
FROM EL4543
1k
SYNC EXTRACTION
AND
LEVEL SHIFT
2k
S1
50Ω
50Ω
51Ω
RJ-45
IMPEDANCE
NETWORK
0.1µF
VIDEO IN
-5V
SYNC
FIGURE 6. INPUT IMPEDANCE MATCHING NETWORK
3
AN1484.0
December 11, 2009
Application Note 1484
Improved Design Using Selectable Range
150ft, 300ft and 450ft (150ft + 300ft)
This design covers from 1,000ft to 1,500ft. Yet, it does
require trade-offs if implemented in a single peaking
circuit. The high frequency losses of the Cat 5 cable
changes dramatically as the cable length increases. Yet,
if the peaking circuits are optimized for a given increment
of Cat 5 cable length, the networks are able to be
connected in parallel, compensating the driver for
different cable runs as needed. By dividing the additional
500ft into two, 1/3 and 2/3’s, we can we can optimize
the peaking for each length, improving the overall
performance. Now, using superposition, the two different
peaking circuits are able to be added, achieving an
overall length of 450ft.
Range selection can be done by using a simple low
capacitance mechanical SPST switch. A simple selector
switch will give us the flexibility to use this circuit in
many applications for cable lengths from 1,000ft to
1,150ft to 1,300ft to finally 1,450ft with minimal
degradation in the video signal.
SUM M ING
NODE
PEAKING CIRCUIT
W ITH SW ITCH
R SPA
LF POLE FOR
FLATNESS
TABLE 1. TYPICAL PEAKING NETWORK VALUES BY
CAT 5 CABLE LENGTH
CAT 5e
FLATNESS
HF
BOOST
SYNC
TIP
HF
BOOST
LIMITER
LENGTH
(ft)
RFLT
(kΩ)
CFLT
(pF)
CHGB
(pF)
RSPA
(kΩ)
RHFL
(Ω)
150
5
75
20
34
510
300
2
130
47
16
255
500
1.43
200
68
11
170
Test Results
R FLT
C FLT
The peaking circuit is designed to peak the desirable
HF elements by using an edge enhancement HF Boost
RC network, a parallel HSYNC pulse flatness RC
network, a parallel DC video amplitude network and a
switch to add peaking as function of the cable length.
The peaking circuit is designed to peak the desirable
HF elements by using an edge enhancement HF Boost
RC network, a parallel HSYNC pulse flatness RC
network, a parallel DC video amplitude network and a
switch to add peaking as function of the cable length
(see Table 1).
C HFB
H F BOO ST
R HFL
SELECTOR
SW ITCH
VIDEO
AM PLITUDE
H F BOO ST
LIM ITER
FIGURE 7. SELECTOR SWITCH PLACEMENT
At this point, we have gone through the quantitative
experimentation and have come up with our
recommended values for the peaking circuit
components. To help isolate these stray capacitances,
the switches should be connected upstream of the
peaking network, before the HF Boost Limiter resistor
and closest to the In-Amp’s summing node right at the
gain resistor.
4
There is the same image at 1500ft with the hermes
board and the extender circuit. Using the Herme’s design
and extended board full range compensation, you can
see in Figure 8, the images are very useful.
FEED BACK
EL4345
IN P U T M A T C H IN G
NETW ORK
+ IN
- IN
R G OR B
SYNC OFFSET
C IR C U IT R Y
+ FB
R E F IN A M P
1/3
EL5378
- FB
G A IN
FEED BACK
150FT
PEA K IN G
CIR C U ITR Y
300FT
P E A K IN G
C IR C U IT R Y
FIGURE 8. COMPLETE BLOCK DIAGRAM
AN1484.0
December 11, 2009
4 9 .9
C at 5
1k
51
2k
NC
OUT1
IN P 1
FBP1
4 9 .9
1k
Sr
4 9 .9
RED
0 .1 µ F
51
510
75pf
2k
FBN1
IN N 1
-5 V
5k
510
16k
47pf
130pf
34k
RED
EL4543
Application Note 1484
20pf
4 9 .9
REF1
O UT1B
3 0 0 ft
Cat 5
1k
51
IN P 2
VSN
510
2k
Sg
GREEN
0 .1 µ F
51
1 5 0 ft
EL5378
4 9 .9
-5 V
IN N 2
OUT2
REF2
FBP2
NC
FBN2
4 9 .9
510
5k
1k
510
16k
EL4543
47pf
4 9 .9
C at 5
1k
2k
IN P 3
51
BLUE
0 .1 µ F
255
51
-5 V
PNN3
OUT3
REF3
FBP3
130pf
34k
20pf
4 9 .9
O UT2B
Sb
4 9 .9
75pf
2k
GREEN
4 9 .9
VSP
3 0 0 ft
510
1 5 0 ft
4 9 .9
N o te : T h e th r e e R G B s w itc h e s
c a n b e a s in g l e s w i t c h t o g r o u n d
NC
FBN3
510
5k
1k
EN’
O UT3B
510
75pf
2k
16k
47pf
130pf
34k
BLUE
EL4543
255
NC
20pf
4 9 .9
1 .4 3 k
11k
68pf
255
3 0 0 ft
200pf
170
510
1 5 0 ft
S in g le N e tw o rk
~ 5 0 0 ft
FIGURE 9. COMPLETE RGB EXTENDER CIRCUITRY
Extending the RGB+HVsync
Output from Your PC
We can use the same peaking network to also extend the
RGBHV from your PC by simply encoding the HSYNC and
VSYNC as is done in the EL4543 but do it descreatly using
simple digital NAND gates (74HC00). We will also need to
encode, then offset and scale these signals so they can
be added on the R, G and B as we did in the previous
circuit (Refer to Figure 10).
5
STD LEVEL BUFFER AND ENCODER
The buffer will shift the input levels to a simple logic level
for the logic encoding. We can use a simple HCMOS logic
device to perform both the Level Buffer and the
Encoding.
OFFSET ADJUST AND SCALER
We need to offset this ground reference logic level Syncs
to a common mode voltage average of 0V. Thus, as with
the previous circuit, we will need to offset the Syncs
negatively to remove the DC offset from the logic device
output.
AN1484.0
December 11, 2009
Application Note 1484
Extending the RGB + HVSYNC
R
75Ω
G
-H SYNC
B
1kΩ
H SYNC
ENCODER AND SCALER/
LEVEL SHIFTER
STD LEVEL BUFFER
R
[= +H SYNC + V SYNC ]
10kΩ
10kΩ
[= -V SYNC ]
5kΩ
V SYNC
-V SYNC
REF
IN-AMP
+ FB
- FB
G
75Ω
1kΩ
REF
IN-AMP
+ FB
- FB
B
10kΩ
R
+ IN
- IN
[= +V SYNC – H SYNC ]
10kΩ
75Ω
1.5kΩ
10kΩ
1.5kΩ
10kΩ
1.5kΩ
10kΩ
74HC00
+ IN
- IN
G
+ IN
- IN
REF
IN-AMP
+ FB
- FB
B
EL5378
VSS = -5V
FIGURE 10. DETAILED PC RGB EXTENDER
RED [+ HSYNC + VSYNC]
Circuit Details
GREEN [- VSYNC]
STD Level Buffer (Figure 10)
The HSYNC and VSYNC coming from the PC are referenced
to ground using a 1kΩ resistor as a standard load. We
use a very low cost logic gate, such as the HC00 on the
HSYNC and VSYNC inputs, setting to known levels as
source to the encoding. The outputs are HSYNC and VSYNC
both inverted and non-inverted.
Encode
We need to buffer the input to the Ref of the EL5378. We
use 10kΩ series resistors at the output of the buffers to
prevent these outputs from loading each other and at the
same time properly encode the HSYNC and VSYNC. The
output of each is 5k in order to simplify the configuration.
Now we have encoded the output of the buffers to the
Sync sum/differences as generated internally by the
EL4543.
Offset and Scale
Scaling and level shifting of the Syncs is done in one
simple resistor divider. Using a simple resistor divider
network tied to -5VDC, we can offset each Sync
combinations as needed before applying them to the
EL5378 Reference inputs. Reference Inputs are as
follows:
BLUE [+ VSYNC - HSYNC]
Figure 10 shows that, all the buffer outputs resistor
networks have the same 5k output impedance and the
10k to -5V. This gives us a 0V output with ±1.66V sync
levels at 3.33k source and with 1.5k to ground giving
±0.5V sync output levels to the Ref.
The HSYNC and VSYNC are now properly encoded
matching the encoding scheme of the EL4543. The
encoded Syncs are applied to the VRef of the EL5378.
Thus, the PC RGB signals are single ended outputs. The
VGA cable RGB lines are connected to a 75Ω termination
resistor and the INP inputs of the EL5378 and the INN
inputs to ground. This configuration with the 75Ω
termination resistor will properly terminate the video
signal. We still need to keep the 2x gain to overcome the
termination loss.
RGB pairs have no standard cable pin out so be sure you
use the same cable pin out as your receiver.
Cable pinout for cat 5 RJ45 input board design should be
correct to insure the sync is on the correct color line so
the sync decode will work.
The cable pin out used is the same as used on the Intersil
EL4543 ‘HERMES-DEMO-BOARDZ’.
KVM RGB/VGA
EL5378
Sync Comp
Board
`
Hermes EL9110
Receiver
1500ft Cat 5
FIGURE 11. PLACEMENT OF THE RGB/VGA SYNC COMPENSATION BOARD
6
AN1484.0
December 11, 2009
Application Note 1484
Notes to Consider
Test Results
1. RGB pairs have no standard cable3 pinout. So be
sure you use the same cable pinout as your receiver.
1. Results for experimentation and testing: The video
band width is about:
2. Cable pinout for Cat 5 RJ45 input board design
should be correct to insure the Sync is on the correct
color line. Otherwise, the sync decode will not work.
160MHz at the 150ft range
3. The cable pinout used is the same as used on the
Intersil EL4543 HERMES-DEMO-BOARDZ, see
Figure 12.
85MHz at the 450ft range
110MHz at the 300ft range
These test results are similar to the EL9111 at max range
of 1000ft.
2. Cat 5E cable skew starts to show up at about 300ft.
We recommend the use of a deskew IC like the
EL9115 or ISL59920 for Cat 5E cables greater than
300ft.
Complete Schematic and PCB Layout
FIGURE 12. KVM RGB w/CMV SYNC-CABLE COMPENSATION SCHEMATIC
FIGURE 13. KVM RGB w/CMV SYNC-CABLE COMPENSATION PCB LAYOUT
7
AN1484.0
December 11, 2009
Application Note 1484
Appendix
Cat 5 cable pair Video color selection Problems and
Solutions:
Problem - The cat 5 cable pairs have different twist
rates to reduce the crosstalk. However, the different twist
rates causes the prop delay to be different for each pair.
Using the pairs for good color video reproduction requires
the colors arrive at the same time so they will align with
each other. The different twist rates causes the colors to
arrive at the monitor at different times. The result is
having three different color images next to each other.
Solution for Deskew ICs - To optimize the
performance of the deskew IC, select the wire pairs to
minimize the RGB skew caused by the cable's different
twist rates. Our testing has shown that the skew is about
24ns to 32ns for nearest skew pairs. Three twisted wire
pairs have been tested at the standard Cat 5 RJ45 pins.
The wire pairs associated with pin pair 1–2 is the shortest
pin pair 3–6 is the middle and pin pair 7-8 is the longest.
Another application for a deskew unit is for shorter up to
500ft cable using the EL4543 as a cable driver and the
application note in the data sheet using the EL5375 used
as the cable comp receiver.
Add Deskew to New Receiver Design
The extra skew from the added 500ft of cable should be
corrected at the cable receiver if possible. It will be easier
to implement at the receiver.
The deskew IC is best placed just before the receiver
VGA plug at the output to the monitor. The RGB lines are
separated from the embedded HSYNC and VSYNC so just
the RGB is available at the VGA plug.
Adding 500’ Cat 5 Skew Problem - A Low
Cost Work Around
Recommendations
For optimum performance, we recommend the shortest
pair (pins 1–2) to be the Blue signal, which will require
the longest delay. The Green (pins 7-8) should be the
longest pair and be the reference or zero delay, thus,
needing no correction. Then, the middle, Red (pins 3–6).
The selection for the RED and Green pairs is not very
critical when using a deskew IC but the Blue wire pair
should have the longest delay.
DeSkew Solution
There are a number of applications where added deskew
is needed. Such as, when a Hermes type KVM
receiver cable comp board is used and a cable longer
than 1000ft is needed, then a pre comp cable driver can
be used but more deskew will be needed. By designing
an IC deskew board unit with computer (VGA cable plugs
on the input and output and a USB plug) then it can be
added to the output of an existing cable receiver to get
the needed deskew added. The deskew unit can be
connected to the input of the pre comp cable driver. This
will allow an extra long cable to be added to a working
system. For new systems the extra IC de skew chip can
be designed in as needed.
FIGURE 14. SKEW AT 500’ NO COMPENSATION
Cable Adapter Xover Board
EL5375
Receiver
(AN1266)
EL4543
Transmitter
250ft Cat 5
250ft Cat 5
FIGURE 15. PLACEMENT OF THE CABLE CROSSOVER BOARD
8
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December 11, 2009
Application Note 1484
Notes:
1. Most of the skew may be canceled by using two
250ft alike Cat 5 cables in series to make 500ft
cable. At the middle of the 500ft cable, you swap the
longest and shortest video lines and at the receiver
swap them back.
2. When a deskew IC is used in the system with over
1k feet then the two cables can be of different
length, as long as the deskew IC can correct the
difference in the two cable lengths. A 1k feet cable
with 500ft cable with longest and shortest video
pairs reversed at both ends of one cable, will have
only 500ft of skew remaining that the deskew IC can
correct.
3. Cable adapter boards, see Figures 16 and 17, may
be used to rearrange the color pairs with existing
systems to get better color performance with the
KVM extender at 1500ft to optimum color pair
assignments. When an added deskew IC is not used
then there will be blue color error but generally will
be small. This will give a low cost solution with good
performance to about 1300ft and reduced
performance at 1500ft on Cat 6 cable.
The cable adapter cable wiring crossover will need to be
designed as needed for the application. The adapter
crossover shown is for Hermes improved pair selection.
The cable adapter board may be configured for color pair
long to short pair reversal. The same adapter is used in
the middle and one end of the cable and just reversed in
direction. The “Cable” labels should go to the same cable.
FIGURE 17. CABLE ADAPTER BOARD
Dielectric Cable Smear (Dielectric Storage
Effect)
If you look close at Figure 18, you will see a smearing to
the right of any Dark to Light or Light to Dark color
transition. This is referred as a cable smear, the result of
the twisted wire insulation dielectric absorption.
The rise time of the cable causes short time constant
smear that is corrected with cable compensation.
However, the dielectric insulation in the cable has
dielectric absorption that causes electrons to be stored in
the dielectric. When there is a voltage across the
dielectric, the electrons are not just on the surface but go
into the dielectric and are stored there. Therefore, when
the voltage is removed, the electrons are slowly
released. In a long cable that is at one voltage for a
number of micro seconds followed by a larger voltage
change, there is a charge or discharge of the electrons
that changes the voltage on the cable that shows up as a
long time constant of a number of micro seconds of
smear. This can only be fixed by a low dielectric
absorption cable.
FIGURE 16. 500’ WITH THE ADAPTER BOARD
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Application Note 1484
FIGURE 18. EXAMPLE OF CABLE SMEAR
ESD Protection
Figure 19 does not have ESD protection shown. ESD
protection should be added as needed with a Schottky
diode, we recommend the BAT54S dual diode be
connected from the plus and minus supplies to the
output and input cables. Transit Voltage Suppressors
should be added if the cables are exposed outdoors.
The placement should be close to the first IC
encountered from the input of the receiver. For short
runs this would be the input to the EL537x and for long
runs requiring deskew, this would be the input to either
the EL9115 or ISL5992x.
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Application Note 1484
+5V
Peaking & Gain Networks
+5V
NC
OUt1
INP1
FBP1
INN1
FBN1
49.9-Ohms
510-Ohms
2K
-5V
Sr
49.9-Ohms
0.1uF
RED
1K
+5V
-5V
51-Ohms
5K
510-Ohms
-5V
EL4543
51-Ohms
Cat 5
1K
OUT1B
+5V
NC
VSP
-5V
INP2
VSN
INN2
OUT2
REF2
FBP2
NC
FBN2
2K
Sg
49.9-Ohms
0.1uF
Green
255-Ohms
300ft
510-Ohms
150ft
EL4543
51-Ohms
1K
49.9-Ohms
-5V
5K
2K
Sb
0.1uF
BLUE
51-Ohms
510-Ohms
2K
INP3
OUT2B
PNN3
OUT3
REF3
FBP3
NC
FBN3
EN’
OUT3B
130pf
20pf
34K
+5V
49.9-Ohms
255-Ohms
+5V
-5V
75pf
16K
47pf
-5V
49.9-Ohms
-5V
510-Ohms
1K
+5V
Cat 5
+5V
+5V
-5V
49.9-Ohms
20pf
34K
+5V
-5V
51-Ohms
130pf
49.9-Ohms
REF1
EL5378
49.9-Ohms
75pf
2K
16K
47pf
RED
Cat 5
1K
Green
EL4543
-5V
49.9-Ohms
51-Ohms
300ft
-5V
150ft
510-Ohms
-5V
+5V
49.9-Ohms
-5V
510-Ohms
5K
1K
510-Ohms
75pf
2K
16K
47pf
130pf
BLUE
Note: The three switches can
be a single switch to ground
20pf
34K
+5V
49.9-Ohms
1.43K
255-Ohms
300ft
510-Ohms
150ft
-5V
11K
68pf
200pf
170
Single Network
~ 500ft
FIGURE 19. COMPLETE COMP CIRCUITRY WITH ESD PROTECTION
Add De-Skew to New Pre Comp Driver
When the cable receiver does not have the range to
correct the added skew, and the added deskew cannot
be added at the receiver, then the additional correction
can be added at the cable driver end. The Cat 5 cable has
a fourth digital control wire pair which can control the
logic from a USB port, for a remote deskew IC at the
cable driver. You can now control the deskew from the
remote KVM station using the fourth cable pair for the
deskew logic control.
The deskew IC is then connected from the of the VGA
board input to the RGB input of the EL5378. The 75Ω
termination resistor stays with the cable. The USB
interface to the deskew IC and the software is needed to
support the remote deskew control. The interface for the
deskew IC is in its data sheet and in the HERMES-DEMOBOARDZ’ documentation.
Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the
reader is cautioned to verify that the Application Note or Technical Brief is current before proceeding.
For information regarding Intersil Corporation and its products, see www.intersil.com
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