HPS/RFL GARD Pro System 1 Hubbell Power Systems, Inc. – RFL™ Products
June 2019 (c)2019 Hubbell Incorporated
RFL™ PLC Hybrid Pro
User Guide
This entire document is the property of Hubbell Power Systems, Inc. (HPS) and may not be reproduced,
transmitted, published or stored in an electronic retrieval system, in whole or in part, by any means –
electronic or otherwise. For permission, contact: customer.serv[email protected]
Hubbell Power Systems, Inc. – RFL™ Products
353 Powerville Road ● Boonton Twp., NJ 07005-9151 USA
Tel: 973.334.3100 ● Fax: 973.334.3863
Email: Customer.Service@RFLelect.com ● www.rflelect.com
Publication Number RF-MCHYBRIDPRO-01
Version 01, Printed in U.S.A.
June 3, 2019
HPS/RFL GARD Pro System 2 Hubbell Power Systems, Inc. – RFL™ Products
June 2019 (c)2019 Hubbell Incorporated
NOTICE
The information in this manual is proprietary and confidential to Hubbell® Power Systems, Inc. (“HPS”). Any
reproduction or distribution of this manual, in whole or part, is expressly prohibited, unless written permission is given by
HPS.
This manual has been compiled and checked for accuracy; however, HPS makes no representation or warranty as to the
accuracy or completeness of the information in this manual. The information in this manual does not constitute a warranty
of performance. HPS reserves the right to revise this manual and make changes to its contents from time to time. We
assume no liability for losses incurred as a result of out-of-date or incorrect information contained in this manual.
HPS shall not have any liability resulting from the use of the information in this manual. This manual does not purport to
cover all details or variations in equipment, nor provide for every possible contingency to be met in connection with
installation, operation, or maintenance of this specific product. Should further information be desired or should particular
problems arise which are not covered sufficiently for the purchaser's purposes, the matter should be referred to your RFL
product representative or RFL Customer Service Department at Customer.Service@RFLElect.com, or by visiting the
website at www.rflelect.com/Support.
NOTES
It is recommended that this product be opened immediately after receiving and inspected for proper operation and signs of
impact damage.
For information regarding product warranty and repairs, please visit the HPS/RFL website at www.rflelect.com/Support or
e-mail the RFL Customer Service Department at Customer.Service@RFLelect.com.
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Table of Contents
1 Safety Instructions ............................................................................................................. 6
1.1 Warnings and Safety Summary ................................................................................................... 6
1.1.1 Safety Summary .................................................................................................... 6
1.1.2 Additional Warnings............................................................................................... 8
1.1.3 Additional Cautions ................................................................................................ 8
2 Introduction ........................................................................................................................ 9
3 Example Applications and Recommended Configurations .............................................. 10
3.1 One Transmitter, One Receiver (FSK) ...................................................................................... 10
3.2 Two Transmitters, Two Receivers (FSK) ................................................................................. 10
3.3 Three Transmitters, Three Receivers (FSK) .............................................................................. 11
3.4 Two Transmitters, Two Receivers (FSK and On-Off/DCB) ..................................................... 12
3.5 Two Transmitters, Two Receivers, Phase-to-Phase Coupled (low-loss method) ..................... 12
3.6 Two Transmitters, Two Receivers, Phase-to-Phase Coupled (high reliability method) ........... 13
3.7 Two Independent Relay Channels, Three-Phase (Mode 1) Coupled......................................... 14
4 Hybrid Module Descriptions ............................................................................................. 15
4.1 Balanced Hybrid ........................................................................................................................ 15
4.2 Skewed Hybrid .......................................................................................................................... 17
4.3 Skewed Hybrid Front Panel Test Switch ................................................................................... 19
4.4 Splitter ....................................................................................................................................... 21
4.5 Splitter/Combiner ...................................................................................................................... 23
4.6 Bypass Hybrid ........................................................................................................................... 25
4.7 1RU Hybrid Chassis .................................................................................................................. 28
5 Testing and Verification ................................................................................................... 30
5.1 Testing the Balanced Hybrid ..................................................................................................... 32
5.2 Testing the Skewed Hybrid ....................................................................................................... 33
5.3 Testing the Splitter .................................................................................................................... 34
5.4 Testing the Splitter/Combiner ................................................................................................... 35
5.5 Testing the Bypass Hybrid ......................................................................................................... 36
6 Compatibility and Part Numbering, New and Legacy Hybrids ......................................... 38
List of Figures
Figure 1 – Grounding ................................................................................................................................ 6
Figure 2 – One Transmitter, One Receiver – (FSK) ............................................................................... 10
Figure 3 – Two Transmitters and Two Receivers (FSK) ........................................................................ 10
Figure 4 – Three Transmitters, Three Receivers (FSK).......................................................................... 11
Figure 5 – Two Transmitters, Two Receivers (FSK and On-Off/DCB) ................................................. 12
Figure 6 – Two Transmitters, Two Receivers, Phase-to-Phase Coupled (low-loss method) ................. 12
Figure 7 – Two Transmitters, Two Receivers, Phase-to-Phase Coupled (high reliability method) ....... 13
Figure 8 – Two Independent Relay Systems, Three-Phase (Mode 1) Coupled ...................................... 14
Figure 9 – Balanced Hybrid, Functional Diagram .................................................................................. 15
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Figure 10 – Balanced Hybrid, Top View ................................................................................................ 16
Figure 11 – Balanced Hybrid, Rear Panel ............................................................................................... 16
Figure 12 – Balanced Hybrid, Front Panel (test points) ......................................................................... 17
Figure 13 – Skewed Hybrid, Functional Diagram .................................................................................. 17
Figure 14 – VSWR / Reflected Power Measurements with Skewed Hybrid Front Panel Test Switch .. 20
Figure 15 – Skewed Hybrid, Top View .................................................................................................. 20
Figure 16 – Skewed Hybrid, Rear Panel ................................................................................................. 20
Figure 17 – Skewed Hybrid, Front Panel (test points) ............................................................................ 21
Figure 18 – Splitter, Functional Diagram ............................................................................................... 21
Figure 19 – Splitter, Top View ............................................................................................................... 22
Figure 20 – Splitter, Rear Panel .............................................................................................................. 22
Figure 21 – Splitter, Front Panel (test points) ......................................................................................... 22
Figure 22 – Splitter/Combiner, Functional Diagram .............................................................................. 23
Figure 23 – Splitter/Combiner, Equivalent Circuit ................................................................................. 23
Figure 24 – Splitter/Combiner, Top View .............................................................................................. 24
Figure 25 – Splitter/Combiner, Rear Panel ............................................................................................. 24
Figure 26 – Splitter/Combiner, Front Panel (test points)........................................................................ 25
Figure 27 – Bypass Hybrid Application ................................................................................................. 25
Figure 28 – Bypass Hybrid, Functional Diagram ................................................................................... 25
Figure 29 – Bypass Hybrid, Functional Schematic ................................................................................. 26
Figure 30 – Bypass, Top View ............................................................................................................... 27
Figure 31 – Bypass, Rear Panel .............................................................................................................. 27
Figure 32 – Bypass, Front Panel (test points) ......................................................................................... 27
Figure 33 – 1RU Hybrid Chassis, Front View, Door Closed ................................................................. 28
Figure 34 – 1RU Hybrid Chassis, Front View, Door Open .................................................................... 28
Figure 35 – 1RU Hybrid Chassis, Rear View ......................................................................................... 29
Figure 36 – 1RU Hybrid Chassis, Dimension ........................................................................................ 29
Figure 37 – Example Terminations, Balanced Hybrid, Insertion Loss Test, TX1 to OUT .................... 31
Figure 38 – Example Terminations, Skewed Hybrid, Insertion Loss Test, LINE to RX ....................... 31
Figure 39 – Measuring Balanced Hybrid Insertion Loss, TX1 to OUT .................................................. 32
Figure 40 – Measuring Balanced Hybrid Isolation, TX1 to TX2 ........................................................... 32
Figure 41 – Measuring Skewed Hybrid Insertion Loss, TX to LINE ..................................................... 33
Figure 42 – Measuring Skewed Hybrid Insertion Loss, LINE to RX ..................................................... 33
Figure 43 – Measuring Skewed Hybrid Isolation, TX to RX ................................................................. 34
Figure 44 – Measuring Splitter Insertion Loss, IN to LINE 1 ................................................................ 34
Figure 45 – Measuring Splitter Output Phase Angle Relationship, LINE 1 to LINE 2 .......................... 35
Figure 46 – Measuring Splitter/Combiner Insertion Loss, IN 1 to LINE 2 ............................................ 35
Figure 47 – Measuring Splitter/Combiner Isolation, IN 1 to IN 2 .......................................................... 36
Figure 48 – Measuring Splitter/Combiner Phase Relationships, IN 1 to LINE 1, LINE 1 to LINE 2 .... 36
Figure 49 – Measuring Bypass Insertion Loss, LINE 1 to LOCAL ........................................................ 36
Figure 50 – RFL Hybrid-Pro Ordering Code/Smart Number ................................................................. 40
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List of tables
Table 1 – RFL PLC Hybrid Pro – Part Numbers and Descriptions .......................................................... 9
Table 2 – Balanced Hybrid Specifications .............................................................................................. 16
Table 3 – Skewed Hybrid Jumper Settings ............................................................................................. 18
Table 4 – Skewed Hybrid Specifications ................................................................................................ 19
Table 5 – Splitter Specifications ............................................................................................................. 22
Table 6 – Splitter/Combiner Specifications ............................................................................................ 24
Table 7 – Bypass Hybrid Specifications ................................................................................................. 26
Table 8 – Chassis Dimensions ................................................................................................................ 29
Table 9 – Balanced Hybrid Loss Characteristics .................................................................................... 32
Table 10 – Skewed Hybrid Loss Characteristics .................................................................................... 33
Table 11 – Splitter Loss Characteristics ................................................................................................. 34
Table 12 – Splitter/Combiner Characteristics ......................................................................................... 35
Table 13 – Bypass Hybrid Characteristics .............................................................................................. 37
Table 14 – New / Legacy Conversion ..................................................................................................... 39
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1 Safety Instructions
1.1
Warnings and Safety Summary
The equipment described in this manual contains high voltage.
Exercise due care during operation and servicing. Read the safety
summary below.
!
1.1.1 Safety Summary
The following safety precautions must always be observed during operation, service, and repair of this
equipment. Failure to comply with these precautions, or with specific warnings elsewhere in this
manual, violates safety standards of design, manufacture, and intended use of this product. HPS/RFL
assumes no liability for failure to comply with these requirements.
Ground the Chassis
The chassis must be grounded to reduce shock hazard and allow the equipment to perform properly.
The Chassis is provided with a rear-panel protective earth terminal, which must be connected to a
proper electrical ground by suitable cabling. The location of the protective earth terminal on the
RFL™ PLC Hybrid Pro system is shown below. Refer to the wiring diagram supplied with the unit for
additional information on chassis and/or cabinet grounding.
Ground wire: 12-14 gauge
Attach the ground
wire to the nearest
panel grounding bar.
Figure 1 – Grounding
A protective earth stud at the right rear of the RFL™ PLC Hybrid Pro system chassis is the main
ground for the RFL™ PLC Hybrid Pro system.
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Do not Operate in an Explosive Atmosphere or in Wet or Damp Areas
!
Do not operate the product in the presence of flammable gases or fumes, or in any area that is wet or
damp. Operating any electrical equipment under these conditions can result in a definite safety hazard.
Keep Away from Live Circuits
Operating personnel should never remove covers. Component replacement and internal adjustments
must be done by qualified service personnel. Before attempting any work inside the product,
disconnect it from the power source and discharge the circuit by temporarily grounding it. This
will remove any dangerous voltages that may still be present after power is removed.
Unrestricted operator access is only permitted to the front of the unit when hazardous voltage is
applied. It is the responsibility of the installer to restrict access to the rear terminal blocks where
hazardous voltage may exist.
Do not Substitute Parts or Modify Equipment
!
Because of the danger of introducing additional hazards, do not install substitute parts or make
unauthorized modifications to the equipment. The product may be returned to HPS/RFL for service
and repair, to ensure that all safety features are maintained.
Read the Manual
!
Operators should read this manual before attempting to use the equipment, to learn how to use the
equipment properly and safely. Service personnel must be properly trained and have the proper tools
and equipment before attempting to make adjustments or repairs.
Service personnel must recognize that whenever work is being done on the product, there is a potential
electrical shock hazard and appropriate protection measures must be taken. Electrical shock can result
in serious injury, because it can cause unconsciousness, cardiac arrest, and brain damage.
Throughout this manual, warnings appear before procedures that are potentially dangerous, and
cautions appear before procedures that may result in equipment damage or service outage if not
performed properly. The instructions contained in these warnings and cautions must be followed
exactly.
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1.1.2 Additional Warnings
WARNING!
Follow all your company’s policies and procedures regarding the installation of AC powered or DC
powered equipment. If there is a conflict between any procedure in this manual and your company’s
safety rules, then your company’s safety rules must take priority.
1.1.3 Additional Cautions
CAUTION
Any installation using an enclosed cabinet with a swing-out rack must be securely fastened to the floor.
This will prevent the cabinet from falling forward when the rack is moved outward.
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2 Introduction
The RFL™ PLC Hybrid Pro hybrids are passive devices used to couple power line carrier (PLC)
signals to/from the transmission line for relay protection communication channels. The hybrids are the
interface between PLC transmitters and receivers in the control building and the line tuner in the
switchyard. They are installed in a 1RU chassis which is typically installed next to the transmitters /
receivers. Generally, HPS/RFL hybrids perform two basic, simultaneous functions:
- Isolate transmitters from transmitters, or transmitters from receivers
o Prevent transmitters from loading/interfering with each other
o Prevent local transmit signals (high level) from appearing as noise to local receive
signals (low level)
- Combine and split signals to set them up for the appropriate line coupling scheme
o Onto one coax cable for single-phase-to-ground coupling
o Onto two or three coax cables for phase-to-phase and 3-phase (mode 1) coupling
An ideal hybrid would perform these functions at no cost. However, a price is paid – a loss of signal
power through the hybrids. The most common hybrid names are also a description of their loss
characteristics: a balanced hybrid is so named because the losses are “balanced” (equal), while in a
skewed hybrid the losses are “skewed” (unequal). Hybrid design and PLC channel design must take
these losses into account to minimize their effect on the channel performance. Improper application of
hybrids will result in excessive channel losses, inadequate isolation between channels, or both.
HPS/RFL hybrids are installed in a 1RU chassis, and each chassis can hold up to 3 hybrids. This is
enough for the most common carrier coupling schemes (occasionally a second hybrid chassis must be
used which can be stacked on top of the other). Below is a listing of the different hybrids HPS/RFL
offers, along with a short description.
Table 1 – RFL™ PLC Hybrid Pro – Part Numbers and Descriptions
Description
Typical Application
RFL P/N
Balanced hybrid
Combine transmitters
RF-107970
Skewed hybrid
Combine transmitters and receivers
RF-107975
Splitter
Phase-to-Phase coupling (low loss method)
RF-107980
Splitter/Combiner
Phase-to-Phase coupling (high reliability method)
RF-107990
Bypass
Linear three-terminal system, or passing signal
around a discontinuity
RF-107985
1RU Chassis
Hybrids are installed in the chassis (up to 3)
RF-106665-1
Blank Panel
Cover a slot
RF-106666-1
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3 Example Applications and Recommended
Configurations
When applying hybrids, the number of transmitters and receivers, as well as the application of each
(DCB/On-Off or transfer trip/FSK), will determine what hybrid configuration will work best for each
system. There are several different ways to connect hybrids but generally there are best practices that
should be observed. The examples shown below illustrate typical hybrid configurations and show how
hybrids work together. The exact performance characteristics of individual hybrids are discussed later
in this manual.
3.1
One Transmitter, One Receiver (FSK)
TX
SK
RX
to/from Line Tuner
Figure 2 – One Transmitter, One Receiver – (FSK)
The configuration shown in FIGURE 2 is the simplest hybrid configuration. Here, a skewed hybrid is
used to combine a single transmitter / single receiver onto one coax cable for connection to a single
tuner. This is a typical single-function, single-phase-to-ground coupled FSK system.
3.2
Two Transmitters, Two Receivers (FSK)
TX1
TX2
BAL
SK
RX1
RX2
to/from Line Tuner
Figure 3 – Two Transmitters and Two Receivers (FSK)
The system shown in FIGURE 3 is an expansion of the system in FIGURE 2. Here, there are two
transmitters, so a balanced hybrid (used to combine transmitters) is added. The output of the balanced
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hybrid is fed into the TX port on the skewed hybrid. Instead of coupling just one transmitter, a skewed
hybrid can pass multiple transmit signals, so long as they are combined with balanced hybrids first.
Note that in this configuration there are also two receivers, but they do not need to be combined with a
hybrid, as is the case for the transmitters. Because receivers are highly selective and are high-
impedance devices, they can be directly paralleled and will not load or otherwise interfere with each
other. Some receivers have an option to provide a termination in the form of a non-inductive resistor
equal to the desired termination value (usually 50 ohms). This option should only be used when no
hybrid is used, for example a single Rx-only device that connects directly to the line tuner. When more
than one receiver is installed, the receivers themselves should be left unterminated. The skewed
hybrid’s RX port provides the required termination for the hybrid circuit. This is an operational
advantage as well – for service and maintenance operations, removing any one receiver from the circuit
will not affect the RX termination value.
FIGURE 3 could represent several different applications. TX1/RX1 might be DTT and TX2/RX2 might
be DCUB or POTT; or, TX1/RX1 and TX2/RX2 might represent a dual-DTT scheme.
3.3
Three Transmitters, Three Receivers (FSK)
TX1
TX2
BAL
SK
RX1
RX2
to/from Line Tuner
BAL
TX3
RX3
Figure 4 – Three Transmitters, Three Receivers (FSK)
FIGURE 4 is a further expansion of the system in FIGURE 3. This system illustrates how the output of
one balanced hybrid can be an input to another balanced hybrid, to accomplish the coupling of more
than two transmitters in a PLC system. Again, the output of the last balanced hybrid is fed into the TX
port on the skewed hybrid.
The system in FIGURE 4 might represent a dual-DTT / DCUB system. Here, it is best to make
TX1/RX1 and TX2/RX2 the DTT channels, so that both will have equal losses, and to make TX3 the
DCUB channel so that DCUB, as the primary protection scheme, has the least loss through the hybrids.
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3.4
Two Transmitters, Two Receivers (FSK and On-Off/DCB)
TX1
TX2/RX2
(2W)
BAL
SK
RX1
to/from Line Tuner
Figure 5 – Two Transmitters, Two Receivers (FSK and On-Off/DCB)
The application in FIGURE 5 is like FIGURE 3 – one balanced hybrid, one skewed hybrid, and two
individual relaying channels. The main difference here is that TX2/RX2 is a 2-wire DCB channel (the
transmit and receive are directly tied together) so there is only one coax connection to/from the unit. In
these cases, the DCB channel TX/RX signals are treated as a transmitter. Like a transmitter, they are
connected to the input of a balanced hybrid.
3.5
Two Transmitters, Two Receivers, Phase-to-Phase Coupled
(low-loss method)
TX1
TX2
BAL
SK
RX1
RX2
SPL
to/from Line Tuner A
to/from Line Tuner B
Figure 6 – Two Transmitters, Two Receivers, Phase-to-Phase Coupled (low-loss method)
FIGURE 6 shows two transmitters and two receivers in a phase-to-phase coupling application. Here,
once the signals have been combined onto a single coax connection (same as the circuit in FIGURE 3),
the signals are “split”. Then, two coax cables are run from the splitter out to the switchyard, adding
redundancy and dependability. There are variations of the connection scheme – some line tuners can be
equipped with a splitter circuit – but the scheme depicted in FIGURE 6 will provide the best reliability
for this phase-to-phase application. This is because there will be two coax cables going out to the tuner,
and ideally these will be run in separate cable trays.
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It should be noted that the signals which are “split” between phases at one end of the circuit are re-
combined at the terminating end of the circuit. Therefore, the splitter does not add any additional loss
to the PLC channel.
It should also be noted that the splitter can be applied with different transmitter/receiver and hybrid
combinations, not just the one shown in FIGURE 6.
3.6
Two Transmitters, Two Receivers, Phase-to-Phase Coupled
(high reliability method)
TX1
SK
RX1
TX2
SK
RX2
SPLITTER /
COMBINER
to/from Line Tuner A
to/from Line Tuner B
Figure 7 – Two Transmitters, Two Receivers, Phase-to-Phase Coupled (high reliability method)
FIGURE 7 shows another variation of a phase-to-phase coupling application. This type of hybrid
scheme is used when two independent relaying channels are being applied. The difference between this
circuit and the one in FIGURE 6 is that there are independent coax cables entering and leaving the
phase-to-phase coupling hybrid. No single point of failure, other than the splitter/combiner itself, is
present (hybrids very rarely fail – they are ruggedized, passive devices).
This circuit introduces significant loss into the channel as compared to the splitter in FIGURE 6, but it
allows the designer to keep the PLC channels and connections functionally and logically separated
while providing good isolation between the channels.
TX1/RX1 and TX2/RX2 might be part of redundant protection schemes. It should be noted that the
splitter/combiner can be applied with different transmitter/receiver and hybrid combinations, not just
the one shown in FIGURE 7.
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3.7
Two Independent Relay Channels, Three-Phase (Mode 1) Coupled
BAL
SPL
Relay System A
SPLITTER /
COMBINER
SPL
Relay System B
to/from center phase
to/from outer phase 1
to/from outer phase 2
Figure 8 – Two Independent Relay Systems, Three-Phase (Mode 1) Coupled
FIGURE 8 shows a “mode 1 combiner” circuit, used in three-phase / mode 1 coupling applications.
This hybrid scheme is typically used only on the most critical lines and at transmission levels of 500
kV and above. It is the most reliable method since the loss of any one phase should not cause the
channel to fail. The losses through this circuit are less than in the splitter/combiner phase-to-phase
application of FIGURE 7. However, the application requires a third set of coupling equipment – tuner,
CCVT, and trap – so it is also the most expensive.
The latest HPS/RFL hybrids simplify the interconnections of this scheme, and the mode 1 combiner
can be supplied as a complete pre-wired system. Contact [email protected] for more
information on the mode 1 combiner.
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4 Hybrid Module Descriptions
This section contains details on the 1RU hybrid chassis and the individual hybrid modules such as
losses, isolation, etc.
For the purposes of this manual, the following assumptions are made:
- All values stated in the “specification” tables are valid for the PLC frequency range, from
30 kHz to 500 kHz
- All stated losses and isolation figures, such as insertion loss and trans-hybrid loss, assume
that all hybrid ports terminated into the specified termination impedance (50 ohms or 75
ohms).
4.1
Balanced Hybrid
BAL
TX1
TX2
> 30 dB
- 3.5 dB
- 3.5 dB
OUT
Figure 9 – Balanced Hybrid, Functional Diagram
The balanced hybrid is most often used to combine two transmitters onto a single coax connection
while providing high isolation (trans-hybrid loss) between the two transmitter inputs. This isolation
prevents the PLC transmitters from mutually loading each other, and from creating intermodulation
distortion products which can interfere with other PLC systems.
A balanced hybrid is “balanced” because its losses are balanced: the loss from each input to the output
is approximately 3.5 dB, meaning each signal loses a little bit more than half its power through the
hybrid. So, a signal at the TX1 port with a level of +30 dBm can be expected to have a level of roughly
+26.5 dBm at the OUT port. This is the price which is paid to achieve the high isolation between input
signals while combining those signals at the output port.
Isolation between input ports is typically greater than 30 dB. For example, a signal at the TX1 port with
a level of +30 dBm can be expected to have a level somewhere below 0 dBm when the same frequency
is measured at the TX2 port. The best isolation values will be seen when bench testing with near-
perfectly matched loads / impedances. In field applications, line tuning imperfections and impedance
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mismatches may lower the isolation between input ports. The minimum isolation required for real-
world balanced hybrid applications is 15 dB.
The LINE port of the balanced hybrid has a gas discharge tube (E2) and slo-blo fuse (F2) for
overvoltage and overcurrent protection.
Table 2 – Balanced Hybrid Specifications
Balanced Hybrid Specifications
Operating Impedance
50 ohms (75 ohms available by request)
Insertion Loss
(either input port to output port)
approximately 3.5 dB
Isolation / Trans-hybrid loss
(input port to input port)
greater than 30 dB
Power Rating
10 Watts, each port
Second Harmonic Distortion
At least 80 dB below the fundamental frequency
Third Order Intermodulation Products
At least 60 dB below the fundamental frequency
Figure 10 – Balanced Hybrid, Top View
Figure 11 – Balanced Hybrid, Rear Panel
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Figure 12 – Balanced Hybrid, Front Panel (test points)
4.2
Skewed Hybrid
SK
TX
RX
> 40 dB
- 0.5 dB
- 13 dB
LINE
Figure 13 – Skewed Hybrid, Functional Diagram
The skewed hybrid is used to combine a transmitter and a receiver (or multiple transmit signals and
multiple receivers) onto a single coax connection while providing high isolation (trans-hybrid loss)
between the transmit and receive ports. This high level of isolation is necessary to prevent the high-
energy transmit signal from leaking over and interfering with the incoming low-level receive signal,
and to prevent the termination impedance of the receiver from loading the transmitter. Since PLC
channels are commonly grouped tightly together in a small frequency band, the transmit signal could
easily appear as noise inside the bandwidth of the receiver. The isolation provided by the skewed
hybrid is what allows a 10-Watt transmitter to be spaced closely next to a much lower-level receive
signal. The skewed hybrid is typically the last hybrid in the hybrid “chain” and the connection to the
line tuner.
A skewed hybrid is “skewed” because its losses are skewed, or unequal: there is a low loss from the
TX port to the LINE port: approximately 0.5 dB. This allows as much transmit power as possible to get
to the power line. A signal at the TX port with a level of +30 dBm can be expected to have a level of
roughly +29.5 dBm at the LINE port.
Alternatively, the loss from the LINE port to the RX port on the skewed hybrid is approximately 13 dB
– much higher than the loss in the transmit path. While this sounds like a lot of loss to the receive
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signal, it is important to remember that the noise is also attenuated by the same amount. So, the signal-
to-noise ratio (SNR) is the same at the LINE port as at the RX port. So long as the absolute signal level
is not too low at the RX port – greater than about -30 dBm for most modern PLC receivers – the
performance of the receiver will not be affected by this high loss from LINE port to RX port. An
example of RX path loss: a signal at the LINE port with a level of +15 dBm can be expected to have a
level of roughly +2 dBm at the RX port.
The isolation between the TX and RX ports is typically greater than 40 dB. For example, a signal at the
TX port with a level of +30 dBm can be expected to have a level somewhere below -10 dBm when the
same frequency is measured at the RX port. The best isolation values will be seen during bench testing
with near-perfectly matched loads / impedances. In field applications, line tuning imperfections and
impedance mismatches may lower the isolation between TX and RX ports. The minimum isolation
required for real-world skewed hybrid applications is 25 dB.
The RX port of the skewed hybrid contains protection circuitry for the receiver’s front-end, and also
contains an optional line termination resistor. There is a transient suppressor (CR1) which limits the
voltage at the receiver. This is permanently installed on the RX path. Optionally, jumper J5 is used to
select or to bypass a 100-ohm current limiting resistance. Current limiting using the resistor is
recommended but not necessary. Jumper J4 is used to select or bypass the RX termination (HPS/RFL
recommends to always terminate the RX connections on the skewed hybrid).
Jumpers J4 and J5 are conveniently located just inside the rear panel of the skewed hybrid card, so they
can be easily accessed by sliding the card out a few inches. It is NOT recommended to make changes to
these jumpers while the system is in-service.
The LINE port of the skewed hybrid has a gas discharge tube (E2) and a slo-blo fuse (F2) for
overcurrent and overvoltage protection.
Table 3 – Skewed Hybrid Jumper Settings
Jumper / Switch
Description
J4 – RX
TERMINATION
Default = IN
OUT: RX port is unterminated
IN: RX port is terminated (recommended)
(Termination = 50 ohms; other values available by request)
J5 – CURRENT LIMIT
Default = IN
OUT: no current limit resistor in RX path
IN: 100-ohm resistor installed in-series with RX path
(recommended)
SW1 – NORM/TEST
Default = NORM
NORM: “HYB OUT” is directly connected to “TO TUNER”;
Hybrid is in normal operation
TEST: “HYB OUT” is disconnected from “TO TUNER”;
Meter/directional coupler is connected to line tuner
RF-MCHYBRIDPRO-01 Hubbell Power Systems, Inc. – RFL™ Products
June 2019 19 973.334.3100
NOTE: If the current limiting resistor is used, the RX termination must be done on the skewed
hybrid. In other words, if J5 is set to “IN”, J4 must also be set to “IN”. If J5 is set to “OUT”, J4
can be either “IN” or “OUT”.
Table 4 – Skewed Hybrid Specifications
Skewed Hybrid Specifications
Operating Impedance (each port)
50 ohms (75 ohms available by request)
Insertion Loss (TX to LINE)
approximately 0.5 dB
Insertion Loss (LINE to RX)
approximately 13 dB
Isolation / Trans-hybrid loss (TX to RX)
greater than 40 dB
Power Rating
10 Watts, each port
Second Harmonic Distortion
At least 80 dB below the fundamental frequency
Third Order Intermodulation Products
At least 60 dB below the fundamental frequency
4.3
Skewed Hybrid Front Panel Test Switch
FIGURE 14 shows the functional circuit of the test switch on the skewed hybrid front panel. The switch
is most commonly used for taking reflected power measurements when the channel is in service. With
the switch in the NORM position, connections are made from HYB OUT to the input of the reflected
power measurement device (instrument), and from the output of the instrument to the TO TUNER port.
Once the connections have been made up and the instrument is ready to measure, the switch is placed
in the TEST position. This diverts the signal energy through the instrument and the measurement is
then recorded. After the measurement has been recorded, the switch is placed back in the NORM
position, and then the cables may be removed. Throughout this process, the power line carrier channel
is never out of service.
RF-MCHYBRIDPRO-01 Hubbell Power Systems, Inc. – RFL™ Products
June 2019 20 973.334.3100
Test Connections Installed, Switch = NORM
(signal path through switch)
TX
SK
RX
HYB
OUT
TO
TUNER
IN
OUT
VSWR METER
coax
cables
TX
SK
RX
HYB
OUT
TO
TUNER
IN
OUT
VSWR METER
coax
cables
Test Connections Installed, Switch = TEST
(signal path through meter)
Figure 14 – VSWR / Reflected Power Measurements with Skewed Hybrid Front Panel Test
Switch
Figure 15 – Skewed Hybrid, Top View
Figure 16 – Skewed Hybrid, Rear Panel
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