Chapter 2. Purpose, Basic Operation & Testing
Copyright © AMETEK Power Instruments
2.1 Purpose & Basic Operation
Hybrids have been used from the beginning with
Power-Line carrier and their basic operation is still
the same as when they originally were developed
many years ago. The hybrid’s primary purpose is to
combine multiple Power-Line carrier (PLC) signals
onto one common coax cable without causing inter-
ference between different transmitters or between
the transmitters and receivers. This is necessary
because two transmitters connected directly
together will load each other down and can cause
signal clipping and intermodulation distortion. Also
a local high powered transmitter can interfere with
a local receiver that is set to receive a weak signal
level from the far-end transmitter. Hybrids solve
these issues by providing isolation between the two
devices being combined while at the same time
allowing them to be combined onto a common coax
without too much signal loss. Isolation is especially
important when there is close frequency spacing
between carrier sets which is most often the case.
Using hybrids allows the minimum frequency
spacing between carrier sets.
Hybrids are completely bi-directional and have
nothing in them to direct signal flow from the inputs
to the output or vice versa. They act as a combiner
in one direction and a splitter in the opposite
direction. So, going in one direction at the transmit-
ting end of the line, the hybrid combines, but at the
receiving end of the line it splits. The hybrid
labelling of the “inputs” and the “output” is for the
local transmitter’s signal direction.
The application of a “balance transformer” used in
phase-to-phase coupling at the transmitting end, for
example, is a hybrid being used backwards as a
splitter instead of a combiner. In this example, the
output becomes the input and the 2 inputs become
outputs.
PLC hybrids are completely passive devices and
consist only of transformers, resistors, capacitors,
and inductors (depending on the type of hybrid).All
components are rated to handle more power than is
specified on their inputs. This type of design adds to
their long life and robustness.
2.1.1 Multiple Transmitters/Receivers
If multiple transmitters and receivers need to be
combined onto one coax cable, then the hybrids can
be stacked together to achieve this. This is
necessary as each hybrid can only combine 2
devices at one time. So, if 3 devices need to be
combined, then normally 2 hybrids will be required,
if 4 devices, then 3 hybrids and so forth. The
exception is that hybrids are not needed to isolate
high impedance receivers from one another as
receivers can be directly connected together.
2.1.2 Hybrid Types
There are two main types of PLC hybrids: Balanced
(B) and Skewed (S). The balanced hybrid has equal
losses from the inputs to the output.And the skewed
hybrid has unequal (skewed) losses from the inputs
to the output. The skewed hybrid is only used for
combining a transmitter and receiver, never for 2
transmitters. When 2 transmitters are combined, the
balanced hybrid is always used. The skewed hybrid
favors the transmit’s side over the receive side with
less than 0.5 dB loss on the transmit side. (See
Table 2–1). This is done to improve the overall
system signal-to-noise ratio by 3dB by getting 3dB
more transmit power out to the line. At the receive
end of the power line, both the signal and the noise
get equally attenuated so more loss on the receive
side doesn’t affect the S/N ratio. At the transmit
end, only the transmit signal gets attenuated as it
goes out to the power-line so we want to keep atten-
uation low.
