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Figure 1: Simplified schematic of high isolation directional
couplers (SYDC-8-122-75+) isolating upstream and downstream
signal traffic in Full-Duplex DOCSIS 3.1 system.
Figure 2: SYDC-8-122-75+
case style
Figure 3: Curves of measured test data for SYDC-8-122-75+ for
Coupling, Isolation and Directivity, swept over DC to 1500 MHz.
develop broadband, high-isolation
75Ω directional couplers to prevent
leakage between forward and return
signal within network nodes. In the
forward direction, downstream signal
is injected through the coupled port
onto the mainline toward subscriber
premises. In the return direction,
upstream signal is injected onto the
mainline toward the head end. The
difficulty is designing a wideband
coupler with sufficient isolation
between the coupled port and the in/
out ports to prevent leakage of the
injected signal into the opposing path.
System requirements call for input/
output operating frequency from
5 to 1218 MHz, coupled frequency
range from 40 to 780 MHz and typical
isolation (In/Out – Coupled Port)
of roughly 30 to 35 dB or better up
to 780 MHz. Achieving this level of
isolation over a frequency range
this wide is challenging, but Mini-
Circuits leveraged our in-house
design expertise to develop model
SYDC-8-122-75+, a surface-mount,
transformer-type 75Ω directional
coupler to support the requirements
for Full Duplex DOCSIS 3.1 systems.
Test data for this coupler exhibits
isolation greater than 40 dB up to
1000 MHz and greater than 30 dB up
to 1220 MHz at the upper limit of the
DOCSIS 3.1 application band. This
design has a coupling ratio of 8.9 dB
with ±0.5 dB flatness over the 5 to 750
MHz range and ±0.9 dB flatness over
the 5 to 1218 MHz range. It provides
75+ an ideal candidate for isolating
upstream and downstream paths in
Full Duplex DOCSIS 3.1 systems.
Conclusion: One Piece of a
Complex Puzzle
Full Duplex DOCSIS 3.1 presents a
number of new challenges that cable
operators and suppliers are working
diligently to overcome. Isolating the
forward and return signal paths at
the node is one of the key challenges
unique to full duplex communication,
requiring innovative problem solving
at the component level. Mini-
Circuits’ SYDC-8-122-75+ enables
system architects to effectively isolate
forward and return signals at the
same frequency and prevent signal
leakage that would otherwise lead to
systemic problems.
The successful implementation of Full
Duplex DOCSIS 3.1 ultimately depends
on a wide and complex variety of
factors. Mini-Circuits is proud to
contribute one piece in the puzzle and
play a role in the collaborative efforts
of the broadband industry to bring the
market faster connectivity. The SYDC-
8-122-75+ high-directivity directional
coupler is an example of our ability to
respond to challenging, application-
specific system requirements with
technical expertise, flexibility and fast
turnaround.
1W RF input power handling, 1.4 dB
typical mainline loss, and 18 dB typical
return loss (input/output/coupling).
It comes housed in a miniature
plastic package (0.38 x 0.50 x 0.25”)
mounted on printed wiring laminate
base with a wraparound terminations
for excellent solderability.
Its exceptional isolation of over wide
frequency range, flat coupling and
combination of other performance
characteristics, make the SYDC-8-122-
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