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THEORY OF OPERATION
The OTX-***-HH-LR8-HS Long-Range Handheld Transmitter combines an LR
Series transmitter and an antenna with an on-board HS Series encoder to form
a highly reliable and secure RF remote-control transmitter. The LR Series
transmitter is a low-cost, high-performance synthesized OOK transmitter. It’s
synthesized architecture delivers outstanding stability and frequency accuracy,
while minimizing the effects of antenna port loading and mismatching. This
reduces or eliminates frequency pulling, bit contraction, and other negative
effects that are common to SAW-based transmitter architectures, providing a
significantly higher level of performance and reliability.
When a button is pressed on the transmitter, power is applied to the internal
circuitry and the encoder is enabled. The encoder then detects the logic states
of the button data lines. These states are formatted into an encrypted message
that is output to the transmitter module. This cycle continues until the button is
released. The encoder data is used to modulate the transmitter, which conveys
the data into free space through the antenna. Once data is received, a decoder
IC is used to decrypt the transmitter’s commands. If decryption is successful, the
decoder’s outputs are set to replicate the transmitter’s button states. These
outputs can then be used to activate whatever external circuitry is required by the
application.
The transmitter is compatible with several Linx receiver products. For
applications where range is critical, the LR Series receiver is the best choice due
to its outstanding sensitivity. When the transmitter is combined with an LR Series
receiver and an HS Series decoder, ranges of up to 1,000 feet are possible.
Applications operating over shorter distances will also benefit from the increased
link reliability and superior noise immunity provided by the LR Series receiver.
1. Characterized, but not tested
Notes
ELECTRICAL SPECIFICATIONS
Parameter
Designation
Min.
Typical
Max.
Units
Notes
POWER SUPPLY
Operating Voltage
V
CC
2.1
3.0
3.6
VDC
–
Supply Current
I
CC
–
3.4
–
mA
–
Power-Down Current
I
PDN
–
5.0
–
nA
1
TRANSMITTER SECTION
Transmit Frequency Range:
F
C
OTX-315-HH-LR8-HS
–
315
–
MHz
–
OTX-418-HH-LR8-HS
–
418
–
MHz
–
OTX-433-HH-LR8-HS
–
433.92
–
MHz
–
Center Frequency Accuracy
–
-50
–
+50
kHz
–
ENVIRONMENTAL
Operating Temperature Range
–
-40
–
+85
°C
1
SECURITY OVERVIEW
The HS Long-Range Handheld transmitter uses the HS Series encoder, which is
based on Cipherlinx™ technology. CipherLinx™ is a high-security encryption
algorithm and wireless protocol designed for remote control and remote keyless
entry applications. It provides a much greater level of security and many more
features than older technologies on the market, such as fixed address or “rolling
code” systems. Additionally, the CipherLinx™ protocol is much more advanced
than the simple PWM method employed by many systems. By utilizing an
advanced serial protocol, CipherLinx™ is able to offer superior noise immunity,
greater range, and greater link reliability, all of which are key factors in a wireless
system.
CipherLinx™ never sends or accepts the same data twice, never loses sync, and
changes codes with every packet, not just every button press. The encryption
that is used in CipherLinx™ is based on the Skipjack cipher developed by the
U.S. National Security Agency (NSA), and is widely considered one of the most
secure ciphers available. The CipherLinx™ algorithm has been evaluated by
Independent Security Evaluators (ISE), a company that has testified before
Congress as experts on electronic security. They concluded that “In short, the
CipherLinx™ protocol in the HS Series is well-designed and is an excellent
choice for applications requiring a secure unidirectional link.”
In addition to this high level of security, CipherLinx™ also offers a number of
features that are unique among remote control products. These include a large
number of data lines, internal key generation, “button level” control permissions,
an optional encoder PIN, as well as the ability for the decoder to identify the
originating encoder.
CipherLinx™ is based on the NSA-designed cipher Skipjack. Skipjack is a block
cipher that has 80-bit keys and 64-bit data blocks. Since each packet is longer
that 64 bits, Skipjack must be employed in an encryption mode. The particular
encryption mode chosen for CipherLinx™ is based on the CMC encryption
mode, so that the resulting cipher is a special kind of function known as a “strong
PRP” (sPRP). The encryption mode uses several invocations of Skipjack to
encrypt the 128 bits in each message. The definition of these terms is quite
involved, but more details can be found in ISE’s evaluation report at
www.cipherlinx.com.
The HS Series uses a 40-bit counter to change the appearance of each
message. This large counter value and the timing associated with the protocol
ensure that the same message is never sent twice and prevents the encoder
from ever losing sync with the decoder.
The user generates the key with the decoder through multiple button presses.
This is ensures that the key is random and chosen from among all 2
80
possible
keys. Since all of the keys are created by the user and are internal to the part,
there is no list of numbers anywhere that could be accessed to compromise the
system.
The user or manufacturer may also set “button level” Control Permissions.
Control Permissions determine how the decoder will respond to the reception of
a valid command, either allowing the activation of an individual data line or not.
The decoder is programmed with the permission settings during set-up, and
those permissions are retained in the decoder’s non-volatile memory.
