KH3 Series
Transmitter Module
Data Guide
! Warning: Some customers may want Linx radio frequency (“RF”)
products to control machinery or devices remotely, including machinery
or devices that can cause death, bodily injuries, and/or property
damage if improperly or inadvertently triggered, particularly in industrial
settings or other applications implicating life-safety concerns (“Life and
Property Safety Situations”).
Table of Contents
1^
1^
1^
NO OEM LINX REMOTE CONTROL OR FUNCTION MODULE
SHOULD EVER BE USED IN LIFE AND PROPERTY SAFETY
SITUATIONS. No OEM Linx Remote Control or Function Module
should be modified for Life and Property Safety Situations. Such
modification cannot provide sufficient safety and will void the product’s
regulatory certification and warranty.
2^
Customers may use our (non-Function) Modules, Antenna and
Connectors as part of other systems in Life Safety Situations, but
only with necessary and industry appropriate redundancies and
in compliance with applicable safety standards, including without
limitation, ANSI and NFPA standards. It is solely the responsibility of any
Linx customer who uses one or more of these products to incorporate
appropriate redundancies and safety standards for the Life and
Property Safety Situation application.
8^
Do not use this or any Linx product to trigger an action directly
from the data line or RSSI lines without a protocol or encoder/
decoder to validate the data. Without validation, any signal from
another unrelated transmitter in the environment received by the module
could inadvertently trigger the action.
All RF products are susceptible to RF interference that can prevent
communication. RF products without frequency agility or hopping
implemented are more subject to interference. This module does not
have a frequency hopping protocol built in.
Do not use any Linx product over the limits in this data guide.
Excessive voltage or extended operation at the maximum voltage could
cause product failure. Exceeding the reflow temperature profile could
cause product failure which is not immediately evident.
Do not make any physical or electrical modifications to any Linx
product. This will void the warranty and regulatory and UL certifications
and may cause product failure which is not immediately evident.
2^
3^
4^
8^
9^
10^
10^
11^
12^
12^
12^
12^
14^
14^
15^
16^
17^
18^
19^
19^
21^
22^
22^
22^
Description
Features
Applications
Ordering Information
Absolute Maximum Ratings
Electrical Specifications
Typical Performance Graphs
Pin Assignments
Pin Descriptions
Module Description
Theory of Operation
Compatibility with the KH2 Series
Encoder Operation
Setting the Transmitter Address
Data Inputs
Enabling Transmission
Input Type Selection
Using LADJ
Power Supply Requirements
Typical Applications
Antenna Considerations
Helpful Application Notes from Linx
Interference Considerations
Pad Layout
Board Layout Guidelines
Microstrip Details
Production Guidelines
Hand Assembly
Automated Assembly
24^
General Antenna Rules
26^ Common Antenna Styles
28^ Regulatory Considerations
30^ Notes
KH3 Series Transmitter Module
Data Guide
Description
1.21 in
The KH3 Series is ideally suited for
(30.73 mm)
volume use in OEM applications such
as remote control and command, and
0.63 in
keyless entry. Housed in a compact SMD (16.00 mm)
package, it combines a highly optimized
RF transmitter with an on-board encoder.
0.106 in
When paired with a matching KH3
(2.69 mm)
Series receiver / decoder module, a
reliable wireless link is formed, capable of
transferring the status of 8 parallel inputs Figure 1: Package Dimensions
over distances of up to 3,000 feet.
Ten address lines provide transmitter uniqueness. No external RF
components are required except an antenna, making integration
straightforward.
Features
•
•
•
•
Low cost
On-board encoder
8 parallel binary inputs
10 addresses for security and
uniqueness
• No external RF components
required
•
•
•
•
•
•
Ultra-low power consumption
Compact SMD package
Stable SAW-based architecture
Adjustable output power
Transmit enable line
No production tuning
Applications
•
•
•
•
Remote control / command
Gate openers
Lighting control
Call systems
•
•
•
•
Remote status monitoring
Home / industrial automation
Remote status / position sensing
Wire elimination
– 1 –
Revised 3/18/2015
Ordering Information
Electrical Specifications
Ordering Information
KH3 Series Transmitter Specifications
Part Number
Description
Parameter
TXM-315-KH3
315MHz Transmitter / Encoder
Power Supply
TXM-418-KH3
418MHz Transmitter / Encoder
TXM-433-KH3
433MHz Transmitter / Encoder
RXM-315-KH3
315MHz Receiver / Decoder
RXM-418-KH3
418MHz Receiver / Decoder
RXM-433-KH3
433MHz Receiver / Decoder
Power-Down Current
EVAL-***-KH3
KH3 Series Basic Evaluation Kit
Transmitter Section
Symbol
Min.
Operating Voltage
VCC
2.7
Average TX Supply Current
ITX
At 3.3dBm
mA
1,2,3,4
1.0
µA
315
MHz
TXM-418-KH3
418
MHz
TXM-433-KH3
433.92
MHz
FC
PO
Harmonic Emissions
PH
−0.3
to
+6.0
VDC
Antenna Port
Any Input or Output Pin
−0.3
to
VCC
VDC
RF Impedance
Operating Temperature
−40
to
+85
ºC
Encoder
Storage Temperature
−55
to
+125
ºC
Data Length
+225ºC for 10 seconds
Exceeding any of the limits of this section may lead to permanent damage to the device.
Furthermore, extended operation at these maximum ratings may reduce the life of this
device.
–75
Output Power
Supply Voltage Vcc
Soldering Temperature
–4
RI N
–1
+75
kHz
2,4
+4
dBm
2,3
–36
dBc
2
Ω
50
26 bits 3x
Average Data Duty cycle
50%
5
Data Input
Logic Low
0.0
Logic High
VCCx0.8
Input Sink Current
Figure 3: Absolute Maximum Ratings
VDC
1.5
IPDN
Center Frequency Accuracy
Absolute Maximum Ratings
5.2
Notes
1,2,4
TXM-315-KH3
Absolute Maximum Ratings
Units
mA
Transmit Frequency
Figure 2: Ordering Information
Max.
2.7
At 0dBm
*** = 315, 418 (Standard), 433MHz
Transmitters are supplied in tubes of 20 pcs.
Typ.
0.6
0.1
0.2xVCC
VDC
5
VCC
VDC
5
5
µA
5
+70
ºC
5
Environmental
Operating Temp. Range
1.
Warning: This product incorporates numerous static-sensitive
components. Always wear an ESD wrist strap and observe proper ESD
handling procedures when working with this device. Failure to observe
this precaution may result in module damage or failure.
– 2 –
2.
Curent draw with 50% mark/space
ratio
Into a 50Ω load
–30
3.
4.
5.
Figure 4: Electrical Specifications
– 3 –
With a 430Ω resistor on LADJ
At 3V and 25ºC
Characterized, but not tested
Typical Performance Graphs
Supply Currnent (mA)
8.00
+85°C
7.00
+25°C
6.00
-40°C
5.00
+25°C
4.00
LADJ = 0Ω
+85°C
3.00
Supply Currnent (mA)
8.00
LADJ = 430Ω
3
3.5
4
4.5
+25°C
5.00
LADJ = 0Ω
4.00
+25°C
3.00
-40°C
+85°C
-40°C
LADJ = 430Ω
1.00
2.5
1.00
2.5
+85°C
6.00
2.00
-40°C
2.00
7.00
5
3
3.5
6
5.5
Supply Voltage (V)
4
4.5
5
5.5
6
Supply Voltage (V)
Figure 7: TXM-433-KH3 Transmitter Supply Current vs. Supply Voltage
Supply Currnent (mA)
8.00
7.00
+85°C
6.00
+25°C
5.00
-40°C
4.00
LADJ = 0Ω
+25°C +85°C
3.00
-40°C
2.00
LADJ = 430Ω
1.00
Transmitter Output Power (dBm)
Figure 5: TXM-315-KH3 Transmitter Supply Current vs. Supply Voltage
11.00
-40°C
9.00
+25°C
LADJ = 0Ω
+85°C
7.00
-40°C
5.00
+25°C
3.00
1.00
+85°C
-1.00
LADJ = 430Ω
-3.00
-5.00
2.5
3
3.5
4
4.5
Supply Voltage (V)
Figure 6: TXM-418-KH3 Transmitter Supply Current vs. Supply Voltage
– 4 –
5
5.5
6
2.5
3
3.5
4
4.5
Supply Voltage (V)
Figure 8: TXM-315-KH3 Transmitter Output Power vs. Supply Voltage
– 5 –
5
5.5
6
3.00
+25°C
9.00
-40°C
LADJ = 0Ω
7.00
+85°C
5.00
-40°C
3.00
+25°C
1.00
+85°C
-1.00
LADJ = 430Ω
-3.00
Transmitter Current (mA)
Transmitter Output Power (dBm)
11.00
-5.00
2.5
3
3.5
4
4.5
5
5.5
2.50
2.00
1.50
1.00
0.50
-7.00
6
-6.00
-5.00
-4.00
-3.00
Supply Voltage (V)
LADJ = 0Ω
9.00
-40°C
5.00
+25°C
3.00
1.00
+85°C
3.00
4.00
7.00
6.00
5.00
4.00
3.00
2.00
LADJ = 430Ω
-3.00
2.00
8.00
+85°C
-1.00
1.00
9.00
+25°C
7.00
0.00
10.00
-40°C
11.00
-1.00
Figure 11: KH3 Series Transmitter Output Power vs. Supply Current at 3.0V
Attenuation (dB)
Transmitter Output Power (dBm)
Figure 9: TXM-418-KH3 Transmitter Output Power vs. Supply Voltage
13.00
-2.00
Output Power (dBm)
1.00
-5.00
0.00
2.5
3
3.5
4
4.5
Supply Voltage (V)
Figure 10: TXM-433-KH3 Transmitter Output Power vs. Supply Voltage
– 6 –
5
5.5
6
0
200
400
600
800
LADJ Resistance (Ohms)
Figure 12: KH3 Series Transmitter Output Power Attenuation vs. LADJ Resistor
– 7 –
1000
1200
Pin Assignments
Module Description
1
LADJ/GND
ANT
24
2
D0
GND
23
3
D1
A9
22
4
GND
A8
21
5
VCC
A7
20
6
TE
A6
19
7
D2
A5
18
A4
17
D4
A3
16
10
D5
A2
15
11
D6
A1
14
12
D7
A0
13
25 26
27
D_CFG
A_CFG1
D3
A_CFG0
8
9
Figure 13: KH3 Series Transmitter Pin Assignments (Top View)
Pin Descriptions
The KH3 Series transmitter / encoder module combines a
high-performance Surface Acoustic Wave (SAW) based transmitter with an
on-board remote control encoder. When combined with a Linx KH3 Series
receiver / decoder, a highly reliable RF link capable of transferring control or
command data over line-of-sight distances of up to 3,000 feet is formed.
The module accepts up to 8 parallel inputs, such as switches or contact
closures, and provides ten address lines for creating unique transmitter
/ receiver relationships. The KH3’s compact surface-mount package
integrates easily into existing designs and is friendly to hand production or
automated assembly.
50Ω RF OUT
(ANT)
SAW
Oscillator
1
Address Inputs
A0-A9
D_CFG
D_CFG0
D_CFG1
Keyed Output
Pin Descriptions
Pin Number
TX Enable
Data Out
Name
GND /
LADJ
I/O
Description
—
Level Adjust. This line adjusts the output power level
of the transmitter. Connecting to GND gives the
highest output, while placing a resistor to GND lowers
the output level.
Data Input Lines. When TE goes high, the module
encodes the state of these lines for transmission.
Upon receipt of a valid transmission, the receiver /
decoder replicates these lines on its output lines.
These lines are pulled to GND internally.
2, 3, 7, 8, 9,
10, 11,12
D0 to D1
I
4, 23
GND
—
Analog Ground
5
VCC
—
Supply Voltage
6
TE
I/O
Transmit Enable Line. When this line goes high, the
module encodes the states of the address and data
lines into a packet and transmits the packet three
times.
13, 14, 15,
16, 17, 18,
19, 20, 21, 22
A0 to A9
I
Address Lines. The state of these lines must match
the state of the receiver’s address lines in order for a
transmission to be accepted. These lines are pulled
to VCC internally.
24
ANT
—
50-ohm RF Output
25
D_CFG
I/O
Data Line Configuration. Determines whether a low
on a data line is interpreted as a zero bit or an open
bit. See the Input Type Selection section. This line is
pulled to GND internally.
26, 27
A_CFG0 /
A_CFG1
Output Isolation
& Filter
RF Amplifier
RF STAGE
ENCODER STAGE
Figure 15: KH3 Series Transmitter Block Diagram
Address Configuration. These lines determine the
address bit type interpretation. See the Input Type
Selection section. A_CFG0 is pulled to GND and
A_CFG1 is pulled to VCC internally.
Figure 14: KH3 Series Transmitter Pin Descriptions
– 8 –
– 9 –
Parallel
Inputs
D0-D7
Theory of Operation
Encoder Operation
The KH3 Series transmitter operation is straightforward. When the Transmit
Enable (TE) line is taken high, the on-board encoder IC is activated. The
encoder detects the logic states of the data and address lines. These
states are formatted into a 3-word transmission, which continues until the
TE line is taken low. The encoder creates a serial data packet that is used
to modulate the transmitter.
The KH3 Series transmitter internally utilizes
the DS Series encoder. The encoder begins
a three-word transmission cycle when the
Transmission Enable line (TE) is pulled high.
This cycle repeats itself for as long as the TE
line is held high. Once TE falls low, the encoder
completes its final cycle and then stops as
shown in the Encoder / Decoder Timing diagram
(Figure 16). When a transmission enable signal
is applied, the encoder scans and transmits the
status of the 10 bits of the address code and the
8 bits of the data serially in the order A0 to A9,
D0 to D7.
The transmitter section is based on a simple, but highly-optimized,
architecture that achieves a high fundamental output power with low
harmonic content. This ensures that most approval standards can be
met without external filter components. The KH3 Series transmitter is
exceptionally stable over variations in time, temperature, and physical
shock as a result of the precision SAW device that is incorporated as the
frequency reference.
The transmitted signal may be received by a Linx KH3 Series receiver
/ decoder module or a Linx LR Series receiver combined with the
appropriate decoder IC. Once data is received, it is decoded using a
decoder IC or custom microcontroller. The transmitted address bits are
checked against the address settings of the receiving device. If a match
is confirmed, the decoder’s outputs are set to replicate the transmitter’s
inputs.
Power On
Standby Mode
No
Transmission
Enabled?
Yes
3 Data Words
Transmitted
No
Transmission
Still Enabled?
Yes
The state of address / data pins can be
3 Data Words
Transmitted
interpreted as ONE, ZERO or OPEN bits,
Continuously
following the logic of the D_CFG, A_CFG0 and
A_CFG1 inputs. See the Input Type Selection
Figure 16: Encoder Flowchart
section for more details. The open bit on the data
input is interpreted as logic low by the decoders since the decoder output
only has two states. The address pins are usually set to transmit particular
security codes by DIP switches or PCB wiring, while the data is selected
using push buttons or electronic switches.
Compatibility with the KH2 Series
The Legacy KH2 Series used encoders and decoders for Holtek® and the
KH3 migrates to the Linx DS Series encoder and decoder. The protocol
and functionality are compatible. There is some difference in the hardware
set-up for the address lines and the data lines. The legacy Holtek®
products used tri-state lines, so high, low and floating were each valid
states. The DS Series has bi-state lines; high and low only. Three lines
have been added to the KH3 module to allow for the selection of how the
address and data line states are interpreted. Please see the Input Type
Selection section for more details.
The KH3 transmitter has been designed to be compatible with legacy
systems. The module has been configured for the most common use of the
KH2 so that it can be placed on existing boards without modification. This
makes the KH3 a drop-in replacement for most applications.
– 10 –
Encoder
Transmit
Enable
Encoder
Data Out
Decoder VT
< 1 Word
3 Words
Transmitted Continuously
2 Words
Check
Check
Decoder
Data Out
Figure 17: Encoder / Decoder Timing Diagram
– 11 –
3 Words
Setting the Transmitter Address
The module has ten address lines. This allows the formation of up to 1,022
(210 – 2) unique transmitter-receiver relationships.
Note: All address lines high or all low is not allowed, so at least one line
must be different from the others.
Because the address inputs have internal pull-up resistors these pins can
be left floating or tied to GND. These pins may be hardwired or configured
via a microprocessor, DIP switch or jumpers. The receiver’s address
line states must match the transmitter’s exactly for a transmission to be
recognized. If the transmitted address does not match the receiver’s local
address, then the receiver will take no action.
lines. Tri-state inputs are connected to ground for zero bits, VCC for one
bits, or left unconnected for open bits. Since the DS cannot match this
operation the D_CFG, A_CFG0 and A_CFG1 lines are provided to select
the desired interpretation. The settings must match on both ends.
Pulling the D_CFG line high configures the data inputs as one and zero.
A high on a data line is interpreted as a one bit and a low on the line is
interpreted as a zero bit. Pulling D_CFG low configures the data inputs as
one and open. A high on a data line is interpreted as a one bit and a low on
the line is interpreted as an open bit. The decoder outputs open data bits
as logic low. This is shown in Figure 18.
D_CFG Configuration
Configuration
Data Inputs
When the Transmit Enable (TE) line goes high, the states of the eight data
input lines are recorded and encoded for transmission. Because the data
inputs have internal pull-down resistors, these pins can be left floating or
tied to VCC. The states of the data lines can be set by switches, jumpers,
microcontrollers or hardwired on the PCB.
The encoder sends the states of the address and data lines three times. If
the TE line is still high, it begins the cycle again. This means that the states
of the data lines are refreshed with each cycle, so the data lines can be
changed without having to pull TE low. There can be up to a 150ms lag
in response as the transmitter finishes one cycle then refreshes and starts
over.
Bit Interpretation
D_CFG
High
Low
0
One
Open
1
One
Zero
Figure 18: D_CFG Configuration
A_CFG0 and A_CFG1 are used to select the bit type for the address lines.
These are shown in Figure 19.
A_CFGO and A_CFG1 Configuration
Configuration
Bit Interpretation
A_CFG1
A_CFG0
High
Low
0
0
One
Zero
Enabling Transmission
0
1
One
Open
The module’s Transmit Enable (TE) line controls transmission status.
When taken high, the module initiates transmission, which continues until
the line is pulled low or power to the module is removed. In some cases
this line will be wired permanently to VCC and transmission controlled by
switching VCC to the module. This is particularly useful in applications where
the module powers up and sends a transmission only when a button is
pressed on the remote.
1
0
Open
Zero
1
1
One
Zero
Input Type Selection
The KH3 Series transmitter incorporates the DS Series remote control
encoder, which is designed to be operable with previous generation
products based on Holtek® encoders and decoders. The Holtek®
encoders and decoders have tri-state input lines but the DS has bi-state
– 12 –
Figure 19: A_CFG0 and A_CFG1 Configuration
D_CFG is pulled low internally so that a high on a data line is transmitted
as a one bit and a low on the line is transmitted as an open bit. A_CFG0
is pulled low and A_CFG1 is pulled high internally so that a high on an
address line is interpreted as an open bit and a low as a zero bit.
This configuration matches the Linx OEM products and the most common
implementation of the legacy KH2 Series. This enables customers using
the KH2 Series to populate the KH3 Series without any PCB modifications
since pins 25, 26 and 27 can be left unconnected.
– 13 –
Using LADJ
Typical Applications
The LADJ line allows the transmitter’s output power to be easily adjusted
for range control, lower power consumption, or to meet legal requirements.
This is done by placing a resistor between GND and LADJ. When LADJ is
connected directly to GND, the output power is at its maximum. Placing a
resistor lowers the output power by up to 7dB, as shown in Figure 12.
Figure 21 shows an example of a basic remote control transmitter utilizing
the KH3 Series transmitter.
VCC
0 ohm
TXE-XXX-KH3
VCC
5
+
6
GND
7
8
9
10
11
The module does not have an internal
Vcc TO
MODULE
voltage regulator; therefore it requires a
clean, well-regulated power source. While it
10Ω
is preferable to power the unit from a battery,
Vcc IN
it can also be operated from a power supply
10µF
as long as noise is less than 20mV. Power
supply noise can affect the transmitter
modulation; therefore, providing a clean
power supply for the module should be a
Figure 20: Power Supply Filter
high priority during design.
GND
D1
A9
GND
A8
VCC
A7
TE
A6
D2
A5
D3
A4
D4
A3
D5
A2
D6
D7
25
12
Power Supply Requirements
D0
A_CFG1
4
GND
ANT
A_CFG0
This is very useful during FCC testing to compensate for antenna gain
or other product-specific issues that may cause the output power to
exceed legal limits. A variable resistor can be used so that the test lab can
precisely adjust the output power to the maximum level allowed by law.
The resistor’s value can be noted and a fixed resistor substituted for final
testing. Even in designs where attenuation is not anticipated, it is a good
idea to place a resistor pad connected to LADJ and GND so that it can be
used if needed.
3
LADJ/GND
A1
A0
24
23
GND
22
21
20
19
18
17
16
1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
15
14
13
12
11
15
GND
14
13
27
2
D_CFG
1
26
GND
100k
GND
+
A 10-ohm resistor in series with the supply followed by a 10µF tantalum
capacitor from VCC to ground will help in cases where the quality of supply
power is poor. These values may need to be adjusted depending on the
noise present on the supply line.
Figure 21: KH3 Series Transmitter Typical Application Circuit
The data lines are connected to buttons. When a button is pressed it takes
the corresponding data line high and the TE line to begin the transmission
process. Since the data pins are internally pulled down to GND, no
pull-down resistors are needed.
Diodes are used to pull the TE line high when any data line goes high, while
isolating the data lines from each other. This makes the transmitter send
data when any button is pressed without affecting any of the other data
lines.
A ten-position DIP switch is used to set the address to either ground or
VCC. Since the address lines are internally pulled up to VCC, no pull-up
resistors are needed.
A resistor is placed on the LADJ line going to GND. This allows the
transmitter output power to be adjusted if needed.
– 14 –
– 15 –
Antenna Considerations
Helpful Application Notes from Linx
The choice of antennas is a
critical and often overlooked
design consideration. The range,
performance and legality of an RF
link are critically dependent upon the
antenna. While adequate antenna
performance can often be obtained
by trial and error methods, antenna
Figure 22: Linx Antennas
design and matching is a complex
task. Professionally designed antennas such as those from Linx (Figure
22) help ensure maximum performance and FCC and other regulatory
compliance.
It is not the intention of this manual to address in depth many of the issues
that should be considered to ensure that the modules function correctly
and deliver the maximum possible performance. We recommend reading
the application notes listed in Figure 23 which address in depth key areas
of RF design and application of Linx products. These applications notes are
available online at www.linxtechnologies.com or by contacting Linx.
Linx transmitter modules typically have an output power that is higher
than the legal limits. This allows the designer to use an inefficient antenna
such as a loop trace or helical to meet size, cost or cosmetic requirements
and still achieve full legal output power for maximum range. If an efficient
antenna is used, then some attenuation of the output power will likely be
needed. This can easily be accomplished by using the LADJ line.
Helpful Application Note Titles
Note Number
Note Title
AN-00100
RF 101: Information for the RF Challenged
AN-00126
Considerations for Operation Within the 902–928MHz Band
AN-00130
Modulation Techniques for Low-Cost RF Data Links
AN-00140
The FCC Road: Part 15 from Concept to Approval
AN-00150
Use and Design of T-Attenuation Pads
AN-00300
Addressing Linx OEM Products
AN-00500
Antennas: Design, Application, Performance
AN-00501
Understanding Antenna Specifications and Operation
Figure 23: Helpful Application Note Titles
It is usually best to utilize a basic quarter-wave whip until your prototype
product is operating satisfactorily. Other antennas can then be evaluated
based on the cost, size and cosmetic requirements of the product.
Additional details are in Application Note AN-00500.
– 16 –
– 17 –
Interference Considerations
Pad Layout
The RF spectrum is crowded and the potential for conflict with unwanted
sources of RF is very real. While all RF products are at risk from
interference, its effects can be minimized by better understanding its
characteristics.
The pad layout diagram in Figure 24 is designed to facilitate both hand and
automated assembly.
Interference may come from internal or external sources. The first step
is to eliminate interference from noise sources on the board. This means
paying careful attention to layout, grounding, filtering and bypassing in
order to eliminate all radiated and conducted interference paths. For
many products, this is straightforward; however, products containing
components such as switching power supplies, motors, crystals and other
potential sources of noise must be approached with care. Comparing your
own design with a Linx evaluation board can help to determine if and at
what level design-specific interference is present.
External interference can manifest itself in a variety of ways. Low-level
interference produces noise and hashing on the output and reduces the
link’s overall range.
High-level interference is caused by nearby products sharing the same
frequency or from near-band high-power devices. It can even come from
your own products if more than one transmitter is active in the same area.
It is important to remember that only one transmitter at a time can occupy
a frequency, regardless of the coding of the transmitted signal. This type of
interference is less common than those mentioned previously, but in severe
cases it can prevent all useful function of the affected device.
Although technically not interference, multipath is also a factor to be
understood. Multipath is a term used to refer to the signal cancellation
effects that occur when RF waves arrive at the receiver in different phase
relationships. This effect is a particularly significant factor in interior
environments where objects provide many different signal reflection paths.
Multipath cancellation results in lowered signal levels at the receiver and
shorter useful distances for the link.
0.07 in
(1.78 mm)
0.065 in
(1.65 mm)
0.046 in
(1.17 mm)
0.096 in
(2.44 mm)
0.10 in
(2.54 mm)
0.10 in
(2.54 mm)
0.274 in
(6.96 mm)
0.61 in
(15.49 mm)
0.14 in
(3.56 mm)
Figure 24: Recommended PCB Layout
Board Layout Guidelines
The module’s design makes integration straightforward; however, it
is still critical to exercise care in PCB layout. Failure to observe good
layout techniques can result in a significant degradation of the module’s
performance. A primary layout goal is to maintain a characteristic
50-ohm impedance throughout the path from the antenna to the module.
Grounding, filtering, decoupling, routing and PCB stack-up are also
important considerations for any RF design. The following section provides
some basic design guidelines.
During prototyping, the module should be soldered to a properly laid-out
circuit board. The use of prototyping or “perf” boards results in poor
performance and is strongly discouraged. Likewise, the use of sockets
can have a negative impact on the performance of the module and is
discouraged.
The module should, as much as reasonably possible, be isolated from
other components on your PCB, especially high-frequency circuitry such as
crystal oscillators, switching power supplies, and high-speed bus lines.
When possible, separate RF and digital circuits into different PCB regions.
– 18 –
– 19 –
Make sure internal wiring is routed away from the module and antenna and
is secured to prevent displacement.
Do not route PCB traces directly under the module. There should not be
any copper or traces under the module on the same layer as the module,
just bare PCB. The underside of the module has traces and vias that could
short or couple to traces on the product’s circuit board.
The Pad Layout section shows a typical PCB footprint for the module. A
ground plane (as large and uninterrupted as possible) should be placed on
a lower layer of your PC board opposite the module. This plane is essential
for creating a low impedance return for ground and consistent stripline
performance.
Use care in routing the RF trace between the module and the antenna
or connector. Keep the trace as short as possible. Do not pass it under
the module or any other component. Do not route the antenna trace on
multiple PCB layers as vias add inductance. Vias are acceptable for tying
together ground layers and component grounds and should be used in
multiples.
Microstrip Details
A transmission line is a medium whereby RF energy is transferred from
one place to another with minimal loss. This is a critical factor, especially
in high-frequency products like Linx RF modules, because the trace
leading to the module’s antenna can effectively contribute to the length
of the antenna, changing its resonant bandwidth. In order to minimize
loss and detuning, some form of transmission line between the antenna
and the module should be used unless the antenna can be placed very
close (
