AN64285 Wirelessusb NL Low Power Radio Recommended Usage and PCB Layout.pdf

AN64285
Wirelessusb™ NL Low Power Radio Recommended Usage and PCB Layout
Associated Project: Yes
Associated Part Family: CYRF8935
Software Version: NA
Related Application Notes: None
To get the latest version of this application note, or the associated project file, please visit
http://www.cypress.com/go/AN64285.
Wireless products are now found in abundance, all around us today, and their usage continues to rapidly expand. This
application note introduces the Cypress CYRF8935 device and a sample PCB design. The appendix section guides you
on antenna selection process.
Contents
Introduction
Introduction ....................................................................... 1
Files List ............................................................................ 1
Understanding the Schematic ........................................... 1
DC Power ..................................................................... 1
MCU Interface .............................................................. 2
Crystal Oscillator .......................................................... 2
RF/Antenna Connection ............................................... 2
Schematic Diagram ........................................................... 3
PCB Layout Notes ............................................................. 4
Bill of Materials .................................................................. 7
Appendix A – Antenna Recommendations ........................ 8
Appendix B – PIFA Dimensions ........................................ 9
Worldwide Sales and Design Support ............................. 11
This note describes the NL RF module reference design,
based on the CYRF8935 device. Various antenna choices
are introduced, enabling designers to easily add 2.4 GHz
wireless linking to a product.
Files List
The following PC files accompany this application note.
File Name
Contents
001-64285.pdf
This file.
NL_Altium.zip
Altium schematic and
PCB layout.
NL_OrCAD.zip
OrCAD schematic of module,
OrCAD reference design.
Antennas.zip
Documents for various antennas
discussed.
Understanding the Schematic
The schematic has four (4) basic categories of circuitry:
DC Power, MCU interface, Crystal Oscillator, and
RF/Antenna Connection.
DC Power
Refer to pins 1, 2, 5, 8, 9, 19, and 22 of the device, on the
schematic.
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Document No. 001-64285 Rev. *B
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WirelessUSB™ NL Low Power Radio Recommended Usage and PCB Layout
The CYRF8935 features an on-chip low dropout (LDO)
regulator that is optimized for the analog mixed signal
(AMS) and radio frequency (RF) needs of the device. The
LDO regulator input pin, VIN, connects to input power,
while the output pin, VOUT, connects to each of the VDD
pins of the device, as shown in the schematic.
It is recommended that VDD bypass capacitors be placed
approximately at the corners of the chip. See the sample
layout shown in the figures.
Note that if the source of DC input power is from a
switching power converter that may have high noise,
additional DC filtering may be needed. Schematic note 3
details this, and provides initial guidance.
Note that a separate VDD_IO pin is provided. Internal to the
CYRF8935 device, this voltage drives the MCU interface
I/O pads. In other words, voltage on this pin sets the
expected I/O voltages. It normally connects to the same
voltage source as the MCU.
MCU Interface
SPI: Refer to pins 13, 15, 16, and 17 of the device, on the
schematic. The primary MCU interface is a conventional
4-wire SPI.
IRQ: Refer to pins 10 and 14 of the device, on the
schematic. The CYRF8935 can generate two IRQ signals
for the MCU. Most applications will only need the PKT IRQ
output. PKT is used to inform the MCU that a packet is
ready to be received, or a packet has been transmitted.
FIFO is an IRQ that indicates the internal FIFO is running
high (for Rx), or low (for Rx), and may need imminent read
or write by the MCU.
In addition, the CYRF8935 expects a reset signal to
initialize the device. This signal derives from a dedicated
GPIO.
Correct crystal loading is checked by measuring the
frequency on BRCLK, pin 18. At power-up, it should
measure 3.000,00 MHz; within approximately 10 ppm to
20 ppm error (It is better if the frequency error is at the
least). Frequency on BRCLK is only accessible on bare
die parts. For packaged parts, crystal loading is checked
by measuring transmit carrier frequency; see datasheet for
instructions. Under room temperature, nominal test
conditions, frequency error should measure no more than
10 to 20 ppm. See datasheet for more in-depth detail
regarding crystal specifications.
For proper startup, it is normally the Rs specification that
is most important. See datasheet for maximum Rs
specification for the crystal.
RF/Antenna Connection
Refer to pins 3 and 4 of the device, on the schematic. The
shunt capacitor and series inductor serve to improve the
impedance match between device and antenna. The
values shown should not be modified. When selecting
these components, choose parts that have low loss at RF.
The chosen parts should exhibit a minimum Q of
approximately 20 or better, at 2.4 GHz.
For EMC compliance testing, direct coaxial cable
connection to the RF lab equipment is often helpful, as
shown by the SMA connector in this reference design.
Without the direct coaxial cable connection, a shielded
and calibrated TEM cell test box may be necessary for
some of the regulatory and EMC compliance testing.
Note
See http://www.temcell.com/ and
http://www.rfshieldbox.com/products.html
Crystal Oscillator
Refer to pins 23 and 24 of the device, on the schematic.
A good quality, low-cost external quartz crystal element is
used as the frequency standard for the RF transceiver. For
error free data transmission and reception, the oscillation
must be free of excess phase noise and jitter. Circuit
operation is explained in the CYRF8935 datasheet.
Appropriate crystal loading capacitors must also be
supplied as detailed in the datasheet.
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Document No. 001-64285 Rev. *B
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WirelessUSB™ NL Low Power Radio Recommended Usage and PCB Layout
Schematic Diagram
Figure 1. Schematic Diagram for the PCB
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Document No. 001-64285 Rev. *B
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WirelessUSB™ NL Low Power Radio Recommended Usage and PCB Layout
PCB Layout Notes
Following are the general guidelines to be observed as a matter of good design practice.
Highly important items are underlined, and their application is illustrated on the layout figures.

RF layout guidelines




Use ground vias liberally
Keep RF traces short for low-loss
Crystal layout guidelines





Solid RF ground plane between RFIC and Antenna.
Minimal ground noise on the crystal load capacitors
Ground via for each capacitor
Ground the crystal case if possible
Keep some separation between XTALi and XTALo. Grounds fill between the crystal leads.
Miscellaneous layout guidelines

Ground vias under or at critical components
a. RFIC
b.
Crystal
c.
Decoupling capacitors
d.
RF transmission line
Figure 2. Top view of PWB. Top layer of PWB (red)
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Document No. 001-64285 Rev. *B
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WirelessUSB™ NL Low Power Radio Recommended Usage and PCB Layout
Figure 3. Top view of PWB, looking through the top layer to view the bottom layer of PWB (blue)
Overall dimensions:
1250 x 850 Mils
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Document No. 001-64285 Rev. *B
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WirelessUSB™ NL Low Power Radio Recommended Usage and PCB Layout
Figure 4. Completed PCB Assembly
Current measurement Test Points
To use, cut the 8-mil jumper trace between
the through-holes; install JP1 header pins.
Connect to ammeter.
Header mounted on
back side.
SMA RF
connector
Fiducials for
automated assembly
(3 each).
FIFO and extra
ground pin not
normally needed.
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Document No. 001-64285 Rev. *B
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WirelessUSB™ NL Low Power Radio Recommended Usage and PCB Layout
Bill of Materials
Qty
====
1
2
4
Part Type
Designator Footprint
========================== ========== ==================
1.0pF
C3
0603
15pF
C7 C8
0603
0.10uF
C1 C4 C5
0603
C6 (see sch. note)
1
1
1
1
680 k
2.2 k
20 k
51
R2
R3
R4
R5
0603
0603
0603
0603
1
2.2nH
L1
0603
1
1
SMA-Female
10-pin hdr, 100-mil
J1
P1
SMA_LAUNCH
HDR2X5
1
CYRF8935
U1
QFN24
1
Quartz xtal 12 MHz
Y1
XTL_49SS
C9 R1
JP1 P2
Y2
0603
HDR1X2
XTL_4X2.5
Not used:
2
Note 1
2
Note 1
1
Note 1
Note 1 Do not install parts labeled “Note 1”.
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Document No. 001-64285 Rev. *B
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WirelessUSB™ NL Low Power Radio Recommended Usage and PCB Layout
Appendix A – Antenna Recommendations
The antenna is normally the biggest factor in achieving successful performance. A rigorous antenna tutorial is beyond the
scope of this paper, but some simple antenna recommendations that can be easily applied to CYRF8935 applications are
introduced.
Virtually any type of good-quality 50 ohms, 2.4 GHz antenna can be used with the CYRF8935. Several choices are shown in
Table 1.
Table 1. Antenna Choices
Antenna type
Picture or Drawing
Wiggle antenna
DC Grounded?
See sch. Note 2
Yes
Description/Notes
Described in Cypress Semiconductor
AN48610 (Doc. no. 001-48610)
Cost: Almost free of cost when added to
existing PCB.
Custom
printed-trace
antenna
This is a specialized antenna, customized to
each application.
Chip antenna
Depends on
the design
No
Cost: Almost free of cost when added to
existing PCB.
Easy to use.
Ensure to read the datasheet and follow all
manufacturer instructions. Manufacturer’s
specification for mounting and layout must be
exactly followed.
Model 2450AT18B100E,
Johanson Technology Inc.
Cost: Can be expensive.
PIFA
Yes
The Printed Inverted-F Antenna (PIFA).
Cost: Almost free of cost when added to
existing PCB.
See Appendix B – PIFA Dimensions for detailed
drawing.
1/2 wave end-fed
dipole
No
Delivers ‘textbook’ 0 dBd performance.
Easy removal and replacement.
Accommodates EMC compliance and end
applications.
Requires RF connect on the board.
Cost: Relatively expensive.
Illustrated: Model W1010, by Pulse.
Antenna designs with inherent DC grounding are generally preferred over non-DC grounded designs. Grounded designs have
inherently better ESD immunity, and the need for the 20 K resistor on CYRF8935 ANT pin is eliminated.
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Document No. 001-64285 Rev. *B
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WirelessUSB™ NL Low Power Radio Recommended Usage and PCB Layout
Appendix B – PIFA Dimensions
For 2.4 GHz. Substrate material is 1/32 inch (0.031) FR-4. Note that ideal dimensions may vary slightly, depending on
application.
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Document No. 001-64285 Rev. *B
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WirelessUSB™ NL Low Power Radio Recommended Usage and PCB Layout
Document History
Document Title: WirelessUSB™ NL Low Power Radio Recommended Usage and PCB Layout - AN64285
Document Number: 001-64285
Revision
ECN
Orig. of
Change
Submission
Date
Description of Change
**
3037038
HEMP
09/23/2010
New Application Note.
*A
3349669
HEMP
09/13/2011
Content updated to QFN-24.
*B
4542074
CSAI
10/17/2014
Updated to new template.
Completing Sunset Review.
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Document No. 001-64285 Rev. *B
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WirelessUSB™ NL Low Power Radio Recommended Usage and PCB Layout
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