AN19219 WirelessUSB Crystal Guidelines.pdf

WirelessUSB™ Crystal Guidelines
Author: Rich Peng
Associated Project: No
Associated Part Family: CYWUSB6934, CYWUSB6935
Software Version: NA
Related Application Notes: None
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AN19219 describes how to design a crystal layout and choose the correct crystal for a WirelessUSB™ system. It also
describes PCB layout, crystal PPM, load capacitance, and frequency measurements.
A properly designed WirelessUSB™ system can easily
operate within a 10 m range. Carefully designed
WirelessUSB systems can operate beyond this 10 meter
range. Many system design parameters will affect the
range of your system. One of the most important of these
is a properly designed clock source.
Table 1. WirelessUSB Crystal Requirements
Nominal frequency
13 MHz
Operating mode
Fundamental mode
Resonance mode
Parallel resonance
Frequency stability
±50 ppm (total, including stability,
temperature, and aging – 5 years)
Series resistance
100 ohms
A good stable clock and its frequency are two of the most
important parts of a wireless system. If the radios in a
wireless system are not operating on the same frequency,
they will not be able to talk to one another.
Load capacitance
10 pF
Trim sensitivity
< 25 ppm/pF
Drive level
100 μW
Clock Requirements
Temperature range
–10 ° C to 70 °C
WirelessUSB requires a clock frequency of 13 MHz. The
output RF frequency ranges from 2.400 GHz to 2.480 GHz
in 1 MHz increments (the channels are 1 MHz apart). This
output frequency is produced by using the input clock as
the frequency reference for a VCO and PLL. The accuracy
and stability of the input clock depend on the external
crystal circuitry. Table 1 and Table 2 show the crystal
requirements for WirelessUSB and WirelessUSB LR,
Table 2. WirelessUSB LR Crystal Requirements
Nominal frequency
13 MHz
Operating mode
Fundamental mode
Resonance mode
Parallel resonance
Frequency stability
±30 ppm (total, including stability,
temperature, and aging – 5 years)
Series resistance
100 ohms
Load capacitance
10 pF
Trim sensitivity
< 25 ppm/pF
Drive level
100 μW
Temperature range
–40 ° C to 85 °C
Document No. 001-19219 Rev. *C
WirelessUSB™ Crystal Guidelines
Clock Frequency
Load Capacitance
WirelessUSB is designed to run with an input clock
frequency of 13 MHz. An input clock running at 5416 Hz
off of 13 MHz will produce an output RF that will be off by
1 MHz or one channel. WirelessUSB will operate in
system with both radios’ input clocks at 13005416 Hz, but
its RF frequency will be 1 MHz higher than expected.
Clock Accuracy
As noted in Clock Frequency, WirelessUSB can operate
with its input clock at 5416 Hz off of 13 MHz, but if it is
trying to talk to another WirelessUSB that has an input
clock of 13 MHz, the two radios will not be able to talk
because they are on two adjacent channels. Even if the
input clocks are 2700 Hz (208 ppm) apart, they are
effectively using two different channels. WirelessUSB
needs a more accurate input clock to effectively
communicate. WirelessUSB needs to be paired with
another WirelessUSB that has an input clock within
±50 ppm of its own frequency for effective communication.
A system using WirelessUSB will operate with higher ppm
difference, but performance will decrease in both range
and interference immunity.
PPM is the abbreviation for “parts per million,” a method of
calculation used to specify the permissible frequency
deviation of a crystal or oscillator. One ppm on a 13 MHz
clock is 13 Hz, and 10 ppm is 130 Hz.
Total ppm clock accuracy is the sum of base ppm,
temperature ppm, aging ppm, and trim sensitivity. Total
ppm for a WirelessUSB system should be less than
±50 ppm.
Base PPM
The base ppm of the crystal being used is also known as
the frequency tolerance. Frequency tolerance is the
allowable deviation from nominal frequency. Tolerance is
usually specified in ± ppm, at +25 °C and a specific load
capacitance. Typical tolerances are from ±10 to 50 ppm.
Temperature PPM
Temperature PPM is also known as frequency stability.
Frequency stability is the allowable deviation, in ppm, over
a specified temperature range. Deviation is referenced to
the measured frequency at +25 °C. Typical frequency
stability numbers range from ±10 to 30 ppm.
Temperature ppm can be de-rated by de-rating the
temperature range of the product.
Aging PPM
Load capacitance is the value of capacitance used in
conjunction with the crystal unit in a parallel resonant
oscillator circuit. In a typical system, the load capacitance
of WirelessUSB and PCB layout is 10 pF. Load
capacitance of WirelessUSB is typically 7 pF, but can vary
10% from radio to radio. Load capacitance also varies
from one layout to the next and depends on signal routing,
pad size, and layer stack-up.
Pullability and Trim Sensitivity
Pullability is the change in crystal oscillator frequency due
to a change in the load capacitance. This is due to the
change in parallel resonant frequency when the load
capacitance is changed. Changing the frequency by
changing the load capacitance is referred to as “pulling.”
The frequency can be pulled in a parallel resonant circuit
by changing the value of load capacitance. A decrease in
load capacitance causes an increase in frequency, and an
increase in load capacitance causes a decrease in
Trim sensitivity is very closely related to pullability. In
practical terms, the two are often interchangeable. Trim
sensitivity is a measure of the incremental fractional
frequency change for an incremental change in the value
of load capacitance. Trim sensitivity is expressed in terms
of ppm/pF.
Typical trim sensitivities range from 5 to 30 ppm/pF.
Crystal Choice Considerations
The easy choice is to pick a crystal with low total ppm.
Crystals with low total ppm can be expensive, so some
trade-offs can be made for lower-cost crystals. In a single
system, all crystals used with WirelessUSB should be the
same type. Crystals of the same type will have similar
frequency stability and aging characteristics. Because
most systems will be in the same environment (especially
HID systems), the temperatures in the system will be
similar. Also, all parts of the system will be built at about
the same time. Using crystals of the same type will
therefore reduce the effects of temperature and aging
Crystal Tuning
WirelessUSB can be tuned. The load capacitance in the
WirelessUSB circuit is adjustable; this will adjust the load
resonant frequency of the whole oscillator circuit. One
drawback of tuning the oscillator circuit is the additional
load capacitance. Too much load capacitance will cause a
slow startup time, and should not be used in systems that
require fast clock startup time.
Aging is the change in the frequency of a quartz crystal
unit with the passage of time. A typical aging ppm is
2 ppm per year. When you are selecting a crystal, you
need to consider how aging ppm will affect product
Document No. 001-19219 Rev. *C
WirelessUSB™ Crystal Guidelines
Clock Frequency Measurements
The LS radio clock frequency can be measured with a
frequency counter on the X13OUT pin. Because this
signal is not used in most systems, and the QFN package
is difficult to probe, a PCB test point is recommended for
this signal. The clock output can be enabled with firmware
by writing the clock register in the LS radio. During normal
operations this pin should be disabled to remove it as a
possible noise source on the PCB and to reduce current
Crystal Layout
The ideal layout would have the crystal on the same side
of the PCB as the radio and placed close to the crystal
signal pins of the radio, with identical crystal trace lengths.
This placement would keep the crystal trace paths short
and reduce parasitic capacitances, which could produce
noise in the system. The two crystal traces should have
matched length, avoid vias, and have good isolation from
noise sources. The WirelessUSB crystal circuit performs
best when the crystal traces are closely matched in both
lengths and parasitic capacitances.
Crystal and radio on same side of PCB.
Avoid vias on crystal traces.
Crystal placed near crystal pin on the LS radio.
Crystal trace paths short as possible.
Match crystal trace path for length and parasitic
Isolate crystal from noise sources.
Crystal layouts should be identical for all radios in the
system. By keeping layouts identical, the parasitic
capacitance on the crystal traces will be similar for each
PCB in the system. Similar parasitic capacitance will
produce similar load capacitance, thus reducing the effect
of trim sensitivity ppm on each radio of the system.
On multilayer PCBs, one way to reduce parasitic
capacitance on the crystal is to void the internal layers
directly beneath the crystal pads (see Figure 1). This is
highly recommended when systems are composed of
PCBs with differing layer counts.
Figure 1. Voiding Internal Layers Beneath Crystal Pads
About the Author
You must carefully design the layout and choose the
correct crystal for a WirelessUSB system. If you follow the
guidelines in this application note, you can easily produce
a stable clock source for WirelessUSB system.
Rich Peng.
Applications Engr Principal
Document No. 001-19219 Rev. *C
WirelessUSB™ Crystal Guidelines
Document History
Document Title: WirelessUSB™ Crystal Guidelines - AN19219
Document Number: 001-19219
Orig. of
Description of Change
New application note.
Added Abstract.
Added Summary.
Applied new template.
No change.
Updated to new template.
Completing Sunset Review.
Document No. 001-19219 Rev. *C
WirelessUSB™ Crystal Guidelines
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Document No. 001-19219 Rev. *C