RF Design Considerations for 802.15.4 Hardware …...RF Design Considerations for 802.15.4 Hardware Development June, 2010 Mark Williams Applications Manager FTF-ENT-F0515 TM Freescale,
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►After completion of this course you should:• Understand the requirements of RF and wireless design• Understand the function of antennas and RF matching networks• Understand the special requirements of RF PCB layout• Understand the benefits of using Freescale’s 802.15.4
reference designs• Understand how to verify your design• Understand the need for, and requirements of, certification
►This course will NOT make you an RF engineer. But you will gain an appreciation for the specialty of RF engineering, and be able to provide guidance for experienced hardware engineers to create successful 802.15.4 designs
►Every trace on a circuit board is a circuit element that cannot be ignored• Circuit traces become transmission lines of a given electrical length• Short lengths of line tend to be capacitive, longer length inductive
►Ground connections are difficult to create since “every trace is a circuit element”
►Any conductor will be a radiator — i.e. an antenna — at RF frequencies
►RF design techniques need to be applied above a few hundred MHz
►Chip components become complex circuits above a few hundred MHz
►High power RF (> 20 dBm) accentuates these issues
► Antennas are used at frequencies as low as 30 Hz (submarine ELF, extremely low frequency) up into the hundreds of GHz (EHF, extremely high frequency). This presentation focuses on 2.4 GHz applications.
► The characteristics of the antenna (gain, impedance, polarization, size, cost, etc.) should suit the application.
► This means that an antenna suitable for one application may not be efficient for another.
►Antenna Gain • dBi is the most common reference used
for antenna gain. This is gain referenced to an “isotropic” antenna, which is a theoretical perfect sphere which radiates equally in all directions (0 dB gain in all directions)
►No such antenna actually exists, but it serves as a good reference standard.
►Sometimes dBd, which is gain referenced to a horizontal dipole in space, is used. Most references define 1 dBd = 2.14 dBi. A horizontal dipole at the correct spacing above a ground plane can have gain greater than 2.14 dBi.
►Gain in one direction is only obtained by reducing gain in another direction. That is, a “high gain” antenna only has gain in one or more directions. In other directions, the gain is significantly reduced (nulls).
►Proper antenna orientation is required to ensure that the gain peak is aimed towards the desired signal.
►So, high gain is not always good(!). For handheld or portable applications where the orientation of the device is not controlled, a high-gain, highly directional antenna may not work very well for the end user.
►High-gain antennas can work very well in fixed applications.
►The horizontal ½ wave dipole is the most basic antenna.
►The radiation pattern is ”doughnut” shaped, a null from the end of each radiator. The impedance at the feedpoint is 73 ohms for a perfect theoretical dipole, at the resonant frequency.
►The dipole is a balanced, differential structure.
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The basic dipole:
Total length is ½ wavelength,(At 2.4 GHz, approx.6.1 cm in air)
► By substituting one leg of the dipole with a conductive ground plane, the remaining part of the dipole ”mirrors” itself in the ground. For a perfect infinite ground plane, the feedpoint impedance is half that of a horizontal dipole, or 36 ohms.
► The ground plane can be the ground plane of a PCB board or metal case.
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A monopole above a groundplane, showing the “mirror”antenna.
►A ground plane needs to be approx. 6 times the wavelength in diameter to act as a theoretically ”infinite” ground plane.
►Anything smaller will change the pattern from the ideal. In general, as the ground plane gets smaller, the peak in the pattern moves up from horizontal, to as high as 30 - 40 degrees above horizontal.
►By bending the monopole into an inverted L shape, the omnidirectional characteristics are improved, but efficiency is slightly lower and impedance is much lower.
►By ”tapping” the monopole we get the inverted F-antenna with good omnidirectional characteristics and a 50 ohm impedance point.
►The F-antenna is a good all-around antenna. It is very commonly used, including on Freescale 802.15.4 evaluation boards.
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Tilted whip and F-antenna:(note the ground plane area)
►Many other antenna types exist. Patch antennas have a patch of metal over a ground plane, and are very directional (used for GPS). Loop antennas are also used in PCB applications. Slot antennas can be thought of as dipoles cut out in metal plates. They are often used in microwave applications.
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Half-wave and full-wave loops:
Half-wave and quarter-wave slot antennas: Patch antennas:A metal patch over a ground plane
►A lot of different chip antennas exist on the market. The usual principles of operation are monopoles, helixes and patch antennas.
►The antennas are made of a low-loss ceramic with high dielectric constant. The high dielectric constant (approx. er=10) shortens the physical length of a wavelength in the material.
►Be careful when comparing size: The actual chip antenna may be very small, but often requires a much larger cleared area and ground plane to be efficient.
►Typical gain ranges from –10 to –2 dBi. Good for very short range applications, such as wireless mouse, keyboard, some remote controls, etc.
►Antenna design requires specialized tools and experience• Don’t change a design unless you know what you are doing and are
ready for multiple board spins and time on the bench►A design is optimized for a particular PCB material and thickness.
Changing either will impact the design►Metal thickness is usually not a concern►Grounding is critical and ground vias are part of the antenna design►Follow a reference design EXACTLY
► Just placing a chip antenna on a PCB layout does not ensure good performance. The bandwidth of the chip itself is very narrow. External matching circuits are used to broaden the response (and center it in-band). Also consider:• Each design must be tuned on the layout and case for which it is placed.• Performance can vary due to production variations.• Consult closely with the manufacturer for applications guidance.
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Johanson Technology, Chip Antenna Layout Considerations for 802.11 ApplicationCenturion Technologies, NanoAntTM Application NoteMurata Manufacturing Company, Application Note of Chip Dielectric Antenna M1 Series/W1 SeriesMitsubishi Materials Corp., TRJC-2F03, Recommended Mount Conditions for 2.4 GHzPulse/Technitrol, Application Note, WLAN 2.4 GHz. AntennaYageo, High Frequency Products (lf28429.pdf)
►What is normally referred to as “antenna” matching is actually matching performed for the benefit of the transmitter.
►The impedance of the antenna is transformed to the TX output impedance, giving maximum power transfer and best overall performance.
►The matching does not usually improve the performance of the antenna — it delivers more TX power into the antenna.
• Return loss: A measure of impedance match. Essentially, the loss in power reflected from the load. Can also be expressed as VSWR.
• Bandwidth: The frequency range over which the return loss is acceptable.• Input impedance: The RF resistance of the antenna at resonance (Fo). The impedance will
vary at other frequencies.• Resonance: Frequency where the antenna impedance is purely resistive
► To tune an antenna, the physical dimensions of the antenna are optimized so that the antenna itself is resonant as intended.
► Antenna tune quality is normally measured by measuring reflected power vs. forward power, or return loss, at the desired frequency(s) of operation. If an antenna is very lossy, it may not work very well but indicate a good return loss. But, in most cases, return loss can be used to determine the center frequency of an antenna.
► Additional matching may be required if the impedance of the antenna, when tuned, is not the same as the TX output impedance — for example, to match a 36 ohm antenna to a 50 ohm RF circuit. Matching is normally accomplished using passive capacitors and inductors that are intended for use at 2.4 GHz.
► In some cases, it may be easier to match and tune at the same time. For example, an antenna that is too short can be matched to 50 ohms in the desired band with series inductance at the input (such as citizens band radio antenna with a large coil at the base).
► In many cases, the RF transceiver will have an interface that is balanced — it has a positive and negative output. This architecture has advantages at the die level, but usually must be converted to single-ended (one side referenced to ground) for most RF circuits. Some sort of conversion circuit is necessary.
► This can be done with a commercial ceramic balun, as shown below (Z100).It can also be performed with a lumped element balun (L/C).
► If the antenna is balanced, such as a dipole, and there are no other single-ended parts such as a TX/RX switch, PA, etc., then it would be possible to stay balanced thoughout and not use a balun (such as the Freescale SARD board).
►Steps to good antenna performance• The antenna should be reasonably clear of metallic objects, but oriented
properly with the ground plane (when one is utilized).• Always check the antenna in its final environment, including the PCB,
components, case, hand effects (if appropriate), battery, etc. Plastic and other materials in the near-field may cause detuning.
• Test actual antenna performance by whatever means available, such as range testing, measuring radiated signal level under controlled conditions, and/or send it out for testing in an anechoic chamber.
• Test samples from volume production to check for production variations.
►Select a chipset vendor with a proven track record• Basic modem performance is a function of the IC design, and an IC
designed for a particular protocol will generally meet the requirements • Range, output power, harmonic and spurious radiation can all be
affected by the design implementation
►Don’t attempt a discrete or low-level integration design unless you know what you are doing.
►Use the manufacturer-recommended reference designs• These have typically been certified and have optimized performance• Follow the design EXACTLY in the RF region of the board
This does not mean the schematic only, but the LAYOUT• Mixed mode regions are ok to modify, as they generally do not
►Non-controlled impedance traces should be kept short• Length traces are lossy and inductive
►Grounding is imperative• Remember the antenna considerations?• Lengths of line are NOT ground but inductors• Consider the concept of “earth” or “chassis” ground
►PC traces are effective radiators at RF frequencies because they are a non-negligible fraction of a wavelength
►Examples of PCB Parasitics:• At 2.4 GHz, a 10 mil wide PCB trace 275 mils long on 32 mil FR4 is equivalent
to a 3.2 nH inductor, +j73 Ohms.• A 10 mil via in 32 mil FR4 is about 0.5 to 1 nH.
• Includes details of recommended footprints, device marking and soldering information for 802.15.4 platforms
• Some of our reference designs and development hardware may deviate slightly from these due to CAD issues or contract manufacturer changes. Either are acceptable
►Soldering of QFN and LGA packages is challenging• Work with a reputable house with experience• Follow their guidelines for solder stencil design• Filled vias are recommended for small vias-in-pads such as with MC13224V• Use a minimum of 5 mils solder thickness to avoid “starvation” of
► MC1321x SRB Development Board: Designed for lab use, code development and experimentation. Lots of stuff….
► MC1321x IPB: Basic RF layout with critical components. Interfaces are pinned out. Designed to be a starting point for OEM designs. Also for add-on to existing hardware
►Numbering Scheme: Platform – Interface, Antenna, Implementation• Interfaces: I = I2C, UART, U = USB • Antennas: P = Printer, C = Chip• Exception: ERB = Extended Range Board
►Perform a basic “smoke” test with a current-limited power supply• Does the unit turn on?• Does it draw the expected current?
►Check the RF performance• Use Freescale’s SMAC software or Test Tool (or your own code) • Check the reference oscillator frequency using a frequency counter
CLKO pin on MC1321x family, TMR2 and connectivity test on MC13224VShould be within 10 ppm of the target frequency
• Measure output power using a power meter or spectrum analyzer May require board modification to make conducted measurementsExpect 1-2 dB variation from data sheet specs and from unit to unit
• Measure the range between two boards using packet error rate (PER) softwareOpen space (outdoors) range for Freescale’s reference designs should be 200-300 meters
►Generally, when electronic hardware will be sold in a country, the certification requirements of that country must be met.
►Certification is usually only required once unless changes are made to the hardware that will affect the RF emission performance.
• Examples: IC changes and software revisions that do not change radio performance are acceptable. (See CFR 47 FCC Part 2.1043)
►Some countries such as Japan require filing notification with the regulatory agency even if engineering or demonstrations will be performed with RF-emitting hardware.
►For operation in the 2.4 GHz band (worldwide):• In the U.S., CFR 47 FCC Part 15.203, 15.205, 15.209 and 15.247• In Canada, RSS-210 which closely follows Part 15• In EU, ETSI EN 300, 301• In Japan, ARIB STD-T66• Other countries generally follow FCC or ETSI
►Consult an expert!• Hire a respected test and certification lab that has experience with unlicensed
band hardware and can file (or help you file) in the countries of interest• Hardware labeling and documentation are critical
►Following are some recommended pre-certification tests that should be performed as part of the design verification.
►FCC Publication 558074 gives more details on measuring digital radios
►Tests are performed using Freescale’s SMAC test mode or connectivity applications in continuous modulation transmission mode (not packet) or PRBS9 random data packet mode
►Similar to spurious conducted; however, measured in a 3 meter RF chamber
►Part 15.247 (c) along with limits in 15.209 and 15.205: 500 µV/m (54 dBµV/m) for average detector since “forbidden bands” are on either side of 2.4 GHz (2.2 – 2.3, 2.31 – 2.39, 2.483 – 2.5 GHz)
►Most OEM labs cannot make radiated measurements, soconducted measurements with antenna gain can be substituted
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Frequency range of Harmonic or SpuriousEmission
Average Limit Converted from Fieldstrength to dBm measured at RF port
Average Fieldstrength Limit of Fundamental (dBμV/m @ 3m)
Peak Field Strength Limit of Fundamental (dBμV/m @ 3m)
Peak Limit Converted from Field strength to dBm measured at RF port
►FCC Publication 558074 specifies continuous modulation mode, not packet
► If the average limit is applied, a duty cycle correction factor can be used per 15.35 (b): On time divided by off time in 100 ms
►Since 802.15.4 is a burst-mode, low duty cycle protocol, a reasonable compromise between ~75 kbps throughput and ZigBee Alliance specification of no more than 10% duty cycle is 17%. One full packet transmitted, no acknowledgement received, the packet is retransmitted 3 more times with no acknowledgement:
• Maximum 15.4 packet contains 133 bytes• At 250 kbps data rate, Tb = 4 µs and TB = 32 µs• 133 byte packet lasts 4.26 ms• 4 transmitted packets is total on time of 17 ms
►Alternatively, peak limits and conditions can be used. The peak limit can be applied with no correction factor.
►Work with an experienced certification lab to help you determine which certification requirements apply to your product
►Realistically assess your RF capability and goals
►Select a proven chipset vendor and follow their reference designs exactly for RF layout
►Perform pre-certification testing or have your certification partner perform the tests to insure you pass before finalizing your design and formally submitting it