AN-1811 Bluetooth Antenna Design (Rev. B)

AN-1811 Bluetooth Antenna Design (Rev. B)
Application Report
SNOA519B – March 2008 – Revised May 2013
AN-1811 Bluetooth Antenna Design
.....................................................................................................................................................
ABSTRACT
This application note is intended for designers using the LMX5251 or LMX5252 Bluetooth® radio chips or
LMX9820A or LMX9830 Bluetooth modules. Antenna design for various applications is described along
with theory, matching circuit description, suppliers and examples.
1
2
3
4
5
6
7
8
9
10
Contents
Introduction .................................................................................................................. 3
Theory ........................................................................................................................ 3
Layout ........................................................................................................................ 7
3.1
PIFA Antenna ....................................................................................................... 7
3.2
Ceramic Dielectric Antenna ....................................................................................... 8
3.3
Examples of 2.4-GHz PIFA Antennas ......................................................................... 10
3.4
LMX9820/A Antennas ............................................................................................ 11
3.5
LMX9830 Antenna ................................................................................................ 12
Matching .................................................................................................................... 12
4.1
Network Analyzer Calibration ................................................................................... 12
4.2
Measurement ..................................................................................................... 13
4.3
Tuning the Impedance ........................................................................................... 14
4.4
PI-Network Matching ............................................................................................. 15
4.5
Matching to a Non-50Ω Active Source/Load Impedance .................................................... 19
Interference Rejection ..................................................................................................... 22
5.1
Filtering ............................................................................................................ 22
5.2
Blocking ............................................................................................................ 24
5.3
Recommended Front-end Layout and Matching ............................................................. 24
Antenna Vendors .......................................................................................................... 25
Comparison Summary .................................................................................................... 26
Points for Consideration .................................................................................................. 26
Examples of Antennas Used With LMX5251/LMX5252 .............................................................. 27
Popular Antenna Types ................................................................................................... 30
List of Figures
1
Fringing Field With Full Ground Plane ................................................................................... 4
2
Fringing Field With Partial Ground Plane ................................................................................ 5
3
Antenna Radiation Pattern ................................................................................................. 6
4
Return Loss .................................................................................................................. 7
5
Printed Inverted-F Antenna (PIFA)
6
PIFA Antenna Placement .................................................................................................. 8
7
Ceramic Dielectric Antenna Placement .................................................................................. 9
8
Typical Chip Dimensions ................................................................................................... 9
9
Typical Chip PCB Footprint .............................................................................................. 10
10
LMX5251 PIFA Antenna .................................................................................................. 10
11
LMX5252 PIFA Antenna .................................................................................................. 11
12
LMX9820A Antenna ....................................................................................................... 11
.......................................................................................
7
All trademarks are the property of their respective owners.
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
AN-1811 Bluetooth Antenna Design
Copyright © 2008–2013, Texas Instruments Incorporated
1
www.ti.com
13
LMX9830 Antenna......................................................................................................... 12
14
Return Loss, No Connection ............................................................................................. 13
15
Smith Chart, Perfect Short ............................................................................................... 13
16
Return Loss, Antenna Connected ....................................................................................... 14
17
Smith Chart, Antenna Connected ....................................................................................... 14
18
Impedance Transformation ............................................................................................... 15
19
Impedance Matching Network ........................................................................................... 15
20
Single Frequency Data Point
21
After Resistance-Conductance Tuning ................................................................................. 16
22
After Conductance-Resistance Tuning ................................................................................. 17
23
Two Possible Matching Networks ....................................................................................... 17
24
Broadband Match .......................................................................................................... 18
25
PI Network .................................................................................................................. 18
26
Component Models........................................................................................................ 19
27
Matching to Non-50Ω Impedance ....................................................................................... 20
28
Setup for Determining Output Impedance.............................................................................. 21
29
RF Filter Performance .................................................................................................... 22
30
LC Filter Response ........................................................................................................ 23
31
Front-end Layout Using Ceramic Filter ................................................................................. 25
32
Front-end Layout Using T PI Pad ....................................................................................... 25
33
Ceramic Chip Antenna for Industrial Remote Control with External PA............................................ 27
34
Printed Antenna—Monopole Yagi-Array for Off-Board Navigation Using GPS ................................... 27
35
Ceramic Chip Antenna for Intelligent Remote Access for Car Lock ................................................ 28
36
Ceramic Chip Antenna for Distance Meter ............................................................................. 28
37
External Antenna—Helix or Monopole for Automotive Integrated Hands-Free Kit
29
38
Printed PIFA Antenna for Automotive Hands-Free Kit
29
39
40
41
42
............................................................................................
...............................
...............................................................
Surface-Mount Chip Antenna (Phycomp AN-2700 or Murata ANCM12G) for Automotive Hands-Free Kit ...
Surface-Mount Chip Antenna (Mitsubishi Materials AHD 1403) for Electronic Whiteboard .....................
GigaAnt Rufa 2.4 GHz SMD Antenna ..................................................................................
Mitsubishi AHD1403 Surface-Mount Antenna .........................................................................
16
30
30
31
31
List of Tables
2
1
Chip Filter Specifications ................................................................................................. 23
2
Blocking Signal Level and Frequency
3
Antenna Vendors .......................................................................................................... 25
4
Antenna Comparison...................................................................................................... 26
..................................................................................
AN-1811 Bluetooth Antenna Design
24
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
Copyright © 2008–2013, Texas Instruments Incorporated
Introduction
www.ti.com
1
Introduction
Any structure that is resonant at 2.45 GHz with bandwidth more than 100 MHz and efficiency >50% can
be considered a Bluetooth antenna. Therefore, a countless variety of antennas can be used, and they are
application-specific. Some common types are:
• Wire Monopole — This consists of a simple wire soldered at one end from which it is fed against a
ground plane. It is trimmed to be resonant at 2.45 GHz and provides good performance and high
efficiency. The disadvantage of this antenna is that it is not low profile because it projects above the
PCB.
• PIFA — The Printed Inverted F Antenna is like a monopole printed on a PCB, but it has a ground point
and feed point along the main resonant structure.
• Helix — Similar to the wire monopole, except that it is coiled around a central core (usually air) making
the physical dimensions smaller. It provides excellent performance, but projects above the PCB.
• Ceramic — Surface mount dielectric antennas are the smallest types of antennas available, because
they are printed on a high-dk ceramic slab, which makes the electric field concentrated allowing the
antenna to be made small while keeping a high resonant frequency.
This application note only describes PIFA and ceramic antennas because they are the most common, lowprofile, smallest, and inexpensive types available.
2
Theory
Printed and surface-mount antennas have certain common properties. Area around and beneath the
radiating element must be kept copper-free. The ground plane must be placed on one side of the radiating
element. Bandwidth is >100 MHz with VSWR <2.5 and efficiency >60%.
The antenna will detune if any object is placed close to it (in its near field). This has an effect of pulling the
frequency, which must be retuned to 2.45 GHz.
An oscillating or constantly accelerating charge is critical in producing propagating waves. A static or nonaccelerating charge will result in a non-propagating electric field. But this is not the only condition for
radiation. For example, consider a printed λg/4 element on microstrip, as shown in Figure 1.
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
AN-1811 Bluetooth Antenna Design
Copyright © 2008–2013, Texas Instruments Incorporated
3
Theory
www.ti.com
Figure 1. Fringing Field With Full Ground Plane
The fringing field around the microstrip due to the ground plane directly underneath the substrate will be
confined to a small area. If a network analyzer is connected to the feed point, it would indicate a high
VSWR and narrow bandwidth. This means very little radiation is being emitted from the microstrip
element.
To increase the radiation emission and achieve greater bandwidth, the ground plane must be moved away
from the microstrip element which makes the fringing field cover more distance, as shown in Figure 2. But
it should be noted that if the ground plane is moved too far, then the fringing field stops altogether, and
there is no radiation. Therefore, the position and size of the ground plane is vital in the design of a good
radiator.
4
AN-1811 Bluetooth Antenna Design
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
Copyright © 2008–2013, Texas Instruments Incorporated
Theory
www.ti.com
Figure 2. Fringing Field With Partial Ground Plane
The antenna could be imagined as an impedance transformer, transforming the impedance of a microstrip
line (50Ω) to that of free space (377Ω), which allows the power to be transferred from a guided wave to a
free-space wave.
The radiation pattern from such antennas in which the physical size is much smaller then wavelength (L
<< λ) is almost symmetrical in all directions, as shown in Figure 3. The pattern can be controlled only
when L is similar or greater than λ.
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
AN-1811 Bluetooth Antenna Design
Copyright © 2008–2013, Texas Instruments Incorporated
5
Theory
www.ti.com
Figure 3. Antenna Radiation Pattern
Input return loss when viewed on a network analyzer looks like that shown in Figure 4, with the full band
covered with VSWR < 2. This gives very good matching into the antenna, however in real conditions when
the antenna is detuned due to handling or placement of components close to it, a VSWR of 3 to 4 is
typical.
6
AN-1811 Bluetooth Antenna Design
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
Copyright © 2008–2013, Texas Instruments Incorporated
Layout
www.ti.com
Figure 4. Return Loss
3
Layout
3.1
PIFA Antenna
The typical length of a 2.45-GHz resonant printed antenna is 20 to 25 mm, depending on the thickness of
the substrate and dielectric constant. Copper clearance is required around the radiating element which is
fed from a point along it, as shown in Figure 5. The position of the feed can be used to control the input
impedance into the antenna. The ground plane required on one side of the antenna is approximately 20
mm wide. If it were any smaller, it will start to reduce the bandwidth at the input. Good design practice is
to have a three-element matching network going into the feed, to give some additional tuning ability if
required. To obtain the exact dimensions of the design, input impedance and bandwidth would have to be
simulated over the frequency band using an antenna simulation package. Alternatively an antenna
manufacturer can be contacted that has the capability to make such a design.
Figure 5. Printed Inverted-F Antenna (PIFA)
The PIFA is placed on the edge of the motherboard PCB, as shown in Figure 6. The area around the
corner is kept copper-free, and any components such as the shielding that come close to the PIFA may
pull its frequency. This can be retuned by milling the end of the radiating element. The
LMX5251/LMX5252 and its surrounding components do not need shielding unless they are very close to
the radiating element.
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
AN-1811 Bluetooth Antenna Design
Copyright © 2008–2013, Texas Instruments Incorporated
7
Layout
www.ti.com
Figure 6. PIFA Antenna Placement
3.2
Ceramic Dielectric Antenna
A ceramic dielectric antenna is smaller than a PIFA or any other PCB antenna because the active element
is wound around a high-dk ceramic slab, which concentrates the electric field. As with a PIFA, a coppercleared area and a ground plane are required, as shown in Figure 7. A smaller ground plane can be used,
at the expense of bandwidth and efficiency.
8
AN-1811 Bluetooth Antenna Design
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
Copyright © 2008–2013, Texas Instruments Incorporated
Layout
www.ti.com
Figure 7. Ceramic Dielectric Antenna Placement
An example antenna from Mitsubishi with details of the ceramic element dimensions is shown in Figure 8.
Figure 8. Typical Chip Dimensions
The land pattern required for mounting on the PCB is shown in Figure 9.
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
AN-1811 Bluetooth Antenna Design
Copyright © 2008–2013, Texas Instruments Incorporated
9
Layout
www.ti.com
Figure 9. Typical Chip PCB Footprint
The ceramic dielectric antenna behaves similarly to a PIFA, in that it can be detuned, has a symmetrical
radiation pattern, and has an efficiency of approximately 70%.
3.3
Examples of 2.4-GHz PIFA Antennas
Figure 10. LMX5251 PIFA Antenna
10
AN-1811 Bluetooth Antenna Design
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
Copyright © 2008–2013, Texas Instruments Incorporated
Layout
www.ti.com
Figure 11. LMX5252 PIFA Antenna
3.4
LMX9820/A Antennas
The LMX9820 and LMX9820A are packaged as shielded LTCC and FR4 modules, approximately 14 × 10
mm in size. The design of its antenna is very similar to that of the LMX5251/LMX5252, but the shielding
makes two differences. First, the metal shield protects the components in the module from the electric field
of the antenna, so it is possible to place the LMX9820A much closer to the antenna element. Second, the
shielding also acts as a ground plane, so less unpopulated ground area is required around the radiating
element.
In the example using a ceramic dielectric antenna shown in Figure 12, the module is placed close to the
radiating element. The electric field from the antenna couples to the surface of the shielded enclosure
producing propagating radiation. If the shield is well-grounded, there will be no adverse effects on the
components inside.
Figure 12. LMX9820A Antenna
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
AN-1811 Bluetooth Antenna Design
Copyright © 2008–2013, Texas Instruments Incorporated
11
Matching
3.5
www.ti.com
LMX9830 Antenna
The LMX9830 is smaller than the 9820/A, approximately 6 × 9mm, however it is unshielded within a
plastic package and so there are some important changes that need to be taken into account. It cannot be
placed as close to the antenna active element, else the E-field may give rise to unwanted coupling effects,
also the E-field from the antenna element will couple though the main body of the module to the ground
plane underneath. PCB ground plane under the module is therefore important.
Figure 13. LMX9830 Antenna
4
Matching
Purchased antennas, such as surface-mount ceramic dielectric antennas, will be matched to 50Ω input
impedance with return loss <-7dB over 100 MHz bandwidth, centered at 2.45 GHz. However, this is only
as measured on the manufacturers test board, in free space. Taking this antenna and putting it on the
application PCB, in which the ground-plane layout may be different or there may be detuning components
such as filters placed nearby, will pull the resonant frequency of the antenna away from 2.45 GHz. The
antenna therefore needs matching to the correct frequency. This can be achieved by means of a threeelement PI network, placed at the input to the antenna. Usually a capacitor pair and an inductor, or an
inductor pair and a capacitor, will give sufficient tuning ability.
There are three steps to matching:
1. The network analyzer must be calibrated accurately with the electrical delay removed.
2. An impedance measurement from 2.300 to 2.600 GHz of the return loss and a Smith-Chart plot.
3. Matching by placing capacitors and/or inductors onto the PCB to see how the impedance is changed.
4.1
Network Analyzer Calibration
The network analyzer should be calibrated for S11, one-port only measurements using the open, short,
and load standard provided. A flat line should be obtained when the standards are removed as shown in
Figure 14.
12
AN-1811 Bluetooth Antenna Design
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
Copyright © 2008–2013, Texas Instruments Incorporated
Matching
www.ti.com
Figure 14. Return Loss, No Connection
Before soldering the semi-rigid cable to the PCB, it is connected to the end of the network analyzer cable,
and the electrical delay is adjusted with the end of the semi-rigid cable shorted. Use a short cable
attachment (less than 5 cm), otherwise the electrical delay will be too long. Electrical delay is adjusted
until it is measuring a perfect short on the Smith Chart as shown in Figure 15.
Figure 15. Smith Chart, Perfect Short
4.2
Measurement
Attach the semi-rigid cable to the PCB and ground it at a point close to the end of the cable. When
measuring the input impedance of the antenna, it is important to have the setup on a wooden or nondetuning surface and to keep your hands away from the setup, otherwise the measurement will be
incorrect. An example of a typical return loss measurement is shown in Figure 16.
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
AN-1811 Bluetooth Antenna Design
Copyright © 2008–2013, Texas Instruments Incorporated
13
Matching
www.ti.com
Figure 16. Return Loss, Antenna Connected
In this example, the resonant frequency of the antenna is 60 MHz too high. At the desired frequency, the
return loss is only -3.3 dB.
Figure 17. Smith Chart, Antenna Connected
4.3
Tuning the Impedance
After taking an accurate measurement of the input impedance, it can be tweaked using the matching
components on the three-element PI network. Marker 1 impedance has to be transformed to 50Ω, as
shown in Figure 18.
14
AN-1811 Bluetooth Antenna Design
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
Copyright © 2008–2013, Texas Instruments Incorporated
Matching
www.ti.com
Figure 18. Impedance Transformation
Starting from the impedance point that needs to be matched (point 1), add a 1.8-nH series inductor to
move from point 1 to 2. A shunt inductor will transform the impedance at point 2 to the center of the chart,
which is the normalized 50Ω impedance point. The matching network is shown in Figure 19.
Figure 19. Impedance Matching Network
This is only a theoretical matching circuit. In reality, the inductors have parasitic resistance and
capacitance, so the impedance will not be transformed as cleanly as shown on the Smith Chart. Also, the
exact values shown above may not be available in a standard kit. Some trial and error is required to get
the exact match required.
4.4
PI-Network Matching
A popular type of matching network is the PI-network, consisting of two shunt components with one series
component in the middle. This provides flexibility for retuning a detuned antenna. Even though only two
components are normally used for matching the load to the source, it allows putting the shunt component
either before or after the series component.
For example, consider the data point on the Smith Chart: (10.2 + j30.1)Ω shown inFigure 20.
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
AN-1811 Bluetooth Antenna Design
Copyright © 2008–2013, Texas Instruments Incorporated
15
Matching
www.ti.com
Figure 20. Single Frequency Data Point
There are two methods for matching to the load. The first technique is to move around a constant
resistance circle from position 1 to 2 by adding a series capacitance and then from 2 to 3 around a
constant conductance circle by adding a shunt capacitance, as shown in Figure 21.
Figure 21. After Resistance-Conductance Tuning
16
AN-1811 Bluetooth Antenna Design
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
Copyright © 2008–2013, Texas Instruments Incorporated
Matching
www.ti.com
The second technique is to move around the conductance circle and then the resistance circle by adding a
shunt and series capacitance respectively, as shown in Figure 22.
Figure 22. After Conductance-Resistance Tuning
The matching networks for the two methods are shown in Figure 23.
Figure 23. Two Possible Matching Networks
To allow both types of matching, a PI-pad must be used with the redundant gap bridged using a zero-Ω
link.
However, so far we have only matched a single-point frequency to 50Ω. In the case of a real passive
device such as an antenna, the entire Bluetooth band has to be matched as closely as possible to 50Ω. At
least three frequency points have to be matched, as shown in Figure 24.
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
AN-1811 Bluetooth Antenna Design
Copyright © 2008–2013, Texas Instruments Incorporated
17
Matching
www.ti.com
Figure 24. Broadband Match
The difficulty with making a broadband match to one frequency point is that the other two will go even
further out! For example, 2.483 GHz can be brought closer to 50Ω by adding a shunt capacitor, but the
2.400 GHz point will move around the conductance circle creating an even larger mismatch at lower
frequencies. A compromise must be found that will suit the entire band. This normally involves using
simulation software such as HP-ADS (advanced design systems). If this is not possible, then a lot of
manual tweaking is needed concentrating on the center frequency point.
First, the input impedance of the detuned antenna is measured using a network analyzer and saved as an
S-parameters block, i.e. frequency points vs. impedance points across the Bluetooth band. This can then
be entered into ADS along with the PI network, as shown in Figure 25.
Figure 25. PI Network
To make the model more realistic, it is more effective to use real components with added parasitics rather
than just pure inductance and capacitance. The models for the components with the parasitics are shown
in Figure 26.
18
AN-1811 Bluetooth Antenna Design
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
Copyright © 2008–2013, Texas Instruments Incorporated
Matching
www.ti.com
Figure 26. Component Models
Data for the parasitic values can be obtained from the component manufacturer.
Starting with the best possible values used to match 2.445 GHz, the model is entered into ADS, and an
optimization procedure is set up to reduce S11 as much as possible in iterative steps from 2.400 to 2.483
GHz. The simulation finely tweaks the PI-pad component values and measures S11. If it is lower, then the
components are tweaked again in the same direction until the best optimized solution is found.
The same procedure can be used for larger matching networks or even active networks, which may yield
better results. However larger circuits will have higher insertion loss due to the parasitic resistance present
within the components.
4.5
Matching to a Non-50Ω Active Source/Load Impedance
If the source and load impedances are both non-50Ω, then they can be matched in much the same way
as a 50Ω impedance as shown in Figure 27.
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
AN-1811 Bluetooth Antenna Design
Copyright © 2008–2013, Texas Instruments Incorporated
19
Matching
www.ti.com
Figure 27. Matching to Non-50Ω Impedance
In this example the load is at 20 + j10 and the source at 84 + j35. A series capacitance and shunt
inductance is required to transform the load impedance to that of the source.
Because these are non-50Ω, they can lie anywhere on the Smith Chart and must be measured using a
vector network analyzer (VNA) to determine their exact value.
Measuring the input impedance of a receiver or passive antenna is simple. A calibrated VNA will display
the value on its screen. However, when measuring the output impedance of a power amplifier or
transmitter, this technique cannot be used because the power being transmitted will completely disrupt the
VNA reading. The technique of conjugate matching using a variable load must be used.
In Figure 28, a variable load and attenuator can provide any desired load impedance to the PA. Therefore,
it can influence its output power which is measured using the power meter attached to the coupled port of
the directional coupler. When a perfect conjugate match is applied to the output of the PA, the power
measured on the power meter will be at its maximum, which signifies the best power transfer conditions.
20
AN-1811 Bluetooth Antenna Design
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
Copyright © 2008–2013, Texas Instruments Incorporated
Matching
www.ti.com
Figure 28. Setup for Determining Output Impedance
When best power transfer is achieved, the variable load and attenuator are fixed so that their impedance
cannot be changed. The PA is detached from the directional coupler and a VNA attached to the input of
the directional coupler to measure the input impedance at this point. The measured impedance is the
conjugate of the output impedance of the PA, or if the impedance measured on the VNA is R + jX then the
output impedance of the PA will be R - jX by definition.
Definition: the term conjugate match means that if in one direction from a junction the impedance is R +
jX, then in the opposite direction the impedance will be R – jX. The condition for maximum power
absorption by a load, in which the impedance seen looking toward the load at a point in a transmission
line is the complex conjugate of that seen looking toward the source.
4.5.1
LMX5252 Impedance Match
In the case of the LMX5252 where the Tx and Rx impedances are different and slightly off 50Ω, two
options are available for the antenna designer, either assume a 50Ω point for a simpler design, in which
case a small miss-match will result causing a small degradation in Tx/Rx power. Or make a matching
network as described above between two non-50Ω points. In the first instance where a 50Ω input/output
impedance is assumed the power loss that will results is as follows;
Worse case Rx input impedance = 32Ω
VSWR at this point = 1.5
Reflection coefficient S11 = 0.2
Return Loss = 10LOG(S11) = 7dB
Through transfer coefficient S21 = SQRT(1-[S11]^2) = 0.98
Power transferred = [S21]^2 = 0.96
Meaning that 96% of the power received by the antenna will be transferred to the receiver even with this
miss-match. To achieve higher power transfer efficiency than this a non-50Ω MN must be used as
described above.
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
AN-1811 Bluetooth Antenna Design
Copyright © 2008–2013, Texas Instruments Incorporated
21
Interference Rejection
5
Interference Rejection
5.1
Filtering
www.ti.com
An additional function of passive components in front of the antenna is to provide RF filtering. They may
be used to create an 83-MHz pass-band window centered at 2.44 GHz for rejecting any unwanted signals
outside the band that may impair the received signal quality. Figure 29 shows how such a filter would look
displayed on a Network Analyzer. It has three main features: within the pass-band is an unwanted
Insertion Loss (IL) which attenuates the transmit and receive signals, outside the pass-band is a wanted
rejection which attenuates interference, and at the edges of the pass-band is the filter roll-off which should
be as steep as possible to form a sharp cut-off between the pass-band and rejection-band.
Figure 29. RF Filter Performance
5.1.1
Filter Types
The simplest type of passive filter is a capacitor and inductor in series. To visualize how this works,
consider the response of a single capacitor and inductor in series to a frequency sweep. Looking at the
capacitor response in Figure 30, at DC it has very high IL, and as the frequency increases its IL
decreases. The inductor has the opposite behavior; at DC its IL is very low and this increases as the
frequency increases. The frequency at which the capacitor or inductor response changes will be
dependent on its value, and the rate by which it changes will be dependent on its “Q” or quality factor.
High Q-factor means rapid response change (steep roll-off) at a given frequency. By selecting the correct
value of the capacitor and inductor, an LC filter can be formed at any desired frequency. But it is important
to note that the higher the frequency of the filter, the lower will be its Q-factor and hence its roll-off.
A 1-pF capacitor in series with a 3.3-nH inductor forms an LC filter with a center frequency of 2.44 GHz.
Using high-Q components yields better roll-off and out-of-band rejection.
22
AN-1811 Bluetooth Antenna Design
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
Copyright © 2008–2013, Texas Instruments Incorporated
Interference Rejection
www.ti.com
Figure 30. LC Filter Response
However, even a well designed LC filter at 2.4 GHz does not provide very good roll-off and out-of-band
rejection. Typically, it will provide 10 dB of rejection below 1 GHz and above 3.5 GHz, however
interference signals will get through at closer frequencies. A better but more expensive solution is to use a
ceramic chip filter. These can be purchased from manufacturers such as Murata and M/A-COM. An
example of a Murata filter is the LFB212G45SG8A166. Table 1lists its electrical specifications.
Table 1. Chip Filter Specifications
Specification
Value
Nominal Center Frequency (fo)
2450 MHz
Bandwidth (BW)
fo ± 50 MHz
Insertion Loss in BW I
1.4 dB max. @ 25°C
Insertion Loss in BW II
1.6 dB max. @ -40 to +85°C
Attenuation (AbsoluteValue) I
30 dB min. @ 880 to 915 MHz
Attenuation (AbsoluteValue) II
30 dB min. @ 1710 to 1910 MHz
Attenuation (AbsoluteValue) III
6 dB min. @ 2110 to 2170 MHz
Attenuation (AbsoluteValue) IV
20 dB min. @ 4800 to 5000 MHz
Ripple in BW
0.8 dB max.
VSWR in BW
2 max.
Characteristic Impedance (Nom.)
50Ω
Power Capacity
500 mW
Min. Operating Temperature
-40°C
Max. Operating Temperature
+85°C
The IL may be slightly worse than an LC filter, but the out-of-band rejection is significantly better (20 to 30
dB). The filter is approximately 2 × 1.5 mm in size, and it is rated over the full automotive temperature
range (-40 to +85°C). A noteworthy but unwanted feature is the in-band ripple. The specification is 0.8 dB,
which means that the IL varies by 0.8 dB within the pass-band. The ripple will get worse as the
input/output impedances presented to the filter deviate from 50Ω, and this will give rise to variable
sensitivity and output power across the band.
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
AN-1811 Bluetooth Antenna Design
Copyright © 2008–2013, Texas Instruments Incorporated
23
Interference Rejection
5.2
www.ti.com
Blocking
The Bluetooth receiver compliance test measures the receiver performance under the effect of a strong
out-of-band interfering signal. A wanted signal is set to 2460 MHz at 3dB above reference sensitivity, and
an interfering signal is applied at the levels shown in Table 2.
Table 2. Blocking Signal Level and Frequency
Interfering Signal Frequency
Power
30 MHz2000 MHz
-10 dBm
2000 MHz2400 MHz
-27 dBm
2500 MHz3000 MHz
-27 dBm
The blocking signal is stepped in intervals of 1 MHz from 30 MHz to 12.75 GHz. Several thousand test
points are used, and at each of these points the bit error rate (BER) of the wanted signal must remain
under 0.1%. A total of 24 exceptions are allowed, because it is very difficult to pass all test points.
Failures are due to insufficient front-end filtering, either due to direct saturation of the front end if the lownoise amplifier (LNA) is not able to tolerate -10 dBm or more commonly due to mixing products entering
the pass-band. Unwanted products are caused by the blocking signal mixing with harmonics of other
signals present near the front end, such as clocks and local oscillators. By eliminating the interfering signal
using filtering, blocking failures can be reduced. Good layout techniques also help avoid the mixing
products.
5.2.1
Blocking Qualification Testing
During Bluetooth qualification, the Bluetooth Qualification Task Force (BQTF) uses the TS8960 test set to
link to the device under test (DUT) and place it on a fixed 2460-MHz receive channel. A signal generator
and combiner is used to produce the interfering signal. The whole setup is controlled with automated test
equipment (ATE), because there are several thousand points to test. This takes up to two days of
continuous measurements. Failures are counted when the BER exceeds 0.1%, however at times the BER
on certain blocking frequencies goes so high that the link is dropped, and a new link must be initialized
before testing can resume. When this happens, one or more failing frequencies may be reported. It is the
responsibility of the DUT manufacturer to test these failing frequencies manually and determine whether
additional filtering is required. During the link failure and re-establishment, the ATE system sometimes
logs more failures than are actually present, so manual testing will also confirm whether these failures are
genuine.
5.3
Recommended Front-end Layout and Matching
The front-end layout shown in Figure 31 or Figure 32 is recommended to provide the best matching and
filtering while at the same time providing flexibility for modifying the circuit as needed to meet the
Bluetooth testing requirements. Figure 31 is for a ceramic filter and blocking capacitor with good out-ofband rejection, dimensions shown here are non-exact. Alternatively the layout shown in Figure 32 maybe
used to form a simpler and cheaper LC filter and to allow matching to the antenna.
24
AN-1811 Bluetooth Antenna Design
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
Copyright © 2008–2013, Texas Instruments Incorporated
Antenna Vendors
www.ti.com
Figure 31. Front-end Layout Using Ceramic Filter
Figure 32. Front-end Layout Using T PI Pad
6
Antenna Vendors
Table 3 lists vendors for off-the-shelf antenna products and custom designs.
Table 3. Antenna Vendors
Vendor
gigaAnt
Mitsubishi
Materials
Products
Contact Information
Small ceramic chips and larger
surface-mount antennas suitable for
mobile phones, headsets and laptops,
printers, etc.
gigaAnt Ideon Science & Technology Park S-223 70 Lund Sweden
For example, 3030A5839-01 Leftside,
3030A5887-01 Rightside.
Phone:+46 46 286 41 77 Web: www.gigaant.com E-mail:
[email protected]
Small ceramic chips for surface-mount.
Suitable for mobile applications
Mitsubishi Materials Corporation Advanced Products Strategic
Company Sales Group, Electronic Components 1-297, Kitabukuro-cho,
Omiya-ku Saitama-city, Saitama, 330-8508 Japan
Phone: +81 48 641 5991 Fax: +81 48 641 5562 Web: www.mmc.co.jp
E-mail: [email protected]
Tyco Electronics Large high-gain printed antennas for
applications such as access points.
Centurion
Tyco Antenna Products/Rangestar 350 Metro Park Rochester, NY
14623 USA
Small ceramic chips for surface-mount.
Suitable for mobile applications
Phone: (585) 272-3103 Fax: (585) 272-3110 Web: www.rangestar.com
PCB surface-mount antennas for
various applications.
Centurion Wireless Technologies PO Box 82846 Lincoln, NE 68501
USA
Phone: (402) 467-4491 Fax: (402) 467-4528 Web: www.centurion.com
E-mail: [email protected]
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
AN-1811 Bluetooth Antenna Design
Copyright © 2008–2013, Texas Instruments Incorporated
25
Comparison Summary
www.ti.com
Table 3. Antenna Vendors (continued)
Vendor
Murata
7
Products
Contact Information
PCB surface-mount antennas
Murata International Sales Dept. 3-29-12 Shibuya, Shibuya-ku Tokyo
150-0002 Japan
For example, M1 series
ANCM12G45SAA072 or W1 se ries
ANCW12G45SAA110TT1.
Phone: +81 3 5469 6123 Fax: +81 3 5469 6155 Web:
www.murata.com E-mail: [email protected]
Comparison Summary
Table 4 compares the features of different antenna types.
Table 4. Antenna Comparison
Antenna Type
8
Profile
Cost
Physical Size
Stub helix or
monopole
Good bandwidth and
High: Projects from the side
efficiency, does not require of the PCB
matching network.
High
2.4 GHz antenna is approximately 15
mm long, but projects. Does not need
ground plane to function.
Surface-mount
ceramic chip
Reasonable performance
on λg/4. Small bandwidth
and reduced efficiency.
Can become detuned
during handling
Low: Can be machine
mounted during assembly,
no more than 0.5 mm thick
Medium
Element for 2.4 GHz is approximately
12 mm long, but needs ground area
and clearance around active region.
Printed inverted-F
or other printed
types
Reasonable performance
on λg/4. Small bandwidth
and reduced efficiency.
Can become detuned
during handling
Lowest: Printed on PCB
Low
Element for 2.4 GHz is approximately
25 mm long, but needs ground area
and clearance around active region.
Points for Consideration
•
•
•
•
•
•
•
•
•
26
Performance
Many types of antennas are available.
Antenna type is chosen to fit the application.
Larger antennas generally have better performance than smaller ones.
Ground plane is always required with printed or ceramic antennas.
Cannot put metal objects such as crystals close to antenna without causing detuning.
The case of a phone or other device will also detune the antenna, so some tuning adjustment ability is
needed.
Matching elements have parasitic values that affect their quality; this is not possible to simulate using
simple simulation software. Some trail and error is therefore required when performing the match or a
sophisticated simulation tool must be used.
The LMX9820A shielding will act as the ground plane for the antenna if placed correctly.
The LMX5251/LMX5252 radio chip must be shielded from the strong electric field only if it is placed
close to the radiating element. Shielding will improve performance but is not always required.
AN-1811 Bluetooth Antenna Design
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
Copyright © 2008–2013, Texas Instruments Incorporated
Examples of Antennas Used With LMX5251/LMX5252
www.ti.com
9
Examples of Antennas Used With LMX5251/LMX5252
Figure 33. Ceramic Chip Antenna for Industrial Remote Control with External PA
Figure 34. Printed Antenna—Monopole Yagi-Array for Off-Board Navigation Using GPS
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
AN-1811 Bluetooth Antenna Design
Copyright © 2008–2013, Texas Instruments Incorporated
27
Examples of Antennas Used With LMX5251/LMX5252
www.ti.com
Figure 35. Ceramic Chip Antenna for Intelligent Remote Access for Car Lock
Figure 36. Ceramic Chip Antenna for Distance Meter
28
AN-1811 Bluetooth Antenna Design
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
Copyright © 2008–2013, Texas Instruments Incorporated
Examples of Antennas Used With LMX5251/LMX5252
www.ti.com
Figure 37. External Antenna—Helix or Monopole for Automotive Integrated Hands-Free Kit
Figure 38. Printed PIFA Antenna for Automotive Hands-Free Kit
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
AN-1811 Bluetooth Antenna Design
Copyright © 2008–2013, Texas Instruments Incorporated
29
Popular Antenna Types
www.ti.com
Figure 39. Surface-Mount Chip Antenna (Phycomp AN-2700 or Murata ANCM12G) for Automotive HandsFree Kit
Figure 40. Surface-Mount Chip Antenna (Mitsubishi Materials AHD 1403) for Electronic Whiteboard
10
Popular Antenna Types
Ceramic chip antennas (Mitsubishi, gigaAnt) are the most popular types being used in Bluetooth products.
These cost about 40 cents/unit.
30
AN-1811 Bluetooth Antenna Design
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
Copyright © 2008–2013, Texas Instruments Incorporated
Popular Antenna Types
www.ti.com
Figure 41. GigaAnt Rufa 2.4 GHz SMD Antenna
Figure 42. Mitsubishi AHD1403 Surface-Mount Antenna
The second most popular type is the PIFA. These have the lowest cost because they consist of a PCB
trace, but are larger and more design-intensive.
SNOA519B – March 2008 – Revised May 2013
Submit Documentation Feedback
AN-1811 Bluetooth Antenna Design
Copyright © 2008–2013, Texas Instruments Incorporated
31
IMPORTANT NOTICE
Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other
changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest
issue. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current and
complete. All semiconductor products (also referred to herein as “components”) are sold subject to TI’s terms and conditions of sale
supplied at the time of order acknowledgment.
TI warrants performance of its components to the specifications applicable at the time of sale, in accordance with the warranty in TI’s terms
and conditions of sale of semiconductor products. Testing and other quality control techniques are used to the extent TI deems necessary
to support this warranty. Except where mandated by applicable law, testing of all parameters of each component is not necessarily
performed.
TI assumes no liability for applications assistance or the design of Buyers’ products. Buyers are responsible for their products and
applications using TI components. To minimize the risks associated with Buyers’ products and applications, Buyers should provide
adequate design and operating safeguards.
TI does not warrant or represent that any license, either express or implied, is granted under any patent right, copyright, mask work right, or
other intellectual property right relating to any combination, machine, or process in which TI components or services are used. Information
published by TI regarding third-party products or services does not constitute a license to use such products or services or a warranty or
endorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual property of the
third party, or a license from TI under the patents or other intellectual property of TI.
Reproduction of significant portions of TI information in TI data books or data sheets is permissible only if reproduction is without alteration
and is accompanied by all associated warranties, conditions, limitations, and notices. TI is not responsible or liable for such altered
documentation. Information of third parties may be subject to additional restrictions.
Resale of TI components or services with statements different from or beyond the parameters stated by TI for that component or service
voids all express and any implied warranties for the associated TI component or service and is an unfair and deceptive business practice.
TI is not responsible or liable for any such statements.
Buyer acknowledges and agrees that it is solely responsible for compliance with all legal, regulatory and safety-related requirements
concerning its products, and any use of TI components in its applications, notwithstanding any applications-related information or support
that may be provided by TI. Buyer represents and agrees that it has all the necessary expertise to create and implement safeguards which
anticipate dangerous consequences of failures, monitor failures and their consequences, lessen the likelihood of failures that might cause
harm and take appropriate remedial actions. Buyer will fully indemnify TI and its representatives against any damages arising out of the use
of any TI components in safety-critical applications.
In some cases, TI components may be promoted specifically to facilitate safety-related applications. With such components, TI’s goal is to
help enable customers to design and create their own end-product solutions that meet applicable functional safety standards and
requirements. Nonetheless, such components are subject to these terms.
No TI components are authorized for use in FDA Class III (or similar life-critical medical equipment) unless authorized officers of the parties
have executed a special agreement specifically governing such use.
Only those TI components which TI has specifically designated as military grade or “enhanced plastic” are designed and intended for use in
military/aerospace applications or environments. Buyer acknowledges and agrees that any military or aerospace use of TI components
which have not been so designated is solely at the Buyer's risk, and that Buyer is solely responsible for compliance with all legal and
regulatory requirements in connection with such use.
TI has specifically designated certain components as meeting ISO/TS16949 requirements, mainly for automotive use. In any case of use of
non-designated products, TI will not be responsible for any failure to meet ISO/TS16949.
Products
Applications
Audio
www.ti.com/audio
Automotive and Transportation
www.ti.com/automotive
Amplifiers
amplifier.ti.com
Communications and Telecom
www.ti.com/communications
Data Converters
dataconverter.ti.com
Computers and Peripherals
www.ti.com/computers
DLP® Products
www.dlp.com
Consumer Electronics
www.ti.com/consumer-apps
DSP
dsp.ti.com
Energy and Lighting
www.ti.com/energy
Clocks and Timers
www.ti.com/clocks
Industrial
www.ti.com/industrial
Interface
interface.ti.com
Medical
www.ti.com/medical
Logic
logic.ti.com
Security
www.ti.com/security
Power Mgmt
power.ti.com
Space, Avionics and Defense
www.ti.com/space-avionics-defense
Microcontrollers
microcontroller.ti.com
Video and Imaging
www.ti.com/video
RFID
www.ti-rfid.com
OMAP Applications Processors
www.ti.com/omap
TI E2E Community
e2e.ti.com
Wireless Connectivity
www.ti.com/wirelessconnectivity
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2013, Texas Instruments Incorporated
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement