Organization

This thesis is devoted to the on-package planar inverted-F antenna for RF SOP application. It consists of five chapters. Chapter 1 gives the introduction of SOP approach and the motivation of this thesis. In Chapter 2, we introduce basic theory of patch antennas and planar inverted-F antennas. Chapter 3 exhibits the design methodology and measurement of the on-package PIFA. In Chapter 4, the coupling effects between the on-package PIFA and RF components are investigated. Chapter 5 shows the conclusions of the excellent antenna in this thesis. The references are attached in the end.

Chapter 2

Theory of Patch Antennas and Planar Inverted-F Antennas

2.1 Patch Antennas

A microstrip device is a layered structure with two parallel conductors separated by a thin dielectric substrate and the lower conductor acting as a ground plane. If the upper metallization is a patch that is an appreciable fraction of a wavelength in extent, the devices becomes a microstrip antenna, as illustrated in Fig 2-1. The patch antenna belongs to the class of resonant antennas and its resonant behavior is responsible for the main challenge in microstrip antenna design-achieving adequate bandwidth.

The fringing fields act to extend the effective length of the patch and are responsible for the radiation. The fringing field and the radiation loss at the open-end of the microstrip line can be represented by the equivalent capacitive component B₀ and conductive component G0, respectively, as shown in Fig 2-2.

2 The patch antenna is usually operated near resonance in order to obtain a real-valued input impedance. Fig 2-3 shows the transmission-line model of the microstrip patch antenna. This model can be used to determine the resonant frequency or the total length of the patch. The input admittance of the fed of the patch antenna is given by

( )

When the total length L of a microstrip patch antenna is

1 2 , the patch antenna is resonant, where λ is the free-space wavelength, λd the wavelength in the dielectric and ε_r the substrate dielectric constant. The expression for the input impedance (reactance is zero at resonance) of the resonant patch is

( )

The patch length L for resonance is given by approximately half wavelength and the patch width W is selected to given the proper radiation resistance at the input, often 50Ω. The far-field components are

( )

The principal plane patterns follow the above equation as

( )

The length of the half-wave patch antenna can be halved by placing a short-circuit plate between the patch and ground plane at the center of the patch where the fields are zero. The modified antenna is called as a planar inverted-F antenna [5], [6].

Fig 2-1 Geometry for the microstrip patch antenna.

Fig 2-2 Equivalent components for the fringing field and the radiation loss.

Fig 2-3 Transmission-line model of the microstrip patch antenna.

2.2 Planar Inverted-F Antennas

There have been frequent requests to develop small antennas for the further miniaturization of portable mobile phone sets. Thus, antennas having small and low profile structures are suitable for mounting on portable equipment. Among these antennas, the planar inverted-F antenna (PIFA) is one of the most promising. The PIFA typically consists of a rectangular planar element, ground plane, and short- circuit plate of narrower width than that of a shortened side of a planar element. Fig 2-4 shows the structure of the PIFA. The PIFA can be considered as a kind of short-circuit rectangular microstrip antenna (or liner inverted-F antenna with the wire radiator element replaced by a plate to expand the bandwidth).

Fig 2-5 shows the transmission-line model of the planar inverted-F antenna. The resonant frequency (or the total length of the radiating element) can be determined by this model. The input admittance of the fed of the PIFA is

( )

When the total length L of the radiating element is

1 2 , the PIFA is resonant, where εr is the substrate dielectric constant, λd the wavelength in the dielectric and λ is the free-space wavelength. The input impedance of the resonant PIFA is given by

( )

The radiation resistance of the PIFA is twice that of the half-wave patch antenna. The resonant frequency determined by the model is exact in case of Ws/W = 1.

As shown in Fig 2-6, the current on the undersurface of the planar element mainly flows to the open-circuit edge on the long side of the planar element in the case of

W-W_s < L. However, the current flows to the open-circuit edge on the short side of the planar element in the case of W-Ws > L. According to the analysis results for the surface current, one can assume that the effective length of the current flow on the short-circuit plate and planar element. Then, in the case of Ws/W = 1, the resonance is expressed by

L H λ

4

+ = (2-12) and in case of Ws = 0, it is expressed by

L W

_{+ +}

H

= (2-13)

λ

4 In case of 0 < W_s/W < 1, the resonant frequency f_r can be expressed by

( )

1 1 1 for 1

r

f r f r f W

= ⋅ + − ⋅

L

≤ (2-14a) and

( )

1 1 1 for 1

k k

r

f r f r f W

= ⋅ + − ⋅

L

≥ (2-14b)

where r = W_s/W, k = W/L, and resonance for frequency f₁ is expressed by (2-12).

Resonance for frequency f2 is expressed by (2-13) [6], [7].

Fig 2-4 Structure for the planar inverted-F antenna.

Fig 2-5 Transmission-line model of the planar inverted-F antenna.

Fig 2-6 Variation of surface current flow on planar element due to size ratio of planar element and width of short-circuit plate.

Chapter 3

Design and Measurement of On-Package PIFA

3.1 Design of On-Package PIFA with and without Dielectric Material

In general, antennas for portable wireless transceivers are based around either a retractable whip monopole or an encapsulated helix because of their simple structures and high radiation gains. However, neither the monopole nor the helical antenna can be neatly integrated with the rest of a wireless transceiver. Recently many patch antennas integrated with package are implemented with the low temperature co-fired ceramic (LTCC) package or the ceramic ball grid array package (CBGA) [8]-[12]. But a patch antenna can not be integrated with package using a single folded copper plate.

Consequently, the preferred candidate is the planar inverted-F antenna.

In this thesis, we utilize the planar inverted-F antenna to achieve the desired antenna integrated with package using a single folded copper plate. As for the rapidly growing wireless market, the demand for compact and low-cost antennas becomes more and more urgent. This designed antenna is applied for IEEE 802.11b/g WLAN.

The required bandwidth should cover the band from 2.4~2.4835 GHz when input return loss is below -10dB. The on-package planar inverted-F antenna is implemented on the FR4 substrate, whose dielectric constant is 4.7, loss tangent is 0.02, and thickness is 0.8 mm. The specifications of the on-package PIFA are listed bellow:

The height of PIFA is less than 3mm.

The package is 15 × 15× 1.5 mm ³. The ground size is 20 × 40 mm ².

The feed of PIFA must be located at the edge of the package..

3.1.1 On-Package PIFA with Dielectric Material

Single-chip wireless transceivers in their bare forms are susceptible to the effect of mechanical stress, environmental change, and electrostatic discharge. Therefore, they are packaged with dielectric materials. We will introduce an on-package planar inverted-F antenna with a ceramic material. The on-package PIFA can be integrated with a wireless transceiver bare chip to achieve a WLAN module. Furthermore, the RF circuits including an antenna are integrated as a single packaged chip.

The on-package PIFA can be realized by a single folded copper plate. The antenna has several advantages of light weight, low-cost and easy fabrication. The feed of the on-package PIFA must be located at the edge of the package. The restriction of the fabrication technology must be taken into consideration. For example, the short- circuit plate and the feed line can not be arranged too close to each other. If the gap is too small, it can not be realized with the technology. The structure of the planar inverted-F antenna is indicated in Fig 3-1 and the lateral views are shown in Fig 3-2.

Fig 3-1 3-D structure of the on-package PIFA with dielectric material.

Fig 3-2a

Fig 3-2b

Fig 3-2c

Fig 3-2 Lateral views of the on-package PIFA with dielectric material (a) x-y plane (b) x-z plane (c) y-z plane.

The length

l

and width

w

of the on-package PIFA determine the resonant frequency, which can be approximated by the formula

0 4( )

f v

=

w l

+ where

v

is the wave velocity in the ceramic material;

l

and

w

are the length and width of the radiating element;

f is the resonant frequency.

0

The radiating element is grounded by a short-circuit strip and fed near the short- circuit strip with the feed line. The input impedance of the antenna can be easily matched to 50Ω by controlling the feed position relative to the short-circuit strip.

In the packaging process, the shielding package and the antenna are filled with a dielectric material whose dielectric constant is 3.7 and loss tangent is 0.018. The other characteristics of PCB and specifications of the shielding package are mentioned above. The dielectric material is extremely critical to the performance of the bandwidth and the gain for the antenna. The patch size of the antenna is 5 × 14.3 mm ² and the height is 3 mm. The short-circuited strip is located on the edge of the patch and 1.5mm apart from the corner. The width of the short pine is 1.5mm and the height is 3mm. The width of the feed line is 1 mm and the height is 4.5mm. The gap between the short pine and the feed line is 1mm.

The configuration of the on-package PIFA is a three-dimensional structure. Thus, we can apply the 3-D full-wave EM simulator Ansoft HFSS 8.0 to simulate the on-package PIFA. The simulated input return loss and the radiation pattern are shown in Fig 3-3 and Fig 3-4, respectively. It has 120MHz bandwidth from 2.39 to 2.51 GHz when the input return loss is bellow -10dB. The bandwidth can cover IEEE 802.11b/g WLAN band from 2.40 to 2.4835 GHz. At x-z plane, the radiation pattern

is omnidirectional. The maximum gain and average gain are 3.52dBi and 1.50dBi, respectively. The gains of each plane at 2.45GHz are listed in Table 3-1.

x-z plane y-z plane x-y plane

Maximum Gain 3.52dBi 3.52dBi 2.54dBi

Average Gain 1.50dBi -2.31dBi -2.32dBi

Table 3-1 The maximum and average gain of the on-package PIFA with dielectric material at 2.45GHz.

Fig 3-3 Return loss of the on-package PIFA with dielectric material.

1.5 2 2.5 3 3.5

Frequency (GHz) -30

-20 -10 0

2.45GHz

2.39GHz 2.51GHz

dB

x-z plane

Fig 3-4a Radiation pattern of on-package PIFA with dielectric material at x-z plane (2.45GHz).

y-z plane

Fig 3-4b Radiation pattern of on-package PIFA with dielectric material at y-z plane (2.45GHz).

x-y plane

Fig 3-4c Radiation pattern of on-package PIFA with dielectric material l at x-y plane (2.45GHz).

3.1.2 On-Package PIFA in Air

As described in section 3.1, the on-package PIFA with dielectric material has the advantage of being integrated with a wireless transceiver bare chip. Besides, we can use the discrete components to complete the RF circuit and the packaging dielectric material can be excluded. The bandwidth and the gain of the antenna can also be increased without the dielectric material. In this section, we removed the filled dielectric material in the shielding package and leave the PIFA in air. Another advantage of dropping the dielectric material is that if the bandwidth has been enough wider than the IEEE 802.11b/g WLAN band, we could reduce the height of the antenna to minimize the package totally size. In addition, the space next to the shielding package is not efficiently used. We extend it to the edge of the FR4 board to form a new shielding package with the size of 15 × 20 × 1.5 mm ³. We have improved the on-package PIFA in Section 3.1 and the new antenna can be still embedded into the shielding package. The structure of the new on-package PIFA is indicated in Fig 3-5 and the lateral views are shown in Fig 3-6.

Fig 3-5 3-D structure of the on-package PIFA in air.

Fig 3-6a

Fig 3-6b

Fig 3-6c

Fig 3-6 Lateral views of the on-package PIFA in air (a) x-y plane (b) x-z plane (c) y-z plane.

The length

l

and width

w

of the on-package PIFA determine the resonant frequency, which can be approximated by the formula

0 4( )

f c

=

w l

+ where

c

is the velocity of light

l

and

w

are the length and width of the radiating element;

f is the resonant frequency.

0

The radiating element is grounded by a short-circuit strip and fed near the short- circuit strip with the feed line. The input impedance of the antenna can be easily matched to 50Ω by controlling the feed position relative to the short-circuit strip.

The patch size of the antenna is 14.5 × 15 mm ² and the height is 2 mm. The short-circuit strip is located on the corner of the patch. The width and height of the shorting strip is 1.5mm and 2mm, respectively. The height and width of the feed line is 3.5 mm and 1.5mm, respectively. The gap between the shorting strip and the feed line is 1mm.

The simulated input return loss and the radiation pattern are shown in Fig 3-7 and Fig 3-8, respectively. It has 150MHz bandwidth from 2.37 to 2.52 GHz when the input return loss is bellow -10dB. The bandwidth can cover IEEE 802.11b/g WLAN band from 2.40 to 2.483 GHz. At x-z plane, the radiation pattern is omnidirectional.

The maximum gain and average gain are 4.92dBi and 2.53dBi, respectively. The gains of each plane at 2.45GHz are listed in Table 3-2.

x-z plane y-z plane x-y plane

Maximum Gain 4.92dBi 4.91dBi 2.85dBi

Average Gain 2.53dBi -1.4dBi -1.16dBi

Table 3-2 The maximum and average gain of the on-package PIFA in air at 2.45 GHz.

Fig 3-7 Return loss of on-package PIFA in air.

Fig 3-8a Radiation pattern of on-package PIFA in air at x-z plane (2.45GHz).

2 2.2 2.4 2.6 2.8 3

y-z plane

Fig 3-8b Radiation pattern of on-package PIFA in air at y-z plane (2.45GHz).

x-y plane

Fig 3-8c Radiation pattern of on-package PIFA in air at x-y plane (2.45GHz).

3.2 Influence of Different Ground Size and Shielding Package

The on-package PIFA in section 3.1.2 is design with the ground size of 20 × 40 mm ² and the shielding package of 15 ×20 × 1.5 mm ³. The RF components can be placed within the shielding package and the baseband circuits are arranged outside.

The ground size and shielding package would be changed when the sizes of circuit components are varied. However, it is well known that the ground size influences the antenna performance and the shielding package is also one part of the on-package PIFA. The change would affect the characteristic of the on-package PIFA. Therefore, we have to investigate the influence of different ground size and shielding package.

3.2.1 Ground Size

The dimensions of the radiating element, the short-citcuit strip and the feed line are fixed and the shielding package is also specified to 15 ×20 × 1.5 mm ³. We alter the ground size to 20 × 35 mm ², 20 × 45 mm ², and 20 × 50 mm ².The input return loss of each case is shown in Fig 3-9. It is observed that the return loss for ground size of 20 × 35 mm ² and 20 × 45 mm ² are much poor. The return loss for ground size of 20 × 45 mm ² has wider bandwidth than the original case. We also compare the radiation patterns of the PIFA with the ground size of 20 × 40 mm ² and 20 × 45 mm ². Their patterns are shown in Fig 3-10. The same radiation patterns occur at x-z plane.

The patterns at y-z plane are almost the same except at the theta 90 degree and -90degree. The reason is that the ground size of 20 × 45 mm ² is longer than the order of 20 × 40 mm ² in the y axis. Therefore, the nulls in the y axis would be more evident.

From above discussion, we realize that the structure of the original on-package PIFA do not have not to be redesigned when the ground size varies from 20 × 40 mm ² to 20

× 45 mm ².

Fig 3-9 Return loss of on-package PIFA with various ground size.

Fig 3-10a Radiation pattern of on-package PIFA with various ground size at x-z plane (2.45GHz)

y-z plane

Fig 3-10b Radiation pattern of on-package PIFA with various ground size at y-z plane (2.45GHz).

Fig 3-10c Radiation pattern of on-package PIFA with various ground size at x-y plane (2.45GHz).

3.2.2 Shielding Package

The original package size is 20 × 15×1.5 mm ³. In this section, we fix all parameters except for the package size. We change the package size to 15 × 15×1.5 mm ³ (case 1), 20 × 20 ×1.5 mm ³ (case 2), and 20 × 25×1.5 mm ³ (case 3). The simulated return loss is shown in Fig 3-11. It shows the bandwidth of case 1 is wider than the original one, but the other two cases are worst than that. We can see that the change of the package size has less influence than the change of ground size. The radiation patterns of case 1 and original one are both shown in Fig 3-12. The radiation pattern has no difference in any plane. The reason is the package size is less than the ground size which is fixed. We conclude that the configuration of the radiation element do not have not to be redesigned when the package changes from 15 × 15×1.5 mm ² to 15 × 20×1.5 mm ³.

Fig 3-11 Return loss of on-package PIFA with various package size

2 2.2 2.4 2.6 2.8 3

Frequency (GHz)

S11

-30 -20 -10 0

dB

15x15

20x15

20x20

20x25

x-z plane

Fig 3-11a Radiation pattern of on-package PIFA with various package size at x-z plane (2.45GHz).

y-z plane

Fig 3-11b Radiation pattern of on-package PIFA with various package size at y-z plane (2.45GHz).

x-y plane

Fig 3-11c Radiation pattern of on-package PIFA with various package size at x-y plane (2.45GHz).

3.3 Measurement

In this section, we carry out the implementation of the on-package PIFA in air, which has been discussed in section 3.1.2, and exhibit the measured results. The on-package PIFA consists of a single folded copper plate, which is shown in Fig 3-12.

The upper and the lower palate of the original copper plate are folded as the package part and the patch part of the antenna, respectively. The final folded copper plate is shown in Fig 3-13. The sizes of the package and the radiating element are 15 × 15×

1.5 mm ³ and 14.5 × 17 mm ², respectively. Fig 3-14 is the photograph of the realized on-package PIFA. The measured bandwidth is 150MHz from 2.37GHz to 2.52 GHz under –10dB return loss, as shown in Fig 3-14. The radiation pattern is also shown in Fig 3-16. The pattern at x-z plane is pretty omnidirectional and the maximum gain and average gain are 1.17 and -0.63dBi, respectively. The gains of all planes at 2.45GHz are listed in Table 3-2. We also investigate the frequency response of the radiation gain at x-z plane as shown in Fig 3-17. The gain has less than 2dBi variation centered at 0dBi. This on-package PIFA hold an excellently linear frequency response.

x-z plane y-z plane x-y plane

Maximum Gain 1.17dBi 3.09dBi 0.61dBi

Average Gain -0.63dBi -2.78dBi -3.17dBi

Table 3-3 Measured maximum and average gain of the on-package PIFA at 2.45 GHz.

Fig 3-12 The original copper plate.

Fig 3-13 The folded copper plate.

Fig 3-14 Photograph of implemented antenna.

Fig 3-15 Measured return loss of the on-package PIFA.

Fig 3-16a Measured radiation pattern of the on-package PIFA at x-z plane (2.45GHz).

2 2.2 2.4 2.6 2.8 3

y-z plane

Fig 3-16b Measured radiation pattern of the on-package PIFA at y-z plane (2.45GHz).

x-y plane

Fig 3-16c Measured radiation pattern of the on-package PIFA at x-y plane (2.45GHz).

( )

Frequency (GHz)

2.30 2.35 2.40 2.45 2.50 2.55 2.60

Gain (dB i)

-10 -8 -6 -4 -2 0 2

Fig 3-17 Gain vs. Frequency characteristic of the on-package PIFA at x-z plane (2.45GHz).

Chapter 4

Coupling between On-Package PIFA and RF Components

4.1 Characteristic of LTCC BPF

Basically, WLAN RF circuits consist of an antenna, a T/R switch, filters, mixers, voltage-control oscillators, a low-noise amplifier, and a power amplifier [13]-[15].

The switch is set up for TX or RX path with appropriate bias. On RX path, the band-pass filter for band selection is followed by a low-noise amplifier. The RF signal amplified by the PA radiates through an antenna when the switch is set up for the TX path. The low-pass filter can suppress the output harmonics of the PA. Among these components, the low-pass filter, the band-pass filter and the matching network of the amplifiers are all passive components. The coupling effect between the on-package PIFA and the RF passive components in the shielding package can be investigated by

The switch is set up for TX or RX path with appropriate bias. On RX path, the band-pass filter for band selection is followed by a low-noise amplifier. The RF signal amplified by the PA radiates through an antenna when the switch is set up for the TX path. The low-pass filter can suppress the output harmonics of the PA. Among these components, the low-pass filter, the band-pass filter and the matching network of the amplifiers are all passive components. The coupling effect between the on-package PIFA and the RF passive components in the shielding package can be investigated by

在文檔中 應用於系統封裝之立體平面倒F天線設計 (頁 15-0)