return loss of the antenna and Fig. 3 shows the axial ratio of the antenna at boresight. It can be seen froin the Figures that by tuiii- ing the diodes on and off, dual-band CP operation can be achieved. The axial ratio is 2.9dB at 4.0GHz and 1.6dB at 4.37 GHz and a frequency ratio of only 1.09 is obtained. Although the return loss at 4.37GHz is -9dB, this can be further improved by adjusting probe position and slot position. Spinning linear radi- ation patterns at xz plane are shown in Fig. 4 at two frequencies, 4.0 and 4.37GHz. As shown in Fig. 4, the pattern shape and beam angle are very similar to each other. Similar radiation pat- terns can be observed in yz plane.

L-

- 4 0 E E e 0

-30 g c

- 2 0 =

- 4 0 1 1 1 - - . 1 L i ~ 1 - -150 -100 -50 0 50 100 150 -150 -100 -50 0 50 100 150

angle, deg angle, deg 1 h

,*' .4.' I I

. , r"

1795141 Fig. 4 Measured spinning linear putterns of untenna (xz plane) U Diodcs ON modc, Ji = 4.0 GHz h Diodes OFF mode, .f; = 4.37 GHz

Conclusion: The switchable slot, a novel technique, is incorporated into a normal circularly polarised patch antenna for dual-band CP operation. This shc tu re has the advantage of low profilc, small, flexible frequency ratio and similar patterns at two bands. This design is suitable for GPS. satellite links and other wireless com- munication applications.

8 IEE 2001 Electronics Letters Online No: 20010695 DOT: 10.1049/~~1:20010695 F. Yang and Y. Rahmat-Samii (Department of Ekectrirul Enginrerinz, University qf Culiforniu ut Los Angeles, Los Anfides, CA 90095-1594, USA) E-mail: rahmat@ec.ucla.cdu

29 May 2001

References

1 POLAR, D.M., and DUFFY, s.M.: 'A dual-band circularly polarized aperture-coupled stacked microstrip antenna for global positioning satellite', IEEE Trans. Antennas Propug, 1997. AP-45, pp. 1618- 1625 JAN, J.Y., and WONG. K.L.: 'A dual-band circularly polarized stacked elliptic microstrip antenna', Microw Opf. Techno/. Lett., 2000, 24,

2

pp. 354-357

pp. 1170-1171

3 HSIEH, G.B., CHEN. M.H., and WONC, K.L.: 'Singk-feed dual b a d circularly polariscd microstrip antenna', Electron. Lett., 1998, 34,

ZHANG, x.x., and YANG. r.: .The study of slit cut on the microstrip antenna and its applications'. Microw Opt. Teclnzol. Lett., 1998, 18, pp. 297-300

5 VIRGA, K.L., and RAHMAT-SAMII, Y.: 'Low profile enhanced- bandwidth PIFA antcnnas [or wirelcss communications packaging', IEEE Trans. Microis. Theory Tech., 1997, 45. pp. 1874-1888

4

Tuning techniques for planar inverted-F antenna

P.K. Panayi, M.O. Al-Nuaimi and L.P. Ivrissimtzis

User terminals of modem mobile comnunication systems require efficiciit, low profile antennas, capable of broadband and multi- band operation. In that respect, planar inverted-F antenna (PIFA) designs have cmerged that explore the trade-off between the height above the ground-plane and the achicvablc efrcctive bandwidth. Further possible bandwidth enhancemcnts are studied, as is multi-band operation by switching and tuning the resonant frequcncy of the PIFA, while maintaining a low height above the ground plane.

ELECTRONICS LETTERS 2ndAugust 2001 Vol. 37

Introduction: Several planar inverted-F antenna (PIFA) configura- tions have been characterised in recent studies [l - 41. In addition to the low-profile implementations that a PIFA allows, its near and far-field directive properties result in reduced absorption by the head tissue of the user and, ultimately, increased radiated efi- ciency. Sufficiently broadband characteristics are achieved at the expense of the effective height of the radiating patch above the ground plane. However, switching or dynamically tuning the reso- nant frequency or band of a low-profile and, inherently, narrow- band PIFA have received less attention. In this Letter we investigate techniques for switching or tuning the resonant fre- quency of the antenna, so that both multi-band and effectively wideband operations are achieved, while maintaining a low pro- file.

ground plane

-k2d

Fig. 1 PIFA confgurulion and equivalent circuit

25 i 70

i. . I

60

E

1 lo

Resonant frequency tuning: A standard PIFA configuration above a ground plane is shown in Fig. I . The position of the feed point and the dimensions of the conductive case and radiating element (its height h, width w, and length 1) are fKed. The resonant fre- quency of an air suspended PIFA is approximately given by [5]

where co is the velocity of light and CL is a constant approximately equal to 0.9. Its relative bandwidth, i.e. the ratio Aj7fo where Afis the frequency range over which the VSWR at the antenna port is less than a pre-specified value (typically, 2: I), is proportional to the height above the ground plane [5]. The PIFA resonates when a shorting pin is introduced, normally at a short distance from the feed point, represented by the eqnivalent circuit of Fig. 1. In our application, i v and l have been selected so that the resonant fre- quency is -YOOMHz, while the height of the PIFA above the ground plane is fixed at 5 mm.

No. 16 1003

To study the extent of the tuning or switching range possible, a second shorting pin, equivalent to a second shunt inductance in the original resonant circuit, is introduced. This shorting pin is positioned at different points along the perimeter of the PIFA patch and the resonant frequency is obtained and plotted in Fig. 2 against the distance from the starting point of each edge. The res- onant frequency increases with increasing distance from the feed point, due to the higher inductance presented to the path of cur- rents on the patch, but it reaches a plateau as the pin approaches the corners. An effective relative bandwidth of -50% is thus achievable. For a TDMA system, electronically switchcd devices such as FETs or pin diodes can replace the shorting pin in practi- cal implementations and can allow switching between bands.

. I I . 5 . . , . , , . , . . . .

frequency, MHz 1825131 Fig. 3 VSWR and bore-.sighf rlil.ective gain for d i f f r m tuning vuhm ilow-band operation)

~ gam Capacitor values ranged from 1.8 pF (highest frequency resonance curve) to 3.0pF (lowcsl resonancc curve)

. . , , . . . . . . . VSWR

frequency, MHz 1825/41 Fig. 4 VSWR und bore-.right direcrive gain .for dfferent tuning valuer (high-band operation) . . . . . . . . . , , VSWR

gain ~~~

Alteiiidtively, the second shorting pin can bc replaced by a var- iable capacitor of fixed position at the antenna perimeter. Fig. 2 also shows the effect of the capacitance value on the resonant fre- quency, when either a tunable capacitor is placcd in the vicinity of one of the corners of the patch. The measurements are shown for capacitance values varying from 3 to 20pF, a range that is easily accommodated with commercially available varactors. At lower capacitance values the resonant frequency approaches the natural resonance of the PIFA, while, as expected, larger values of added

capacitance reduce the resonant frequency of the antenna. With this approach, an elrective bandwidth of the order of 15-20% can be achieved.

In both cases, a second resonance, not shown in Fig. 2, occurs at approximately twice the frequency of the first one.

Dual-bund tunable PIFA: Employing the tuning and switching pos- sibilitics described, a dual-band PIFA has been designed Lo oper- ate in the 88&960MHz and 171C1880MHz band. The position of the fixed short pin for this antenna is at 5mm from corner 1. Two capacitors have been used: the first one at 25 mi from cor- ner 1 has a fixed value that is selected to allow the antenna lo res- onate at approximately the mid-frequency for the two bands of operation. The second capacitor is a tuning capacitor, placed at 20" Prom corner 2, in series with a large fuced capacitance for DC blocking purposes.

The VSWR and the normalised bore-sight gain of the dual- band fine tuned PIFA for low- and high-band operation are shown in Figs. 3 and 4 for discrete tuning capacitor values. In general, capacitance values ranging from 1.8 to 3.0pF have been adequate to accoinmodate the width of both bands under consid- eration. Tuning can be achieved either mechanically or electroni- cally with similar results.

Conclusion: A PIFA operating over a wide bandwidth over two bands of operation using simple tuning techniques has been designed and demonstrated. These techniques allow low-profile implementation of an inherently narrow-band radiator for dual- and wide-band operation.

8 IEE 2001 Electronics Letters Online No: 20010692 D 01: IO. 1049/z:2OO 10692 P.K. Panayi and M.O. AI-Nuaimi (School of Electronic.y, (/niver.sity uf Glamorgan, Pontypridd, Mid-Gluinorgan. CF37 I DL, United Kingdom) L.P. Ivrissimtzis (Agera Systems, Microelectronics House, Kingswood. Kings Kirk, A.sro1, Berkshire. SLI IDL, Uniled Kingdoin)

29 March 2001

References

ROWELL, c.K., and MUKCH, K.D.: 'A capacitively loaded PIFA for compact mobile telephone handsets', IEEE Trun.?. Antennas Proprig., 1997: 45, pp. 837-842 LIU. z.D.: and HALL. P.S : 'Dual-band antenna for hand held portable telephones', Electron. Lett., 1996, 32, pp. 609-610 LIU, Z.D., HALL. P.S., and WAICE. D.: 'Dual-frequency planar inverted- F antenna'. IEEE Truns. Arrtennu.s Propug., 1997, 45; pp. 1451- 14% ROWELL, c . ~ , and MURCH. K.D.: 'A compact PIFA suitable for dual-frequency 90011 800-MHz operation', IE'EE Trans. Antennus Proprig., 1998, 46, pp. 596-598 OGAWA. K., and OWANO, T.: 'A diversity antenna for very small 800- MHz band portable phone', IEEE Trans. Antennas Propug., 1994, 42, pp. 1342-1345

ASMD with duty cycle correction scheme for high-speed DRAM

Seong-Jin Jang , Young-Hyun Jun , Jae-Goo Lee and Bai-Sun Kong

4n analogue synchronous mirror delay with duty cycle correction scheme (ASMDCC); to impi-ove the data transmission performance between DRAM and system, is proposed. The ASMDCC achievcs duty cycle correction and clock synchronisation at once within two clock cycles, by using a half value current source. The simulation results show the duty cycle of the intcmal clock is stabiliscd with lcss than 5100ps deviation from 50% for the wide duty cycle range.

Introduction: As the demand for high-speed systems increases, the skew between output data and the system clock becomes even more critical for the correct transfer of data. Phase-locked loops (PLLs) 'and delay-locked loops (DLLs) are typical internal clock

1004 ELECTRONICS LETTERS PndAugustPOOI Vol. 37 No. 16