Wide range suppressed harmonic response compact microstrip balun

Int. J. Electron. Commun. (AEÜ) 66 (2012) 45–48

Contents lists available at ScienceDirect

International Journal of Electronics and Communications (AEÜ) journal homepage: www.elsevier.de/aeue

Wide range suppressed harmonic response compact microstrip balun Vamsi Krishna Velidi ∗ , Subrata Sanyal Department of Electronics and Electrical Communication Engineering, Anechoic Chamber Lab, Indian Institute of Technology Kharagpur, Kharagpur 721302, West Bengal, India

a r t i c l e

i n f o

Article history: Received 23 April 2010 Accepted 26 April 2011 Keywords: Balun Microstrip Open-stub Harmonic suppression Bandwidth

a b s t r a c t In this paper, simple shunt open-stub units are used to design a compact balun with suppressed harmonic response. The stopband response of shunt stubs has been utilized to suppress the harmonic passbands. Design guidelines are presented with equations and graphs based on transmission line model. To validate theoretical predictions, an experimental prototype microstrip balun, with suppressed harmonic passbands up to six times the operating frequency, and occupying only 25% of the conventional balun circuit area is presented. © 2011 Elsevier GmbH. All rights reserved.

1. Introduction Baluns are important microwave components, used in many applications such as balanced antennas, push–pull amplifiers, balanced mixers and microstrip leaky-wave antennas, to convert unbalanced single input signals into balanced differential signals. Several planar balun designs were presented based on coupled lines [1], and Wilkinson power dividers [2]. Baluns using transmission line sections [3–6] are popular because of the simplicity. In [3], a stub loaded balun structure was presented. Much research is focused on miniaturization of balun [3] and making it suitable for dual-band [4] or wideband applications [5,6]. Suppression of balun harmonic response is rarely studied. Size reduction in [3] can be up to 50% only if the open-stubs are bent inside the interior ring. Further, the balun in [3] exhibits spurious passband response at the second harmonic in addition to the response at odd-harmonics of the operating frequency. Balun harmonic response may result in signals that could interfere with other equipments. Use of additional bandstop filter to eliminate the harmonics increases the passband insertion loss, circuit size and cost. In this paper, a compact balun with wide range of higher order harmonic suppression is presented. This is realized by replacing the transmission line sections of the conventional balun with their equivalent bandstop units (BSU). Design equations are derived using transmission-line model. Size reduction achieved here is larger than that in [3] along with the additional up to sixth harmonic

∗ Corresponding author at: Department of Electronics and Electrical Communication Engineering, Anechoic Chamber Lab, Indian Institute of technology Kharagpur, Kharagpur 721302, West Bengal, India. Tel.: +91 3222 281460; fax: +91 3222 282299. E-mail address: [email protected] (V.K. Velidi). 1434-8411/$ – see front matter © 2011 Elsevier GmbH. All rights reserved. doi:10.1016/j.aeue.2011.04.012

passband suppression. Further, the achieved equal magnitude (within ±0.15 dB) and 180◦ phase (within 5◦ ) balance bandwidths are larger than those of [3]. 2. Theoretical analysis The conventional balun considered here is a modified rat-race coupler [6] with open circuit at the uncoupled port 3 as shown in Fig. 1. The terminating impedance of the ports is Z0 (=50 ). A signal input to port 1 is coupled with equal magnitudes to ports 2 and 4. The coupling between the ports is 180◦ out of phase, as required for balun operation. The balun is characterized by six quarter-wavelength transmission line sections, having √ characteristic impedances Z = Z0 2. The basic BSU, equivalent to a conventional transmission line is shown in Fig. 2. It is composed of two shunt open-stubs (Z2 ,  2 ) separated by a transmission line section (Z1 ,  1 ). Similar method was reported in [8]. However, the design is only for a special case  1 =  2 , where all lines are of the same characteristic impedance. In this paper, on the other hand, generalized design equations for generating the parameters of the transmission line equivalent two-stub BSU are derived using transmission-line model calculations. By equating the ABCD parameters of the BSU to those of the transmission line section, the equivalence is obtained as Z1 = Z sin csc1

(1)

tan 2 sin  Z2 = Z cos 1 − cos 

(2)

From (1), it is observed that the series arm impedance Z1 of the BSU is independent of  2 . When Z = 70.7  and  = 90◦ , the impedance solutions (1) and (2) are plotted in Fig. 3(a). The impedance Z1 increases with decreasing  1 . In contrast, the

46

V.K. Velidi, S. Sanyal / Int. J. Electron. Commun. (AEÜ) 66 (2012) 45–48

S-parameters (dB)

(a) 0 -10

S31

-20 -30 -40 -50 0

2

3 f/f0

4

5

6

4

5

6

5

6

θ =60,° θ =30° 1 2

-10 -20

S21

-30 S31

-40 -50 0

Fig. 2. Transmission line and its equivalent circuit of a shorter transmission line bandstop unit with two shunted open stubs.

S11

1

2

3 f/f0

(c) 0 S-parameters (dB)

open-stub impedance (Z2 ) decreases with decreasing  1 . For any fixed  1 , the value of Z2 also decreases with decreasing  2 . The BSU produces a transmission zero at a frequency where the open-stubs are quarter-wavelength long (90◦ ). For example, the normalized frequency response of BSU when  1 = 60◦ and  2 = 30◦ and for a port impedance of 70.7  is illustrated in Fig. 3(b). A transmission zero at 3f0 is observed in the transmission response, which is symmetric when  2 is integer multiple of  1 . This stopband property is used to suppress the harmonic passbands of the balun. The length of the BSU is determined by  1 and the out-of-band rejection is controlled by  2 . The parameters of BSU, equivalent to the conventional single quarter wavelength section (Z = 70.7  and  = 90◦ ) are first obtained. The entire balun is then realized by cascading six such BSUs. Fig. 4(a) shows the circuit computed normalized magnitude responses of the conventional balun. Fig. 4(b and c) shows the responses of the proposed harmonic suppression balun realized using BSUs with  1 = 60◦ ,  2 = 30◦ and  1 = 45◦ ,  2 = 22.5◦ respectively. In each case, the corresponding impedances (Z1 , Z2 ) of BSU are obtained from Fig. 3. Note that the implemented stub

S11

1

(b) 0 S-parameters (dB)

Fig. 1. Conventional balun based on modified rat-race coupler [6].

S21

θ =45,° θ =22.5° 1 2

-10 -20

S21

-30

S31

-40 S11

-50 0

1

2

3 f/f0

4

Fig. 4. Circuit computed S-parameters of (a) conventional balun and (b and c) present harmonic suppression balun implement using bandstop units with different 1 , 2 .

Fig. 3. (a) Variation of Z1 and Z2 with  1 for the bandstop unit equivalent transmission line having Z = 70.7 ,  = 90◦ and (b) computed responses of the bandstop unit when  1 = 60◦ and  2 = 30◦ .

V.K. Velidi, S. Sanyal / Int. J. Electron. Commun. (AEÜ) 66 (2012) 45–48

47

Table 1 Comparison of simulated and measured results.

Return loss S11 (dB) Insertion loss S21 (dB) Insertion loss S31 (dB) Phase difference (S31 − S21 ) (◦ ) No. of harmonics suppressed Mag. balance (0.5 dB) BW (1.0 dB) BW Phase balance (5◦ ) BW

Fig. 5. Photograph of the proposed fabricated balun with integrated bandstop units.

S-parameters (dB)

Measured Fullwave

-10 -20

S21

Harmonics suppression

-30

1

2 3 4 Freequency (GHz)

5

6

Fig. 6. Simulated and measured responses of the proposed balun.

impedance for the proposed balun is Z2 /2. When  2 = 30◦ (22.5◦ ) the open-stubs produce transmission zero at 3f0 (4f0 ), where the electrical length is 90◦ . As the response being symmetrical about the transmission zeros, the harmonic passbands are suppressed effectively up to 4f0 (6f0 ). The number (N) of suppressed harmonic passbands is given by

 90  2

16.7% 23.3% 39%

Ref. [3]

Present work

Better than −20 ≈−3 ≈−3 ≈180 140

Better than −20 −3.56 −3.5 177.3 150

230

390

50% if the stubs are bent No

75% Yes (up to 6)

3. Experimental results

-50 0

N=2

17% 24.3% 32%

S31

S11

-40

Measured −20.9 −3.56 −3.5 177.3 6

Table 2 Comparison with best reported balun design.

Return loss S11 (dB) Insertion loss S21 (dB) Insertion loss S31 (dB) Phase difference (S31 − S21 ) (◦ ) Magnitude balance within ±0.15 dB BW (MHz) Phase balance within 5◦ BW (MHz) Size reduction

0

Fullwave Sim. −19.7 −3.63 −3.58 180.7 6



−1

(3)

Since the individual BSU has a passband at most up to 1.4f0 (see Fig. 3(b)), a raise in balun S21 response up to 5 dB is observed near 1.4f0 . However, this is not a significant passband as the return loss S11 is also 5 dB around 1.4f0 and hence will not affect on the overall performance of the proposed balun.

(b) 0

Phase difference (deg)

Magnitude balance (dB)

(a)

The proposed balun is realized in microstrip form at 1 GHz and fabricated on a low-cost FR4 board with thickness of 1.58 mm, dielectric constant of 4.3, and a loss tangent of 0.022. The line lengths of each BSU, corresponding to the electrical lengths  1 = 45◦ and  2 = 22.5◦ , are 21.7 mm, 9.7 mm respectively. The line widths of the series lines (Z1 = 100 ) and open-stubs (Z2 /2 = 20.7 ) are 0.7 mm and 10.6 mm respectively. The geometry of the proposed compact balun is shown in Fig. 5. The balun has an area that is one fourth of the conventional geometry and hence the size reduction is 75%. Superficially the geometry of the present balun appears like that in [7]. However, the main difference arises from the basic unit that is used to replace the conventional transmission line section. In [7], the structure is a ratrace hybrid coupler based on stepped impedance resonator (SIR) units. The lower impedance sections (larger line widths), of each SIR, are placed in the interior ring geometry. In contrast, here, the open stubs of each BSU are placed in the interior ring. Hence the operation is entirely different. Fig. 6 shows good agreement between the fullwave IE3D simulated and the measured results. An Agilent 8510C vector network

-1 -2 -3

Measured Fullwave

-4 -5 0.8

360 270 180 90 0

0.9

1

1.1

1.2

Frequency (GHz)

1.3

Measured Fullwave

0

0.5

1

1.5

Frequency (GHz)

Fig. 7. Simulated and measured (a) magnitude and (b) phase balance responses of the proposed balun.

2

48

V.K. Velidi, S. Sanyal / Int. J. Electron. Commun. (AEÜ) 66 (2012) 45–48

analyzer is used for measurements. The measured operating frequency is 1.06 GHz. The measured return loss is 20.6 dB; the insertion losses S21 and S31 are −3.56 dB and −3.5 dB respectively, at the operating frequency. The measured phase difference between the output ports is 177.3◦ . Second to sixth harmonics are suppressed by at least 30 dB. The magnitude and phase balance responses are shown in Fig. 7. Table 1 shows the comparison of full-wave simulated and measured results of the proposed balun for various parameters. The equal magnitude balance is within ±0.15 dB over a bandwidth of 150 MHz, while the 180◦ phase balance is within 5◦ over a bandwidth of 390 MHz. These bandwidths are larger than those achieved in [3], where the magnitude and phase balance bandwidths (within ±0.15 dB and 5◦ ) are 140 MHz and 230 MHz respectively. Table 2 compares the performance of the proposed balun with the reported balun [3]. The bandwidth of the proposed compact balun can further be improved using the short-circuited stub at port 2 as described in [5].

[3] Park M-J, Lee B. Stubbed branch line balun. IEEE Microw Wireless Compon Lett 2007;17(March (3)):169–71. [4] Zhang H, Peng Y, Xin H. A tapped stepped impedance balun with dual-band operations. IEEE Antennas Wireless Propag Lett 2008;7:119–22. [5] Li J-L, Qu S-W, Xue Q. Miniaturized branch-line balun with bandwidth enhancement. Electron Lett 2007;43(August (17)):931–2. [6] Bex H. New broadband balun. Electron Lett 1975;11(January (2)):47–8. [7] Kuo J-T, Wu J-S, Chiou Y-C. Miniaturization of rat-race coupler with suppression of spurious passband. IEEE Microw Wireless Compon Lett 2007;17(January (1)):46–8. [8] Velidi VK, Bhattacharya A. Miniaturized planar 900 hybrid coupler with unchanged bandwidth using single characteristic impedance line. In: 2008 China-Japan Joint Microwave Conference. 10–12 September 2008. p. 396–9. Vamsi Krishna Velidi received the B.Tech. degree in electronics and communication engineering from Jawaharlal Nehru Technological University, Andhra Pradesh, India, in 2003, the M.Tech. degree in RF and microwave engineering (department of E&ECE) from Indian Institute of Technology (IIT) Kharagpur, Kharagpur, WB, India, in 2008, and is currently working toward the Ph.D. degree at IIT Kharagpur. From 2003 to 2006, he worked as a faculty member. He has authored or coauthored 15 international journal papers. His current research interests include the design, analysis and experimental characterization of compact high performance planar passive devices in PCB microstrip/coplanar waveguide

4. Conclusion The design and implementation of a microstrip balun with compact size along with harmonic suppression is presented. Bandstop filter units, equivalent to conventional lines, are embedded in the balun design to achieve a wide range suppression of up to six harmonics passbands by at least 30 dB. The design is simple and straightforward as it is based on transmission line model. The proposed balun has wider balance bandwidths, wide range harmonic suppression and is more compact than the stubbed branch-line balun of [3]. References [1] Ang KS, Robertson ID. Analysis and design of impedance transforming planar Marchand baluns. IEEE Trans Microw Theory Tech 2001;49(February (2)):402–6. [2] Zhang Z-Y, Guo Y-X, Ong LC, Chia MYW. A new wideband planar balun on a single-layer PCB. IEEE Microw Wireless Compon Lett 2005;15(June (6)):416–8.

technologies.

Subrata Sanyal received the B.E. degree from the Indian Institute of Science (IISc) Bangalore, Bangalore, India, in 1975, and the M.Tech. and Ph.D. degrees from the Indian Institute of Technology (IIT) Kharagpur, Kharagpur, India, in 1977 and 1987, respectively. From 1977 to 1980, he was an Electronics Engineer with the Radio Astronomy Group, Tata Institute of Fundamental Research (TIFR), Ooty, India. From 1990 to 1992, he was a Post-Doctoral Research Fellow with Queen Mary and Westfield College, London. Since 1984, he has been a faculty member with IIT Kharagpur, where he is a professor. He has authored or coauthored over 17 IEEE journal papers. His research interest has been in the area of electromagnetic scattering and passive RF components.