A Compact Dual-Band Differential Negative Group Delay Circuit With Wideband Common Mode Suppression

A compact dual-band differential negative group delay (DNGD) circuit with wideband common mode suppression is proposed. Two small 180° phase shift swap structures of double-sided parallel striplines (DSPS) are adopted to realize intrinsic wideband common-mode suppression. The dual-band DNGD characteristic is achieved by the DSPS coupled asymmetrically with two different-length open-circuited resonators. The DNGD times and center frequencies of the lower and upper bands can be tuned independently. To verify the proposed DNGD circuit topology, a prototype is designed, fabricated, and measured. From the measured results, DNGD times are −1.60 ns and −1.43 ns at the center frequencies of 1.227 GHz and 1.575 GHz, respectively. The differential insertion loss is lower than 3.55 dB, the differential return loss is larger than 13.3 dB and the common-mode suppression is over 30.8 dB in both DNGD bands.


I. INTRODUCTION
In recent years, with the rapid development of high-speed electronic systems, balanced/differential circuits have been utilized widely due to their excellent performance in terms of the immunity against environmental noises and electromagnetic interferences, compared with the single-end counterparts [1]. To combine the balanced active circuits and differentially driven antennas to build up fully balanced transceivers, various balanced passive components have been proposed, such as balanced filter [2], balanced coupler [3], and balanced power divider [4].
The group delay (GD) is a key parameter in microwave circuits and systems [5]. In the pseudo-range measurement of the global navigation satellite systems (GNSS), the distortion of the group delay will cause the increase of the system measurement error [6]. For these reasons, the GD equalization technique based on the negative group delay (NGD) has been proposed [7]- [10]. In [11], a reconfigurable negative group delay circuit (NGDC) based on the branch-line coupler was proposed. By simply adjusting the capacitance values, both the transmission gain/loss and group delay values of the NGDC can be independently adjusted as desired. But it needs to insert two amplifiers in the transmission paths to compensate for the transmission loss. Various approaches for the implementation of RF passive devices integrated with NGD characteristics have been presented [12]- [17]. In [12], a novel and generalized planar coupler with four independent arbitrary terminated resistances, unequal power division, and wideband positive and negative group delays was proposed. In [13], a full-passband linear-phase bandpass filter was presented and the NGDC was utilized to suppress the salient group delay at the edge of the passband of the traditional bandpass filter for achieving the full-passband linear-phase characteristic. Several kinds of RF power dividers integrated with NGD characteristics can be achieved using the coupling matrix approach [14]- [17]. All mentioned RF passive devices focus on improving the phase characteristics of their devices by integrating NGD characteristics. However, these RF passive counterparts are single-ended and unsuitable for use in balanced systems to reduce interference from common-mode noise.
Furthermore, with the rapid development of various wireless systems for multiple-access and multifunctional applications, several dual-band NGDCs have been designed in [18]- [21] to satisfy the GD compensation for multiband devices. In [18], [19], the frequency ratio of the upper and lower bands is decided by the characteristic impedance of the transmission lines. However, the upper and lower NGD operation frequencies f 1 and f 2 rely on the frequency f 0 = (f 1 + f 2 ) /2. To independently control the center frequency and NGD time, a dual-band NGDC with the defected structures was proposed [20]. The center frequency can be changed by the defected structures and the NGD time of each band can be tuned by the loaded resistors. But it has to face the problem of large insertion loss (IL). In [21], a dual-band NGDC with multi-coupled lines was proposed. The disadvantage is that the circuit dimension is slightly larger. In addition, an approach to applying the NGD circuit to a multi-band system is detailed in [22], but higher-order and in-series-cascade multi-stage realizations increase the complexity of the circuit design. In order to achieve a differential circuit with commonmode noise suppression, negative group delay properties can also be implemented to improve the phase behavior of the circuit, two balanced circuits integrated with differential negative group delay (DNGD) characteristics have been presented [23], [24]. However, the common-mode suppression (CMS) is poor. To the best of our knowledge, the dual-band DNGD circuit has not been reported before. Therefore, it is necessary to design a dual-band DNGD circuit to accommodate various wireless systems for multiple-access and multifunctional applications and to achieve excellent common-mode noise suppression in balanced systems.
In this paper, a compact dual-band differential negative group delay circuit (DNGDC) with wideband common-mode suppression is proposed. The proposed DNGDC is composed of two small 180°phase shift swap structures and the double-sided parallel striplines (DSPS) coupled asymmetrically with two different-length open-circuited resonators. Excellent common-mode suppression and the DNGD characteristic can be integrated into this dual-band balanced circuit, which has a flexible design of operation frequencies (f 1 and f 2 ) and DNGD times. The analysis and results of the proposed dual-band DNGDC are given and discussed. And it can be applied in dual-band balanced GPS L1 band and L2 band system to shorten the delay lines and suppress the common mode interference.

II. DESIGN OF THE PROPOSED DUAL-BAND DNGDC
The configuration of the proposed dual-band DNGDC is shown in Fig. 1. It is originally implemented with two small 180°phase shift swap structures and the asymmetrically coupling between the main transmission line and the λ/2 When the length of the TL1 (L 1 ) is increased from 37.18 to 45.18 mm, the center frequency of the lower DNGD band varies inversely to L 1 . The change of the DNGD value and differential insertion loss (|S ddBA |) at the center frequency of the lower DNGD band is negligible. Meanwhile, during the width of the TL1 (W 1 ) is increased from 2 to 6 mm, the center frequency of the lower DNGD band is also increased. On the contrary, the DNGD absolute value and differential insertion loss at the center frequency of the lower DNGD band are decreased with the increase of W 1 . Furthermore, the performances in the upper DNGD band are less sensitive to the open-circuited DSPS resonator TL1. On the contrary, the open-circuited DSPS resonator TL2 has a negligible effect on the lower DNGD band and a similar effect on the upper DNGD band, as shown in Fig. 3. Therefore, by tuning (L 1 , W 1 ) and (L 2 , W 2 ), the independent adjustment of the lower and upper DNGD bands can be achieved.  When the coupling length L 3 is increased from 18.18 to 26.18 mm, only the differential group delay (DGD) and |S ddAA | in the lower DNGD band have minor changes. When the coupling length L 4 is increased from 10.12 to 18.12 mm, the DNGD absolute value and differential insertion loss at the center frequency of the upper DNGD band are increased. Fig. 5 shows the effects of the coupling gaps (S 1 and S 2 ) on the performance of the proposed DNGDC. During S 1 is increased from 0.13 to 0.33 mm, the DNGD absolute value at the center frequency of the lower DNGD band is decreased and the performances in the upper DNGD band are unchanged. During S 2 is increased from 0.1 to 0.3 mm, the DNGD absolute value at the center frequency of the upper DNGD band is decreased and the performances in the lower DNGD band are unchanged. The gaps of the coupling sections S 1 and S 2 have independent effects on the lower and upper bands. Therefore, when tuning S 1 and S 2 , the center frequency is slightly changed, but the independent adjustment of DNGD time for both bands can be achieved. Fig. 6 gives the effects of the main transmission line on the performance of the proposed DNGDC. When the width of the main transmission line (W 3 ) is increased from 3 to 7 mm, the DNGD absolute values at the center frequencies of     54.1 mm (around 0.41λ g × 0.30λ g , where λ g is the guided wavelength of 50-transmission line at f 1 ). The fabricated dual-band DNGDC prototype is measured with an Agilent N5230A network analyzer. Fig. 8 shows the measured results of DGD and mixed Sparameters, along with simulated ones for comparison. For f 1 of 1.227 GHz, the measured DGD and |S ddBA | are −1.60 ns and −2.49 dB, respectively. The lower DNGD bandwidth (the bandwidth for DGD less than 0 ns) is 1.47% (1.217-1.235 GHz). The differential return loss (RL) is better than 14.5 dB (i.e., |S ddAA | < −14.5 dB) in the lower DNGD band. For the upper band, the measured DGD and |S ddBA | at f 2 = 1.575 GHz are −1.43 ns and −3.55 dB, respectively. The upper DNGD fractional bandwidth (FBW) is 1.46% from 1.562 to 1.585 GHz. The differential RL is better than 13.3 dB in the upper DNGD band. As shown in Fig. 8, the measured |S ccBA | is less than −30.8 dB from 1.05 to 1.75 GHz, wideband and large common-mode suppression is obtained. There are some discrepancies between the simulated and measured results, which are mainly due to inaccurate dielectric constant and loss of the used FR4 substrate in the prototype and fabrication tolerance.
The comparison of the proposed dual-band DNGDC with previous works is shown in Table I. Compared with [18]- [21], [23], [24], the proposed balanced dual-band DNGDC with outstanding CMS has a flexible design of operation frequencies (f 1 and f 2 ) and DNGD times. Furthermore, the proposed DNGDC has a small insertion loss, compact circuit structure, and moderate figure of merit (FOM), where the FOM is defined as (1)

IV. CONCLUSION
In this paper, a compact dual-band DNGDC with wideband CMS has been presented. Two small 180°phase shift swap structures of DSPS deliver excellent common-mode rejection properties with a compact structure. The dual-band DNGD characteristic is generated by the DSPS coupled asymmetrically with open-circuited resonators. Compared to the existing NGDCs, the proposed dual-band DNGDC is integrated with excellent CMS and has the advantage of flexible design of operation frequencies and DNGD times. So it can be applied in dual-band balanced GPS L1 band and L2 band system to shorten the delay lines and suppress the common-mode interference.