I. INTRODUCTION

The video streaming applications develop rapidly between mobile devices and TVs, especially for the next generation TFT-LCD devices with Ultra HD (UHD) 4K2K (3840 ´ 2160) resolution. As a result, high-speed interface becomes a critical design. For example, Fig. 1 shows the UHD display system with intra-panel interface, where the length and width are two times of the FHD resolution. As the TFT-LCD panel becomes larger, the transmission distance between timing controller (TCON) and column drivers (CD) becomes longer. Thus, the dielectric loss and skin effect results in much more serious in high-speed transmission, which will cause inter-symbol interference (ISI) issue. In order to solve above problems, an adaptive equalizer is indeed for the display system.

Several adaptive methods for the continuous time equalizer (CTLE) have been proposed. The conventional architecture with two high-pass filters (HPF) will cause an unbalancing-swing problem between input and output of the slicer^[1]. Although the enhanced low-frequency gain method with two low-pass filters (LPF) can adjust the low-frequency gain of the equalizer^[2]. The highfrequency gain tuning range will be limited because the trade-off between high frequency and low frequency compensations in the equalizer filter. In order to solve the unbalancing swing problem, the spectrum balancing technique has been proposed^[3,4], which removes the slicer and sensing the energy of equalizer’s output directly with HPF and LPF for lower power applications. However, this method works only if the data is random and fixed data rate. For the intra-panel interface of the display system, we must find a more robust design for the equalizer.

In order to solve the problems above, an adaptive equalizer with swing control technique was proposed^[5-7]. In this work, we adopt a dual control technique using a swing-matching balancer for the slicer and an adaptive controller to tune the gain and bandwidth for the frontend equalizer. By generating swings that match the equalizer output swings, it won’t have the unbalancing swing problem. Furthermore, in order to overcome the various transmission distance between TCON and CD of UHD 4K2K large display panel. We add a fine tune loop to adjust the low-frequency gain by 3-bit resistor-string DAC, which can not only ensure the equalizer have an enough wide bandwidth for various loss but also overcome the variations of process, voltage, and temperature (PVT).

Section II presents the architecture and operation of the proposed adaptive equalizer. Section III describes the circuit in detail. Section IV and V show the measurement results and conclusion, respectively.

II. ARCHITECTURE

Fig. 2 shows the block diagram of the proposed adaptive equalizer, where the equalizer’s filters followed by a slicer^[7]. The high-frequency compensation is sensing signal X and Y by two pairs of band-pass filters (BPF), which measuring the energy in a narrow range of frequencies to promote the accuracy. Then the rectifiers extract the high-frequency power. After that, the difference between these two signals is amplified by the error amplifier to generate a feedback signal (Vctrl_High) for the equalizer filter. In order to eliminate the unbalancing swing problem that cause by the signal level and circuit differences between signal X and Y, we generate a feedback signal (V_{ctrl_Swing}) to control slicer tail current. Besides, another signal (V_{ctrl_Adj}) is generated to overcome the various loss cause by different lengths of FR4 board (from 23 cm to 123 cm) and PVT variations by a 3-bit resistor string DAC.

In order to prove the system stability, we assume there is only one compensated loop to adjust the equalizer by BPF. At the stable condition, we can find out the spectrum at X and Y as blow:

where K_X and K_y are the scaling factors of the signals X and Y, and T_b is the bit period. As shown in Fig. 3, it measure the energy in narrow range of $f_{1}$ and $f_{2}$ by BPF, and we can express as:

In this condition, the output of X and Y carry similar spectra, which will cause poor adaptation, even if the error in the adaptive loop is approach zero. Because the spectrum of X between $f_{1}$ and $f_{2}$ is determined by the voltage swing of X. Besides, the voltage swings between X and Y must be different, which cause by characteristics of different length of transmission line and TX driver circuit.

The unbalancing problem can be alleviated by adjusting the equalizer low-pass frequency gain directly, but it will limit the high-frequency gain relatively^[2]. Increasing a swing control loop as the architecture in Fig. 2. We assume the low-pass characteristics of the signal X and Y is also captured in (1) and (2), which have the - 3-dB frequencies of $f_{p1}$ and $f_{p2}$ , respectively. Thus, for the swing control loop it can be expressed as:

where $f_{0}$ is the -3-dB frequency of the “assumable” LPF. When the stable state satisfy the following conditions, $K_{X} \approx K_{Y}$ , and $f_{p 1} \approx f_{p 2}$ . We can modified (4) as blow:

As shown in Fig. 3(b), the result shows the signal X (after equalizer) smoothly than in Fig. 3(a). In this way, we can achieve the same goal without LPF for saving chip area and power consumption. Furthermore, it won’t limit the tuning range of high-frequency gain, which can improve the accuracy of the adaptation^[5].

Unlike the conventional spectrum-balancing technique only works for random and fixed data rate. In the proposed architecture, the control for the high-frequency gain won’t interfere by unbalancing swing problem, but also suits for any coding scheme at the transmitter including the clock-embedded coding technique for intrapanel interface. As the advantages above, the architecture is very suitable for the next generation UHD 4K2K display panel, which have a various transmission distance.

III. CIRCUIT DESCRIPTION

IV. SIMULATED AND MEASURED RESULTS

The adaptive equalizer was fabricated in 0.18-μm CMOS technology. The die photograph of the chip is shown in Fig. 8. The total chip area is 0.47 mm², and the core area is 0.12 mm². The S21 parameters of various lengths of FR4 board are shown in Fig. 9, from - 6 dB to - 24 dB at 1.5 GHz. Fig. 10 shows the simulated frequency response of the whole system, the - 3 dB bandwidth from 225.6 MHz to 1.5 GHz after equalization. To evaluate the circuit performance, the prior work using the die on package can reach to 1.5 Gb/s^[7]. Alternatively, we adopted the die on PCB to reduce the parasitic effects of interface and the data rate of the proposed equalizer can achieve to 3 Gb/s. The test channel was a 1.23-m trace on the PCB with 24-dB loss at 3 Gb/s. Fig. 11(a)-(d) show the measured eye diagrams before and after equalization at 1.5-, and 3-Gb/s 2⁷ -1 PRBS pattern for 0.56-m and 1.23-m channel lengths, respectively. The measured equalized signals have only 0.356-UI peak-to-peak jitter at 3-Gb/s. The equalizer chip consumes 27-mW with 1.8-V power supply at 3-Gb/s, excluding the output buffer. Table 1 shows the performance summary and the comparison with the prior works^[2-4,6,8-11].

V. CONCLUSIONS

An equalizer with adaptive swing controller for the next generation 4K2K resolution TVs has been presented. It can improve the adaptation accuracy without degrades the boost tuning and input swing ranges. Furthermore, adding a 3-bit resistor string DAC for the equalizer filter is not only able to compensate for wide range of various lengths, but also immune from the PVT variations. The chip was fabricated in 0.18-μm CMOS process. The proposed adaptive equalizer shows the output peak-topeak jitter is 0.356-UI at 3-Gb/s of 24-dB channel loss (1.23-m PCB length).