A BAND-PASS FILTER IN 0 , 18 μ m CMOS FOR 2 , 4 GHz READER-LESS RFID TRANSPONDER

Original scientific paper Radio Frequency Identification (RFID) based systems are ubiquitous nowadays. Band pass filters always play an important role in overall performance of an RFID transponder. In this paper, a band pass filter with a transistor only active inductor is presented for compact high performance reader-less RFID transponder. Post layout simulation result reveals that the centre frequency of the filter can be set to 2,42 GHz frequency with a bandwidth of 38 MHz. The filter core occupies an area of 0,004 mm and dissipates only 1,3 mW at 1,5 V supply voltage. CEDEC 0,18 μm CMOS technology in Mentor Graphics environment has been used for the design of the proposed filter.


Introduction
RFID is a lucrative automatic system for storing as well as remotely retrieving information through contactfree electromagnetic communication in a cost effective way [1].It consists of three basic attributes; tags, readers and associated data-processing equipment [2].The tags, attached to the objects from which information is to be retrieved, contain identification i.e.Electronic Product Code (EPC) and storage for keeping information which is read by a reader.Finally the information is processed by computers.Almost every country spends a considerable amount of money to develop RFID for different applications [3][4][5][6].
An RFID tag can be passive (draw power from the reader), active (battery powered) or semi-passive (require battery-but the tag lies inactive until a signal is received from the reader).A reader reads tag data by using wireless communication and typically, these transmissions occur in UHF band or High Frequency band [7].Current technology lets the researchers to use microwave band for nowadays [8].The frequency band of RFID is determined by the purpose of their usage.Such a communication system offers many advantages over other similar systems like magnetic strip, bar code etc. [9].
The IEEE 802.11 standard in the 2,4 GHz band has emerged as a popular technology for wireless access.The IEEE 802.11 protocol-based wireless communication is robust, resistant to interference and has more range and a lower bit error rate (BER) at data rates as high as several Mbps [10].IPv6 protocol is going to replace the IPv4 in the near future, for global unique address structure [11].In contrast to WLAN, an RFID network is less secure, but more expensive.Reader is the highest priced component in an RFID network [12].To eliminate the vendor specific monopoly business and unnecessary establishment investment, a novel RFID using WiFi has been proposed where the integrated Wireless Network Interface Cards (WNIC) of laptops or PCs can be replaced for readers [13].The Electronic Product Code (EPC) would be mapped in the IPv6 protocol that is only 128 bits.As a result the complete scenario of concurrent RFID system will be changed and the users will be able to use a common RFID tag for their applications and the readers will be no more required.So overall system cost will be less.Current trend of the IC design led the researchers to fabricate low cost, low power and compact wireless communication systems [14,15].An RF filter is an important block for RFID transceivers as shown in Fig. 1.The filter performance greatly influences the overall performance of the transceiver.On-chip spiral inductors are usually utilized to realize the RF bandpass filters.But such spiral inductors are not recommended due to resistive loss and the capacitive coupling to the substrate.Technical Gazette 24, 1(2017), 31-34 Moreover, they are not suitable for RF circuits where high Q factor and high inductance are required.Active inductor is one of the promising solutions to overcome these limitations [16,17].Therefore, in this study, an active inductor based bandpass filter design is proposed for 2.4 GHz ISM band RFID transponder.

Methodology
The idea of an active inductor is derived from a wellrecognized gyrator concept as shown in Fig. 2 [18], which is a two-port circuit usually consisting of a couple of transconductors connected in negative feedback.An active inductor is a circuit which consists of a combination of CMOS transistors and works as an alternative to the spiral inductor.The active inductors are preferred because of their large inductance values, high quality factor, low chip area and tenability.But the usage of active inductors in RF bandpass filter encounters a few difficulties like poor noise performance, inadequate dynamic range and relatively higher power consumption [19].In this circuit, M 1 and M 2 act as a non-inverting transconductor which is labeled as G M1 whereas transistor M 3 works as an inverting transconductor which is labeled as -G M2 .Thus, G m1 and -G m2 build up a gyrator.This gyrator along with the parasitic capacitor C 3 at Node 3, works as an equivalent inductor, L eq = C 3 /(G m1 G m2 ) at Node 1.To tune the center frequency and Q factor of the inductor, varactors, M I and M Q are added, respectively, to the proposed active inductor circuit as shown in Fig. 5.

Figure 5 Active inductor with tuning functionality
The proposed band pass filter is framed with three phases as shown in Fig. 6.The input buffer generates a current which is fed to the active inductor based tank circuit to produce the desired output voltage.To avoid the change in centre frequency and Q factor tuning of the inductor due to load, an output buffer is incorporated.The complete circuit schematic of the proposed band pass filter is shown in Fig. 7. 3

Results and Discussions
The bandpass filter is designed and simulated in CEDEC 0,18 µm CMOS process parameters with 1,5 V power supply.Design Architect (DA-IC) and IC station tools of Mentor graphics is used to measure the performance of the band pass filter.In this study, centre frequency, bandwidth, Q factor, power dissipation of the filter are evaluated from the post layout simulation result.The AC analyses of the bandpass filter circuit are shown in Fig. 8, from which it is evident that the proposed band pass filter circuit can operate at the centre frequency of 2,42 GHz having a quality factor of 63,81 and bandwidth of 37,97 MHz.The comparison of centre frequency with and without varactors M I and M Q is shown in Fig. 9. Without a varactor M I , it shows that the bandpass filter circuit operates at the centre frequency at 2,29 GHz.While without both varactor M I and M Q , it shows that the bandpass filter circuit operates at the centre frequency at 2,36 GHz.It shows that two MOS varactors, M I and M Q are added for tuning the centre frequency and quality factor respectively.Fig. 10 shows the centre frequency tuning of the proposed bandpass filter circuit by varying VIF.Gate voltage for transistor M 4 is varied from 0,70 V to 0,90 V.It shows that the chosen value for VIF equal to 0.85 V will result in 2,42 GHz centre frequency of the proposed bandpass filter circuit.The total power dissipated by the filter including all the associated circuitries is 1,3 mW.The power consumption of the filter will be less if it is used alone excluding the input and the output buffer.The layout design for the bandpass filter is shown in Fig. 11.It occupies a die area of 0,004 µm 2 .The performance of the proposed bandpass filter is abridged in Tab. 1.A comparison of this work with the previous works in literature is presented in Tab. 2. From the assessment it is obvious that the proposed filter, including the input and the output buffer, dissipated only 1,3 mW power at 1,5 V Technical Gazette 24, 1(2017), 31-34 supply voltage.The die area is also very small which is only 0,004 mm 2 without pad and matching networks.The Q factor and bandwidth of the filter are also competitive with previous works.The filter also got the privilege of adjustable center frequency ranging from 1,91 to 3,31 GHz.

Conclusion
RFID technology, being a reliable wireless communication system, offers many advantages over other similar systems.As a result its utilization for different applications is increasing day by day.In this paper, a design of a fully integrated bandpass filter for 2,45 GHz RFID tag front-end in 0,18 µm CMOS process has been presented.The post layout simulation result demonstrates that the filter can operate at 2,42 GHz center frequency and achieve less power dissipation at low supply voltage of 1,5 V when compared with previous works.Such a filter is essential for high performance analog front-end of a readerless active RFID tag.

Figure 1
Figure 1 Block diagram of RFID tag IC

Figure 2
Figure 2 Basic concept of a gyrator where, G mx s are the transconductances of the amplifiers.An active inductor is a circuit which consists of a combination of CMOS transistors and works as an alternative to the spiral inductor.The active inductors are preferred because of their large inductance values, high quality factor, low chip area and tenability.But the usage of active inductors in RF bandpass filter encounters a few difficulties like poor noise performance, inadequate dynamic range and relatively higher power consumption[19].

Figure 6
Figure 6 Block diagram of the proposed inductor-less band-pass filter

Figure 8
Figure 8 AC analyses of the bandpass filter circuit

Figure 9 Figure 10
Figure 9 AC analyses of the bandpass filter circuit with and without a varactor

Table 1
Summary of the proposed bandpass filter circuit

Table 2
Performance comparison of the proposed bandpass filter with previous works * including pad