gm-c filter on 5.8ghz

8/21/2019 Gm-C filter on 5.8GHz

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Design of Low Power CMOS Band-pass Gm-C Filter

for 5.8 GHz RF Transceiver of ETC SystemYan Li1,2, Hang Yu1,2, Lai Jiang1,2, Zhen Ji1,2

1.Shenzhen City Key Laboratory of Embedded System Design, Shenzhen University2.College of Computer Science and Software Engineering, Shenzhen University

Shenzhen, 518060 China

Abstract-Since electronic tolling collection (ETC) allowsvehicles passing through without slowing down which improvesgreatly transportation efficiency, it is chosen as the basictechnology for new national highway network construction. Thekey component of the ETC system is a 5.8 GHz RF transceiverthat enables wireless communication between toll booth andvehicles. A 1-V Gm-C sixth order band pass filter is designed in a0.18 mm CMOS process for this purpose. Based on a pseudodifferential operational transconductance amplifier (OTA), awide tuning range and large input voltage swing is achieved. Acommon-mode feed forward (CMFF) circuit is introduced toreduce the distortion caused by common mode signal. The filteris implemented as a cascade of three identical second-orderblocks. The power consumption is about 456 W.

I. I NTRODUCTION

With the rapid increase of automobiles, traffic congestion is

now a prominent problem in many major cities in China. One

way to alleviate the traffic problems is to employ information

and wireless communication technologies on transport

infrastructure and vehicles to construct urban intelligent traffic

control system. Floating car data (also known as floatingcellular data), inductive loop detection and video vehicle

detection have been developed for traffic measurement [1-3]

and radio frequency identification (RFID) technology is

widely applied traffic control. Electronic tolling collection

(ETC) is one of the main applications. On the contrary of the

traditional manual in-lane toll collection process, the ETC

allows drivers to pass through a toll booth without stopping

their cars. Thus, a registered car will be electronically debited

when it drives through a toll gate at traffic speed. Toll booth

throughput is largely increased and the traffic congestion is

minimized. Taking a long-term view, operation costs will be

also lower since additional investment in toll booth and the

number of staff dedicated to the toll collection process can be

both reduced. Based on the above advantages, the ETC is now

widely applied to enforce congestion pricing in city centers [3-

5]. According to the advantages of the ETC, China's

transportation authority plans to construct a national ETC

based highway network with the goal of improving

transportation efficiency [6]. The Standardization

Administration of the People's Republic of China (SAC) has

released the Chinese standard of Dedicated Short Range

Communications (DSRC), which set its working frequencies

range from 5.8 GHz to 5.9 GHz [7].

A typical ETC system involving toll gate and vehicle is

given in Fig. 1. Relied on RFID technology, a road-side unit

(RSU) is installed at the toll gate and an on-board unit (OBU)

is installed on the vehicle. When a vehicle is passing through

the toll gate, the RSU works as a base station that

communicates with the OBU via DSRC. In order to decrease

interference from neighbor lanes, a high gain and narrow beam-with antenna should be used in the RSD design. For the

OBU, since it is required to be installed inside a car and is

battery powered, the characteristics such as easy-to-portable

and low-power must be concerned.

Figure 1. ETC based system. The RSU is at the toll gate and the OBU isinside the car.

The basic issue to ensure a fast automatic toll collection

process is to realize high quality two-way communication

between the OBU inside moving object and the RSU of the base station. A 5.8 GHz RF transceiver that complies with

China's ETC standard [7] is the key component for both part

of such a system. In particular, in order to meet the

requirement of the OBU, the power consumption of the

transceiver should be reduced as much as possible. Smart

wake-up strategy that only actives the chip during toll

collection process should be implemented and low-voltage

VLSI circuit design techniques should be applied. In the

transceiver, a filter is necessary to reject surrounding interface

of the input signal. Since Gm-C filters operate on open loop

topology, good frequency responses of signal transfer can be

obtained [8]. Compared to active R-C filters, the tuning range

OBUinside

RSU

RSU RSU

Antenna

OBU

Wake up

Information exchange

OBU Antenna

OBUinside

RSU

RSU RSU

Antenna

OBU

Wake up

Information exchange

OBU Antenna

This work is partially supported by the project 60901016supported by NSFC, the project 801000172 supported by SZU

R/D Fund and the SRF for ROCS, SEM.



of Gm-C filter is easily achieved by changing DC-bias of the

Gm cell. In this work, a 1-V sixth order band-pass filter is

design to meet the size and power limits of the 5.8 GHz RF

transceiver. The filter is implemented by cascading three

identical second order Gm-C blocks which is based on pseudo

differential operational transconductance amplifier (OTA).

The OTA operates in both weak inversion and strong

inversion regions and a wide transconductance tuning range is

achieved. In section II, the design issue about the pseudo

differential OTA, which is the basic element of a Gm-C filter,

is discussed. The filter design is described in section III.

Section IV gives the simulation results and the conclusion is

given in Section V.

II. PSEUDO-DIFFERENTIAL OTA CIRCUIT

Since a pseudo differential transconductance avoids the

voltage drop across the tail current source, this structure is

suitable for low power supply application. However, the

common mode signal is equal to the input differential signal in

such architecture so that carefully control on common mode

signal is required. In this work, a common mode feed forward

(CMFF) method introduced in [9] is used. An additional

transconductance with non-differential inputs is applied for

common mode cancellation, as shown in Fig. 2. The added

transconductance has the same common mode signal as that of

the original one (Gma = Gmb) but a zero differential

transconductance. Thus, the common mode signal of the

original transconductance can be rejected by subtraction the

output current at the output nodes.

Figure 2. Principle of CMFF for common mode cancellation.

The CM rejection ratio (CMRR) of the proposed CMFF

architecture is given in [9] which is

(1)

, where τ is the time constant of the current mirror pole. From (1),

the CMRR is dominated by the matching errors at low

frequencies.

The circuit implementation of the CMFF architecture is

given in Fig. 3. The W/L ratio of transistors M3 and M4 is

half of the transistors M1 and M2. They sense the input

common mode signal and then send it to the main pseudodifferential pairs through the current mirror formed by the

transistors M5 to M10. The current flowing through transistor

M5 is mirrored to the main pseudo differential pairs and then

the cancellation of the input common-mode signal is obtained.

In order to improve output impedance, cascade current mirrors

are used here. Moreover, a high-swing structure [10] is

applied for achieving a large output swing. Transistors M11

to M13 are added to ensure that the transistors M1 to M4

operate in the triode region.

Figure 3. Circuit implementation of the CMFF architecture.

Since the transistors M3 and M4 operates in the triode

region, the current IM is given by

I M = 2 K (W/L)3,4V CM V DS (2)

, where K is the technology parameter which equals to μCOX.When the second order effect of the transistors is not

concerned, the transconductance of the stage can be estimate

by the following formula:

(3)

Vb

Vi+

Vi–

Io+ I

o–

VDD

M1 M2 M3 M4

M5M

6M7

M8M

9M

10

Vt

M13M

12M11

IM

Vb

Vi+

Vi–

Io+ I

o–

VDD

M1 M2 M3 M4

M5M

6M7

M8M

9M

10

Vt

M13M

12M11

IM

m11,12

1,2ds

DS 1,2

m

g

g 1

V K(W/L)G

+=

+ –

– +

+ –

+ –

Vcm + Vin/2

Vcm – Vin/2

io+

io –

A

B

+ –

– +

+ –

+ –

Vcm + Vin/2

Vcm – Vin/2

io+

io –

+ –

– +

+ –

+ –

Vcm + Vin/2

Vcm – Vin/2

+ –

– +

+ –

+ –

Vcm + Vin/2

Vcm – Vin/2

io+

io –

A

B

⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜

⎝

⎛

+−+

= sτ

G

GG

sτ 1CMRR

ma

mbma



The value of the GM is tunable by changing VDS via the

adjustment of Vt. A linear relationship between the Gm value

and the control parameter is achieved.

III.

FILTER DESIGN According to the power and space limits of the OBU, direct

conversion structure is chosen to realize the 5.8 GHz RF

transceiver. In a direct conversion structure, an important issue

is that clock leakage to the inputs will cause unwanted DC

bias shift. The solution is to introduce an intermediate

frequency (IF) instead of using directly the base band. Taking

into account both the power consumption, the IF is set to 10

MHz. A band-pass filter with ω 0 equaling to 10 MHz is then

indispensable to get the useful signal. For entirely rejection of

the unwanted background, a sixth order filter is chosen.

By biquad synthesis method, the sixth order band-pass filter

is built by cascading three identical second order blocks. Each

block is a Gm-C filter based on the OTA discussed in section

II. The architecture proposed in [11] is used for the second

order filter design. The schematic is given in Fig. 4.

Figure 4. System schematic of the second order filter.

The transfer function is as follows:

(4)

As shown in (4), the ω 0 and Q of the filter can be adjusted

independently by the gm1 to gm4, which are linearly controlled

by Vt, as indicated in Fig. 3. However, in order to achieve the

desired ω 0 , a large value of capacitor is required. Thus, the

grounded impedance must be scaled down using the

techniques described in [12]. The principle of impedance

scaling is illustrated in Fig. 5. In this architecture, because the

equivalent ac impedance of the diode connected transistor M1

is 1/gm (seen from point A), the total impedance seem from Vi

is Z+1/gm. When a large (W/L) ratio of M1 is chosen, the ac

current flowing through Z is ir =Vi / Z . Note that the (W/L)

ratio between M1 and M2 is 1/N , the total current at the input

node should be: (N+1) ir . Therefore, an effective capacitance

equaling (N+1) C can be obtained when a capacitor C is used

as the impedance.

Figure 5. Impedance scaling.

IV. SYSTEM VALIDATION

The whole system was implemented using TSMC 0.18 μm

CMOS technology. Simulated by the Spectre, the I-V transfer

curve of the CMFF OTA is given in Fig. 6. When Vid change

from -0.5 V to 0.5 V, a good linearity is obtained.

Figure 6. I-V transfer characteristics of the CMFF OTA.

Figure 7. The gain and phase responses of the sixth order filter.

+ –

– +

+ +

– –Vid

+ –

– +

+ +

– –Vobp

C1

C1

C2

C2

gm1gm2 gm3 gm4

+ –

– +

+ +

– –Vid

+ –

– +

+ +

– –Vobp

C1

C1

C2

C2

gm1gm2 gm3 gm4

+ –

– +

+ +

– –Vid

+ –

– +

+ +

– –Vobp

C1

C1

C2

C2

gm1gm2 gm3 gm4

2C

1C

m4 g

m3 g

)s

2C m2

g ( 2 s

)s

1C

m1 g (

H(s)

++

=

M4

M3

M2M

1

Vb1

VDD

Vi

it

ir

Z

A

1 : N

M4

M3

M2M

1

Vb1

VDD

Vi

it

ir

Z

M4

M3

M2M

1

Vb1

VDD

Vi

it

ir

Z

A

1 : N

0

-20

-40

-60

-80 M a g ( d B )

180

-180

0

-90

90

P h a s e ( d e g r e e )

105

106

107

108

109

Frequency (Hz)

-500 -250 0 250 500

0

5

10

15

20

25

C u r r e n t ( µ A )

Vin(mV)-500 -250 0 250 500

0

5

10

15

20

25

C u r r e n t ( µ A )

Vin(mV)



The gain and phase responses of the sixth order filter are

given in Fig. 7. The design requirement is achieved. The

power consumption of the system is about 456 μW.

V.

CONCLUSION A low power sixth order filter for the 5.8 GHz transceiver of

the ETC system is designed. The Gm-C cell based structure is

used for reduced system power consumption. A pseudo

differential OTA employing CMFF technique is designed. The

sixth order filter was realized by cascading three identical

second order filters that based on the OTA. Simulation results

show that the system satisfies the design requirements.

R EFERENCES

[1] T. Monahan, “‘War Rooms’ of the Street: Surveillance Practices inTransportation Control Centers,” The Communication Review 10, 2007,

pp. 367–389.[2] A. Gühnemann, R.P. Schäfer, K.U. Thiessenhusen and P. Wagner,“New Approaches to Traffic Monitoring and Management by FloatingCar Data,” The 10th World Conference on Transport Research, 2004.

[3] P.J. Tarnoff, D.M. Bullock, E. Stanley et al., “Continuing Evolution ofTravel Time Data Information Collection and Processing,”Transportation Research Board Annual Meeting, 2009.

[4] H. Kanoshima, “Development of next-generation road services by public and private joint research,” 8th International Conference on ITSTelecommunications, ITST, 2008.

[5] S. Lin, W.H. Xiong and J.M. Xu, “Research on decision-makingorientated index and method of ETC evaluation system,” Urban Traffic1, pp. 81–86, 2008.

[6] Guidelines of the eleventh five-year plan (2006-2010) for nationaleconomic and social development, 2005.[7] GB/T 20851-2007, “Electronic toll collection-Dedicated short range

communication Interface with Roadside Unit and Lane Controller”,2007.

[8] K. C. Kuo and A. Leuciuc,“A linear MOS transconductor using sourcedegeneration and adaptive biasing,” IEEE Trans. Circuits Syst. II, vol.48, no. 10, pp. 937–943, Oct. 2001.

[9] F. Rezzi, A. Baschirotto, and R. Castello, “A 3 V 12–55MHzBiCMOS pseudo-differential continuous-time filter,” IEEE Trans.Circuits Syst. I, vol. 42, no. 11, pp. 896–903, Nov. 1995.

[10] P. J. Crawlet and G. W. Roberts, “Designing operationaltransconductance amplifiers for low-voltage operation,” in IEEE Znt.Symp. Circuits Syst.(ISCAS ’94), London, U.K., pp. 1455-1458, 1994.

[11] Y. P. Tsivids, “Integrated continuous-time filter design -- An overview,”IEEE J.Solid-State Circuits, vol. 29, no. 3, pp. 166-176, 1994.

[12] P. E. Allen, D. R. Holberg, “CMOS analog circuit design,” Beijing:

PHEI, 2002.

gm-c filter on 5.8ghz

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