A CMOS 0.35µm, 1.5V Multi-band RF Q-enhanced LC Bandpass filter

Aymen Ben Hammadi, Mongia Mhiri, Kamel BesbesMicroelectronic and Instrumentation LR

FSM, University of MonastirMonastir, Tunisia

benhammadiaymen@gmail.com, Mongia.Mhiri@fsm.rnu.tn, Kamel.Besbes@fsm.rnu.tn

Abstract— In this paper, a 2nd order tunable RF CMOS LC bandpass filter is presented. Q-enhancement and tuning are also considered. The Q-tuning is implemented through an adjustable negative conductance generator circuit. The simulated results show that using a 0.35 µm AMS CMOS process, the filter operates at center frequency varying from 1.69 GHz to 2.33 GHz frequency band under 1.5 V supply. A set of varactors were in assistance to achieve the coarse and exact tuning simultaneously.

I. INTRODUCTION

The tremendous advances in telecommunications that concern both the mobile phone sector, and the local networks, as the satellite positioning led to the proliferation of norms and standards [1]. A challenge of research today is to design devices that are reconfigurables, that is to say, be ordered to switch their characteristics from one standard to another [2].

One of the most critical reconfigurable functions is filtering radio frequency. It requires a broadband agreement in order to adapt the templates associated with different standards. Indeed, the most widely used filters are the ones tosurface acoustic wave (SAW). These filters are not tunable, and remain one of the most bulky passive devices of RF front-end [3-4].

Active filtering seems to be of solution for these two constraints [5]. Several ways are possible: resonators offset, recursive and transversal filters, Gm-C filters, etc ... For us, we develop on the Q-enhanced LC bandpass filters.

In this paper, we outline the highlight features for designing our bandpass filter in section II. The architecture of the Q-enhanced LC filter is exposed in section III. Section IV presents the simulation results of the RF filter. And finally in section V, conclusions are drawn.

II. Q-ENHANCED LC FILTER DESIGN

A simplified Q-enhanced filter is presented in Fig. 1; RPrepresents equivalent parallel loss resistance which is dominated by the inductor one.

The negative conductance represents negative resistance which is used in boosting the quality factor in a lossy LC tank. Using positive feedback is the basic idea of Q-Enhanced LC filter. For canceling losses presented by RP, performed on the lossy inductor, a negative resistance can be added, yielding the “parallel mode” Q-enhanced circuit shown in Fig. 2 [6], [7], where:

RsR )Q20(1p Q(1

LQ2

0

Q201

Lp LLLL11 Q

where Q0 is the self-tuning quality factor of the inductor.

In this approach, the negative resistance has been implemented as a transconductor with positive feedback to cancel losses represented by RP. The effective parallel resistance Req and the effective quality factor Qenh of the LC resonator can be found from:

RpRpgm11Rp

gm

1Req g11R

and

Q0Rpgm11

XLp

ReqQenh g1

R

and can be made arbitrarily high by a suitable choice of gm.

At large Q enhancements however, gm must be carefully controlled to avoid oscillation (Qenh �∞), and circuit tolerances and temperature coefficients become critical issues, requiring the addition of some form of tuning mechanism into the filter design. In addition, the process of Q enhancement creates a regenerative amplification of circuit noise, lowering the resonator’s dynamic range.

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978-1-4577-2209-7/11/$26.00 ©2011 IEEE

A. Frequency tunning To make a filter for multi-standard systems, varactors are

an adequate solution to cover a wide frequency range and thus meet many of the standards. One form of this solution is presented in Fig. 3 [8]. It consists in using, instead of a variable capacitor, storage of four parallel varactors which gates are connected to one terminal.

The drain, the source and the bulk of the varactor CV1 are related to the control voltage Vctr. For varactors CV2, CV3 and CV4 the drain, the source and the bulk are controlled by a digital control word.

B. Q-enhancement A primary method for increasing the Q of non-ideal on-

chip LC tank is the use of negative resistance, implemented by active devices, as shown in Fig. 4. But this is achieved at a cost of higher power consumption and noise, presented by the active devices.

A differential topology is used for our balanced circuit which is better in high frequency operation [9], using a cross-coupled transistor pair M1, 2 (gate inputs are connected to opposite drain Outputs). This topology achieves a positive feedback that compensates losses in the LC tank [8]. The voltage to current ratio indicates the effective negative resistance at the terminals M1, 2 as shown in Fig. 4.

The impedance seen between the two terminals Vds1 and Vds2 is expressed by R = -2/gm with the simple analysis given by equation (5):

G1

gm

2Vdsgm

Vds2i

V 2dsV 1dsiVR

G2

gVV

iV

III. FILTER ARCHITECTURE Figure 5 shows the proposed second order Q-enhanced LC

bandpass filter. The basic concept in improving the LC filter is to use an LC resonator and to provide partial compensation for its losses by incorporating a negative conductance generator [10], [11].

The input signal is fed to the filter using an input differential transistor pair M1 and M2. The filter’s center frequency is tuned by changing control voltage Vctr and the digital control word for the equivalent varactors capacitance. Negative resistance due to NMOS transistors helps increasing the Q factor of the filter.

The filter’s Q is tuned by changing the tail current of the negative conductance circuit built with two cross-coupled NMOS. The varactors are PMOS capacitors operating in accumulation mode within the tuning range.

IV. SIMULATION RESULTS AMS CMOS 0.35μm technology has been used for

simulation of the bandpass LC filter. As shown in Fig. 6, 7, 8 and 9, the center frequency of the filter can be tuned over

Figure 1. Simplified Q-enhanced LC Filter

Figure 2. Parallel mode Q-enhancement

Figure 3. Schematic of the MOS varactors

Figure 4. Negative resistance implanted by cross-coupled MOSFETs

a 640 MHz range, from 1690 MHz to 2330 MHz. This result is achieved when using varactors in accumulation mode, where a capacity ranges from 2.67 pF to 5.05 pF. Reasonable selectivity is provided for DECT, DCS 1800, PCS 1900 and UMTS applications, while drawing 4mA current from a 1.5V supply.

To obtain such filter characteristics, the control voltage of varactors, Vctr, changes from 0.1 V to 1.5 V, and IQ, the bias current, is varied between 0.5 mA and 4 mA.

CV1

Digital Word

CV4 CV3 CV2 Vctr

D0 D1 D2

i

IQ

Vds2 Vds1

M1 M2

Y

(5)

L

Rs -2/gm Rp Lp C C

-

+

Vout Vin C Rp -R L Gm

-

+

Figure 5. Q-Enhanced LC filter

1.65 1.70 1.75 1.80 1.85 1.90 1.95 Frequency (GHz)

-10

-5

0

5

10

Volta

ge M

agni

tude

(dB)

v db(8,2)

Combination 0,0,0

Figure 6. Frequency response of the filter for first combination.

1.85 1.95 2.05 2.10 2.20 Frequency (GHz)

-10

-5

0

5

10

Volta

ge M

agni

tude

(dB)

v db(8,2)

Combination1,1,0

Figure 7. Frequency response of the filter for second combination.

Figure 10 shows frequency responses where the Q-factor varies up 80 to 157 when the current IQ varies from 0.5 up to 4 mA for a constant center frequency.

The power consumption of the filter is 2.96 mW at 2.1 GHz. The output power of the filter versus input power is shown in figure 11. The 1-dB compression point at the filter’s input is -33.5 dBm.

1.75 1.80 1.85 1.90 1.95 2.00 2.05 2.10 Frequency (GHz)

-10

-5

0

5

10

Volta

ge M

agni

tude

(dB)

v db(8,2)

Combination 1,0,0

Figure 8. Frequency response of the filter for third combination.

1.95 2.05 2.15 2.25 2.35 2.45 Frequency (GHz)

-5

0

5

Volta

ge M

agni

tude

(dB)

v db(8,2)

Combination 1,1,1

Figure 9. Frequency response of the filter for fourth combination.

The three main contributors to the nonlinearity of the filter are the negative conductance generator, the varactor and the input gm stage. The nonlinearity analysis of the circuit demonstrates that the contributions of the negative conductance generator and the varactor are much more pronounced than that of the input stage.

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Vol

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Mag

nitu

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Vol

tage

Mag

nitu

de (d

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Frequency (GHz)

Frequency (GHz) Frequency (GHz)

Frequency (GHz)

Combination 1, 0,0 Combination 0, 0,0

Frequency (GHz)

Combination 1, 1,0 Combination 1, 1,1

VDD

+ M2

IP

M4 M3 M7 M8

M6 M5

IQ

L1

VDD

-Vin

Vctr Vout+ Vout-

M1

R2 R1

C1 C2 L2

VDD

There are several ways to characterize the noise performance of RF circuits, including the noise spectral density referred to the input, the spectral density of output noise and the noise factor. From these parameters, we can determine the noise of our circuit, which is about 16.65 dB. Fig. 12 shows the noise spectral density figure of the Q-enhanced filter.

All simulations of the filter frequency response are using an open circuit load, while noise figure and compression point are simulated using a 50 Ω termination.

A summary of the simulated results and those of another work [3] is given in Table I.

V. CONCLUSION In this paper we have presented techniques to make

possible on-chip LC filters achieving the range of centre frequency from 1.69GHz to 2.33GHz.They are adapted for filter constraints for cellular standards (DECT, DCS1800, PCS 1900 and UMTS). Results have confirmed Q enhancement with negative resistance and the frequency tuning with PMOS varactor array. The improvement of automatic tuning dealing with the distortion in the filter's frequency response is considered as a future work. The design of the multi-band RF CMOS LC bandpass filter is implemented to demonstrate the proposed techniques and to evaluate the potential of such filters. Broader applicability of the techniques presented, such as negative resistance, or varactors array, can be planned because it is easy to fabricate, to control the wide tuning range. Finally, we expect to apply our design features to other kinds of RF on-chip circuits.

2.00 2.05 2.10 2.15 2.20 -10

-5

0

5

10

Volta

ge M

agni

tude

(dB)

v db(8,2)

Figure 10. Frequency response of the filter versus IQ(Q-tunning)

-55 -50 -45 -40 -35 -30 -25 -20-15

-10

-5

0

5

10

15

Out

put p

ower

(dBm

)

Input power (dBm)

Figure 11. 1-dB compression point

0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Noise

Spe

ctral

Dens

ity (n

V/Hz

^)

inoise(mag)onoise(mag)

Figure 12. Noise spectral density of the filter

TABLE I. PERFORMANCE SUMMARY OF RF BANDPASS FILTER

Measured value Parameter This work [3] Process AMS 0.35µm CMOS 0.35 µm CMOS Filter order 2 2 Center frequency

1.69 GHz-2. 33GHz 1.93 GHz-2.19 GHz

Quality factor 80-157 20-170 Average -3dB Bandwidth

10.53MHz-29.25MHz 53.8 MHz

Peaking passband gain

10.99dB-13.91dB -

Supply voltage 1. 5V 1.3 Power 2.96 mW 5.2 mW Noise figure 16.65dB 26.8 dB IP1 dB -33.5 dBm -30 dBm

REFERENCES [1] C. Andriesei, L. Goras, F. Temcamani, B. Delacressonière, "Wide

Tuning Range Active RF Bandpass Filter with MOS Varactors", Romanian Journal of Information Science and Technology (ROMJIST), Volume 12, pp. 485-495,Number 4, 2009.

[2] A.Tasic, W.A. Serdijn, J.R. Long. Adaptive multi-standard circuits and systems for wireless communications. IEEE Magazine On Circuits and Systems, vol. 6, no. 1, pp. 29-37, Quarter, 2006.

[3] F. Dügler, E. Sánchez-Sinencio, and J. Silva-Martinez, “A 1.3-V 5 mw fully integrated tunable bandpass filter at 2.1 GHz in 0.35-_mCMOS,” IEEE J. Solid-State Circuits, vol. 38, no. 6, pp. 918–928, Jun. 2003.

[4] W.B. Kuhn, D. Nobbe, D. Kelly and A.W. Orsborn, “Dynamic range performance of on-chip RF bandpass filters,” IEEE Transactions on Circuits and Systems-II: Analog and Digital Signal Processing, vol. 50, No. 10, pp. 685–694, October 2003.

[5] F. Temcamani, B. Delacressonière, M. Dousti, J-L. Gautier, " Filtrage actif dans les systèmes de communication : atouts et défis", Conférence invitée aux 4èmes Journées Franco-Maghrébines des Microondes et de leurs Applications et à TELECOM 2005, Rabat,, March 2005.

[6] Y. P. Tsividis, “Integrated continuous-time filter design,” in Proc. IEEE CICC,pp. 641-647.1993.

[7] S. Pipilos and Y. Tsividis, “Design of active RLC integrated filters with application in the GHz range,” in Proc. IEEE ISCAS, pp. 5.645-5.648.1994.

[8] A.W. Orsborn, “Noise analysis and automatic tuning of Q-enhanced LC bandpass filters,” M.S. thesis, Dept. Electr. Comput. Eng., Kansas State Univ., Manhattan, KS, 2001.

[9] W. B. Kuhn, F. W. Stephenson, and A. Elshabini-Riad, “A 200- MHz CMOS Q-enhanced LC bandpass filter,” IEEE J. Solid-State Circuits, vol. 31, no. 8, pp. 1112–1122, Aug. 1996.

[10] Y. P. Tsividis, “Integrated continuous-time filter design—An overview,” IEEE J. Solid-State Circuits, vol. 29, pp. 166–167, Mar. 1994.

[11] D. Li and Y. Tsividis, “Active LC filters on silicon,” IEE Proc.: Circuits, Devices, Syst., vol. 147, no. 1, pp. 49–56, Feb. 2000.

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P1dB= -33dBm

Input power (dBm)

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