开关电容网络SC的噪声分析

TH03-4
Takafumi Nasu, Kiyomichi Araki
ANTENNA
l S
LNTA
T
D DSM D
A AC
DSP D T
a CURRENT
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1.
DECIMAT-ING
FILTERING
DIGITAL
Fig.
Schematic of discrete time receiver
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©)2007 IEEE.
61
2007 Korea-Japan Microwave Conference
BANK A
SA(1)
LO
SA(2)
DUMP
CR
ei(t)
DUMP
(2R,,
V~~~~~~~~~
CH
LNTA
BANK B
SB(2)
SB(1)B
2.-..R
RES
Fig. 5. Noise flow graph of read
out section
DUMP
From Fig. 5, the noise transfer function is derived as follows.
Hi
This noise transfer function has a first order IIR characteristic. As a simulation result, histogram of discrete noise input and power spectral density of the output are shown in Figs. 6 A. Noise analysis model of read out section First, we consider the on-resistances of DUMP, SA (2) and and 7. We assume that fLo =40MHz, decimation ratio = 0.2pFCB = that and Ron= input SB (1) which are connected in series while DUMP is at on- NFrom 2, CR 6,= we can confirm 2PF discrete noise 500Q. Fig. state. In this period, either SA (2) or SB (1) is at on-state. We assume that each on-resistance value is the same and their qnoisel (n) also follows Gaussian distribution. And, its variance noise sources follow an independent identical Gaussian distri- is kTCRCB/(CR ± CB), so it depends on only capacitance bution. Its noise analysis model is expressed as Fig. 4. The value. It does not depend on on-resistance value. The spectrum noise sourceenl (t) is the white Gaussian noise source whose of the output from buffer capacitor in Fig. 4 has a low pass remains on-state. On the other hand, characteristic is power spectral density is n[V2/Hz]. k > Boltzmann' ~~the wide bandif DUMP noise iS vreatl under sam Dled since a low ass .is series on. y g constant, T is an absolute temperature and 2Ron time constant is much smaller than the clock (sample) period resistance value. The voltage of the buffer capacitor (5) while DUMP is at on-state.in the discrete time system in Fig. 4. The power spectral density of the sampled noise is increased with respect to the t continuous-time noise spectrum by the under sampling and VB(L,r>CRtB(TDUMP,n-C1)-R(TDUMP,n-1) becomes approximately white[12]. Its power spectral density ±CR'R(TDUMPnl)±CB,B(TDUMPn1) CR+CB is kTTDUMPCRCB/(CR + CB). It does not depend on onrj (5) resistance value, and it increases as sampling period becomes + 2CB1R t+(n-l)TDUMP J@-)DUMP en t(t)e ()DUMTP dte longer. Since input noise spectrum is white, shape of the (.t. TDLMP output spectrum depends on the frequency characteristic of where TDUMP is a period of DUMP, time index n means (7). Horizontal axis in Fig. 7 shows signal frequency after nth period of DUMP. And, Ti is a time constant, which is down-conversion. If we assume that RF carrier frequency spectrum located in the desired signal 2RonCRCB/(CR+CB). Tl is set much smaller than TDUMP. is 400.5MHz, noise on bandwidth centering 500kHz overlaps with desired signal. In (5), vR(TDump, n - 1) is always zero since rotating capacitor is reset after read out. The noise flow graph can be B. Noise analysis model of decimation section expressed as Fig. 5 as we consider that the DSM is a discrete Intothis section, we consider the on-resistances correspondtime system which outputs when DUMP becomes off-sta l~~~~~ng SA (1) and SB (2) which are located between CH and , X Where dscretenoise nput qoisei Ti) isCR. We assume that current in the SCF section does not leak to TA according to LO's phase since output impedance of q ~(n)___ 1)~~~~ TA is sufficiently large. So, the transfer function of the noise t'(n1)TDUMP - TDM T dt'e 2Ti (6) source which is located between CH and CR does not depen 1R~ J(f)2 )TD U MP ei(t')e
Noise Analysis of Switched Capacitor Networks in Direct Sampling Mixers
Graduate School of Engineering, Tokyo Institute of Technology Ookayama, Meguro-ku, Tokyo 152-8552, Japan, Email: {nasu,araki} @mobile.ee.titech.ac.jp Telephone: +81-03-5734-3594, Fax: +81-03-5734-3594
I. INTRODUCTION to current, then inputted into the DSM. While LO is at onRecently, the technology innovations in Si-CMOS allow RF state, history capacitor CH and either rotating capacitors CR circuits to have functions, such as oscillation, amplification and which are located in BANK-A or -B are charged. Analog down-conversion[l]-[3]. In RF-CMOS technology, attention filtering is performed by this operation. And, since a period is currently focused on Digital RF Processor(DRP) which of SA(l), SB(2) is N times as long as a LO's, the current is enables one chip receiver including RF circuit and digital base integrated N times while either SA(1) or SB(2) is at on-state. band circuit[4]-[6]. In DRP technology, receiver operates as FIR and N decimation is provided by this. The signal transfer a discrete time receiver. Discrete time receiver architecture is function of the DSM can be expressed as follows[II]. shown in Fig. 1. In the discrete time receiver of [4], the Direct T IIR1 FIR IIR2 (1) Sampling Mixer (DSM)[7]-[9] performs down-conversion, decimation and filtering. Down-conversion is performed by where charge sampling[I0]. Filtering and decimation are performed O1 in the Switched Capacitor Filter (SCF) section. Operation of IIR1 (2) YCTHO 7 CR±CH(IZN) the AD-converter is eased by decimation. There are three noise sources in the DSM. First is the IIR2 C(3) CR + CB (1 - ZN) Tranceconductance Amplifier (TA) which is located in the first N stage of the DSM. Second is the clock jitter which disturbs FR control of the MOS switches in the SCF section. Third is the 1 zon-resistance of the MOS switch. However, exact analysis has and TLO is a period of LO, gm is a transconductance of TA. not been performed yet. To solve this problem, we focus on- In case of direct sampling method, TLO is made equal to a resistances of the MOS switches and analyze its influence on period of RF carrier. In case of Low-IF method, TLO is shifted the desired signal. Off-resistance is considered as a slowly from a period of RF carrier a little. varying offset voltage and is neglected in the noise spectrum III. NoiSEANALYSIS OFSCFSECTION since it presents in conjunction with the capacitors extremely From Fig. 2, the SCF section is composed of capacitors long time constants. This paper is organized as follows. Section II presents and MOS switches. Because of its architecture, it has noise operation of the DSM. Noise analysis of on-resistance in the sources, which are on-resistances of the MOS switches. In SCF section is presented in section III. Low noise design and Fig. 2, although on-resistance exists in the each switch, the influence on Noise Figure (NF) of the DSM including TA is transfer process depends on each location. Since the different presented in section IV. Finally, section V concludes the paper. noise sources are uncorrelated, their contributions to the output noise spectrum can be evaluated separately. In this section, II. OPERATION AND TRANSFER FUNCTION OF DSM we classify the noise sources according to their location and Architecture and timing diagram of the basic DSM are derive each transfer function independently. And, we analyze shown in Fig. 2, 3. Signal received at antenna is converted their influence on the desired signal.
Abstract-In order to improve the Noise Figure (NF) of the Direct Sampling Mixer (DSM), thermal noise corresponding to on-resistance of the MOS switch, one of the noise sources in the DSM, is analyzed, and the noise transfer function is derived. As a result, at the output terminal of the DSM, its influence on the desired signal is sufficiently small as compared with the influence I which Tranceconductance Amplifier (TA) in the first stage of the DSM causes. In the DSM, internal noise from Switched Capacitor Filter (SCF) section can be neglected. Index Terms-Direct Sampling Mixer, discrete time receiver, Switched Capacitor Filter, noise analysis.
RtCj
_
B
C
CB., VB()
T CR
RES
Fig. 4. Noise analysis model of read out section
wisel
Fig. 2. Architecture of basic DSM
LO
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C
(n)
SA(1), SB(1)
C,
|
4-.
Ai TLO
0.
SA(2), SB(2)