ARTICLE IN PRESS

Microelectronics Journal 39 (2008) 1860– 1866

Contents lists available at ScienceDirect

Microelectronics Journal

0026-26

doi:10.1

� Tel.:

E-m

journal homepage: www.elsevier.com/locate/mejo

A low-noise mixer with an image-reject notch filter for 2.4 GHz applications

Nam-Jin Oh �

Department of Electronic Engineering, Chungju National University, 123 Geomdan-ri, Iryu-myeon, Chungju, Chungbuk 380-702, Republic of Korea

a r t i c l e i n f o

Article history:

Received 21 March 2007

Received in revised form

3 April 2008

Accepted 16 April 2008Available online 9 June 2008

Keywords:

CMOS

Heterodyne

Image rejection filter

Low-noise mixer

Noise figure

92/$ - see front matter & 2008 Elsevier Ltd. A

016/j.mejo.2008.04.007

+82 43 8415162; fax: +82 43 8415160.

ail address: onamjin@cjnu.ac.kr

a b s t r a c t

This paper presents a low noise first down-conversion mixer with a notch filter for the heterodyne

receiver. The notch filter connected to the output node of the mixer driver stage plays a role of image

rejection at an image frequency, thereby suppressing the sideband image noise and improving the mixer

noise performance. Targeted for 2.4 GHz industrial–scientific–medical band applications, a simple

source-degenerated down-conversion single balanced mixer with the filter is implemented. The

measurement results of the proposed down-conversion mixer shows about 3.0 dB improvement of

single-side band noise figure, about 2.9 dB power conversion gain improvement, and 25 dB image

suppression compared to those without the filter dissipating 4 mA from a 2.5 V supply voltage.

& 2008 Elsevier Ltd. All rights reserved.

1. Introduction

While research and development for a direct conversion radiotransreceiver has been active recently, heterodyne radio architec-ture has been widely adopted for several decades, and is stillprevalent for many commercial products.

Fig. 1 shows a conventional heterodyne radio receiver front-end which comprises an radio frequency (RF) band pass filter(BPF), a low-noise amplifier (LNA), an image rejection filter (IRF), adown-conversion mixer, and an intermediate frequency (IF) filter.The down-conversion mixer is used for frequency translation of aRF signal down to an IF by mixing the RF signal from the LNA withthe local oscillator (LO) signal. The front-end blocks such as LNAand down-conversion mixer mainly affect the receiver noise figure(NF). Since the LNA predominantly affects the receiver NF, it isrequired to have a low NF to receive very weak signals. The nextnoise contributor is a down-conversion mixer. A low NF mixerrelaxes the gain requirement of the preceding LNA. A mixer withhigh-power gain reduces the noise contribution from the follow-ing IF stages [1]. Usually, the filters are implemented externally.They make several decibels of signal losses in the signal path andthereby degrade the overall receiver NF.

In this paper, a simple down-conversion single balanced mixer(SBM) with a notch filter [2,3] targeted at 2.4 GHz applications ispresented to achieve low NF, high-power gain, and an imagerejection. The emphasis is on the image rejection filter imple-mentation, and the noise analysis of the proposed mixer.

ll rights reserved.

2. Mixer design considerations

For a cascaded system as shown in Fig. 1, it can be shown thatthe LNA must have enough gain to suppress the noise contributionby the following stages such as down-conversion mixer and IFfilter. The system noise factor expressed in linear scale can bewritten as

NF ¼1

LRFþ

NFLNA � 1

LRFþð1� LIRÞ

LRFGLNALIRþ

NFMIX � 1

LRFGLNALIR(1)

where LRF and LIR are the insertion losses of the RF filter and theimage rejection filter, respectively, NFLNA and NFMIX are the NFs ofthe LNA and the down-conversion mixer, respectively; and GLNA isthe power gain of the LNA.

Based on the typical heterodyne receiver specifications andassuming LRF ¼ 3 dB, NFLNA ¼ 1.5 dB, GLNA ¼ 15 dB, LIR ¼ 6.0 dBand NFMIX ¼ 13 dB, the calculated overall system NF is 8.9 dB. Ifthe NF of the mixer NFMIX ¼ 10 dB, the system noise performancecan be significantly improved to 7.22 dB.

Analyzing the linearity for the cascaded system, the linearity ofthe latter stages becomes increasingly important and hencesufficiently high linearity of a mixer is required. To improve thereceiver linearity, the gain of the LNA also should not be too high,which in turn leads to the NF degradation of the receiver.Lowering the mixer NF can compensate the degradation of thereceiver NF.

The next consideration is an image problem. Many heterodynereceivers adopt double conversion architecture with high IF overseveral hundred megahertz to provide an inherent image rejec-tion, and employ low-side mixing (LO has lower frequency than

ARTICLE IN PRESS

LNA

Mixer

RFIN

IFOUT

LO

RF FilterImageRejectFilter

IF Filter

This work

Fig. 1. Conventional heterodyne radio receiver front-end.

fRF

3fLO

fLO

fIF

N.-J. Oh / Microelectronics Journal 39 (2008) 1860–1866 1861

RF) for the first down conversion [4,5]. However, the imagerejection ratio cannot be obtained enough at GHz range highfrequencies. So, an integrated image reject mixer for the firstdown conversion has advantage of suppressing the imagefrequency and noise furthermore.

Frequency

Pow

er5fLO

Fig. 2. Frequency translations of white noise in the driver stage (dotted line) and

RF signal translations by mixing with LO (solid line).

3. Mixer noise analysis

Usually, the active mixers (in silicon technology) have a single-side band (SSB) NF greater than 10 dB. The mixer is noisy becauseside band noise is translated from multiple frequency bands tothe output and the input RF signal power is translated tomultiple frequency bands by frequency mixing as shown inFig. 2. Thus, the mixer inherently has high NF compared to thoseof amplifiers.

The mixer noise contributing components consist of driverstage noise, switching pair noise, LO signal noise and thermalnoise from the load resistor. Among the noise contributingcomponents, the driver stage noise dominates the mixer noiseperformances.

3.1. Noise for a nondegenerated single balanced mixer

Fig. 3 shows a nondegenerated active SBM, and a small-signalequivalent circuit of the RF input driver stage. The noise sourcesare modeled in the input and output. Assuming that the gain ofthe driver stage and its output noise are constant across allfrequencies, the total output noise current spectral density at thedrain of the driver stage is

Sa ¼ 4kT½ðRs þ RgÞg2m þ gðkþ xÞgd0� (2)

where k is Boltzmann’s constant, T is the absolute temperature, Rs

is the source resistance, Rg is the gate resistance, gm is thetransconductance, g is a bias-dependent factor, gd0 is the zero-biasdrain conductance, k is the combined parameter for the drainchannel current noise and the correlated induced gate noise, and xis the parameter for the uncorrelated induced gate noise [6]. From(2), the first term is due to the source resistance and polysilicongate resistance, the second term is due to the induced gate noiseand drain channel current noise.

Assuming the LO switching is square wave-like, the LOfrequency and its odd harmonics will downconvert the respectivenoise components to the IF. Since the mixer conversion gain is 2/p,the mixer output noise current spectral density is

i2no;M1 ¼ nSa2

p

� �2

(3)

where

n ¼ 2 1þ1

32þ

1

52þ � � �

� �¼

p2

� �2(4)

is the noise increase factor and originates from the harmonicamplitudes of the square-wave [7,8].

From (3), it can be shown that the mixing process increasesthe noise contribution by a factor of (p/2)2 or 3.9 dB. As shown inFig. 2, the first term of (4) is noise at fLO7fIF downconverted by thefundamental LO, the second term is noise at 3fLO7fIF down-converted by the third harmonic of the LO, and so on. The first andsecond term account for 81% and 9% of the noise transferred to theoutput, respectively. The remaining 10% comes from the higher-order LO harmonics7fIF [9].

The output noise current spectral density due to the switchnoise and the output load resistor (if two load resistors RL areused) is

i2no;sw;RL¼ 8kT

gI

pAþ

1

RL

� �(5)

where I is the tail fixed current, A is the LO amplitude [7]. In (5),the noise contribution of the switches can be minimized with alarge LO signal.

ARTICLE IN PRESS

IF IF

LO LOX

M2 M3iD1

RFM1

CX

v2ns

i2ng i2nd

v2n,Rg

Rs

Rg

gg Cgs vgs gmvgs

iD1

CX

+

-

Fig. 3. (a) A nondegenerated active single balanced mixer and (b) small-signal equivalent circuit of the RF input driver stage for noise calculation.

N.-J. Oh / Microelectronics Journal 39 (2008) 1860–18661862

By summing up (3) and (5), the total output noise currentspectral density of the nondegenerated SBM is given as

i2no ¼ 4kTnððRs þ RgÞg2m þ gðkþ xÞgd0Þ

2

p

� �2

þ 8kTgI

pAþ

1

RL

� �. (6)

3.2. Noise for a single balanced mixer with degeneration

Fig. 4 shows the proposed image rejection low noise mixerwith source degeneration and a notch filter, and a small signalequivalent circuit of the RF input stage. The differential outputsare combined as a single-ended output with a power combiner[10].

In Fig. 4(b), the notch filter nullifies the parasitic capacitanceby parallel resonating at the signal frequency, thus makes highimpedance at node X and thereby increases the signal contribu-tion of M1. Also, the filter boosts the image rejection by seriesresonating at the image frequency, thus makes low impedance atnode X and thereby reduces the image frequency gain [11].Including the parasitic capacitance CX, the input impedance of thefilter, Zf, can be written as

Zf ðsÞ ¼s2C1L1 þ 1

sðs2CXC1L1 þ C1 þ CXÞ. (7)

The filter has imaginary zeros at

oz ¼ �1ffiffiffiffiffiffiffiffiffiffiC1L1

p (8)

and imaginary poles at

op ¼ �

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiC1 þ CX

CXC1L1

s(9)

Fig. 5 shows the simulated transfer function of the driver stagefor the nondegenerated single balanced and source-degeneratedactive mixers with and without the notch filter. With includingthe notch filter, the noise at the image frequency would berejected, and about half of 81% noise translated from fLO7fIF issuppressed. The noise translated from 3fLO7fIF, 5fLO7fIF and so on,which corresponds about 19% can be significantly suppressed.Thus, at least over 50% (or 3 dB) noise performance can beimproved in the proposed mixer.

In Fig. 4, the overall transconductance of the degenerated inputstage is given by

Gm ¼gm

oCgsðRs þ oTLsÞ¼

oT

oRs½1þ ðoTLs=RsÞ�(10)

where the cut-off frequency oT is equal to gm/Cgs. From (10), theoutput noise power density is dependent on frequency [6]. Thus,the output noise current density is reduced at higher frequencies,thereby resulting in smaller noise translation to the IF andimproving the mixer noise performance. Also, the noise contribu-tion from the image frequency can be suppressed by the notchfilter.

At the RF frequency, the output noise component due to theresistances of Rs (source resistance), Rg (polysilicon gate resis-tance), and Rl (series resistance of the gate inductor) is

Sa;Rs ;Rl ;RgðoRFÞ ¼

4kTo2T

o2RFR2

s ½1þ ðoTLs=RsÞ�2ðRs þ Rl þ RgÞ. (11)

ARTICLE IN PRESS

L2 = 11nH L3= 8.5nH

C2 = 5pF C3 = 2pF

VDD

40�m × 3 40�m × 3

40�m × 3

LO LO

M2M3

X

RF

Lg = 15.5nH

Ls = 1.5nH

M1

iD1

CX

Zf

L1 = 9nH

C1 =1.7 pF

Combiner

Switching

Driver

Filter

v 2ns

Rlv2

n,Rg

i2ng i

Rs

Lg lg RR

gg Cgs vgsgmvgs

iD1

CX

L1

C1

Ls

Filter

RL RL

IF

2nd

Fig. 4. (a) Proposed image rejection low noise mixer with source degeneration and a notch filter and (b) small-signal equivalent circuit of the RF input driver stage for noise

calculation.

N.-J. Oh / Microelectronics Journal 39 (2008) 1860–1866 1863

The output noise component due to the induced gate noise anddrain current noise is

Sa;ing ;indðoRFÞ ¼

4kTgðkþ xÞgd0

½1þ ðoTLs=RsÞ�2

. (12)

The output noise current spectral density translated from the RFfrequency to the IF due to the driver stage is

i2no;RF ¼ ðSa;Rs ;Rl ;RgðoRFÞ þ Sa;ing ;ind

ðoRFÞÞ � ð2=pÞ2. (13)

Including the noise sources in the switches, and the loadresistor, the total output noise current spectral density of the

inductor degenerated mixer is given by

i2no ¼4kT

½1þ ðoTLs=RsÞ�2

o2T

o2RFR2

s

ðRs þ Rl þ RgÞ þ gðkþ xÞgd0

!

�2

p

� �2

þ 8kTgI

pAþ

1

RL

� �. (14)

By comparing (6) and (14), it can be seen that the noisetranslation from the image frequency and the higher-order LOharmonics is suppressed.

For the double balanced mixer, the noise sources from thedriver stage and switches become twice as compared to the SBM.Another noise from the driver stage at the IF can feed through to

ARTICLE IN PRESS

Frequency (MHz)2490

Noi

se F

igur

e (d

B)

9

10

11

12

13

14

15

16NF with filterNF without filter

Frequency (MHz)

Gai

n (d

B)

5

6

7

8

9

10

11

12Gain with filterGain without filter

2500 2510 2520 2530 2540 2550

2490 2500 2510 2520 2530 2540 2550

Fig. 7. Measured (a) NF and (b) conversion gain.

Frequency

Gai

n (d

B)

-30

-20

-10

0

10

Nondegenerated mixer without filterNondegenerated mixer with filterDegenerated mixer without filterDegenerated mixer with filter

fRFfLOfIM 3fLO 5fLOfIF

Fig. 5. Simulated transfer function of the driver stage.

Fig. 6. Fabricated chip micrograph.

N.-J. Oh / Microelectronics Journal 39 (2008) 1860–18661864

the output. Since the driver stage has some gain at the IF, theoutput noise of the mixer would increase if a single-ended outputis taken, and can be suppressed by taking the IF outputdifferentially since the IF noise is common-mode [8].

The proposed technique can be an effective way to improve NFof a mixer, especially when the low-side mixing is adopted as inthe case of double conversion receiver.

4. Measurement results

A SBM with a notch filter is implemented in 0.18mm CMOStechnology as shown in Fig. 6. For measurements, the IF outputcombiner and the image reject notch filter are implemented withexternal inductors and capacitors on printed circuit boards.

The measurement is taken with a NF meter. A balun isused to transform the single-ended LO signal to differential. BPFsare used for the RF input to measure the SSB NF and for the LOinput to stop the noise contribution from the LO signal generator,respectively. Also, an IF bandpass filter is used to prevent thesaturation of NF meter input. The degeneration inductor Ls isimplemented by using a bond-wire which has inductor value of0.8 nH/mm.

The mixer consumes about 4.0 mA from a 2.5 V supply. Thelow-side mixing is used for the measurement as in the cases ofwidely adopted double conversion heterodyne receiver. The LOand IF frequencies are 2026 and 484 MHz, respectively.

The applied LO input power is �3 dBm. Because the LO port ismatched to higher impedance rather than 50O, the LO swing canbe assumed to be large. The input port is matched with a returnloss less than �10 dB. The measurement results of SSB NF andconversion gain are plotted in Fig. 7 with an RF input frequencysweep.

The measurement results show that over 3.0 dB improvementof SSB NF, and over 2.9 dB conversion gain increase are achieved.The image frequency of 1,542 MHz is applied to the RF input tomeasure the image suppression. Around 25 dB image suppressionfor the proposed mixer with the notch filter is achieved comparedwith 7 dB without the filter as shown in Figs. 8 and 9, respectively.The measured input IP3 of the mixer with and without the notchfilter is about �9.9 and�4.6 dBm, respectively, as shown in Fig. 10.Table 1 summarizes the measured results of the proposed mixerwith and without the notch filter compared to those of otherresults.

5. Conclusion

This paper presented a low-noise down-conversion SBM with adetailed noise analysis for the 2.4 GHz double-conversion receiverapplication. A notch filter is connected to the output node of thedriver stage of the mixer to suppress the image noise. With theimage rejection notch filter and source degeneration inductor,lower NF, higher gain, and a partial image rejection are achieved.While consuming 4 mA current from 2.5 V supply, the proposedmixer achieves conversion gain of 10 dB, noise figure of 10.3 dB,

ARTICLE IN PRESS

Fig. 8. IF spectrum without the notch filter (a) from the RF frequency and (b) from the image frequency. Image suppression is about 7 dB.

Fig. 9. IF spectrum with the notch filter (a) from the RF frequency and (b) from the image frequency. Image suppression is about 25 dB.

Table 1Performance comparison of the proposed degenerated mixer

Topology This work [11] [12] [13] [14] [15]

SBM+IRF LNA+IRF LNA+IRF SBM SBM SBM

Supply voltage (V) 2.5 1.8 1.8 1 1.25 1.8

Current (mA) 4 3.7 4 0.5 0.3 6

Gain (dB) 10 – 18a 20.4a 7 �4.5

Input IP3 (dBm) �9.9 – 2 – 10 �11.5

NF (dB) 10.3 4.3 1.4 13.1b o20 14.6

Operating frequency (GHz) 2.510 5.25 5.8 2.4 0.9 5.8

Image suppression (dB) 25 16 16 – –

Technology 0.18 mm CMOS 0.25mm CMOS 0.18 mm CMOS 0.18mm CMOS 0.25 mm CMOS 0.18 mm CMOS

a Voltage gain.b Measured at 50 MHz IF.

N.-J. Oh / Microelectronics Journal 39 (2008) 1860–1866 1865

ARTICLE IN PRESS

Input Power (dBm)-40

Out

put P

ower

(dB

m)

-80

-60

-40

-20

0IIP3 with filterIIP3 without filter

IIP3=-4.6 dBmIIP3=-9.9 dBm

-30 -20 -10 0

Fig. 10. IIP3 of the proposed degenerated mixer with a notch filter compared to

that without the filter.

N.-J. Oh / Microelectronics Journal 39 (2008) 1860–18661866

image suppression of 25 dB, and the input third-order interceptpoint of �9.9 dBm.

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