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wireless receiver


Introduction

In wireless communication receivers,the received radio frequency (RF)signal should be downconverted to baseband and sampled for further processing in digital domain.The received signal is processed ?rst in the analog domain and passed on to the digital domain,and thus analog-to-digital and digital-to-analog conversions should occur somewhere in the receiver path.So,there are choices of receiver architectures according to where to put the analog-to-digital converter (ADC)and the digital-to- analog converter (DAC)and how to downconvert the received signal.It ranges from the traditional heterodyne architecture to digital-IF architectures.In this chapter, some of the important receiver architectures such as heterodyne,direct conversion, low-IF,and digital-IF receiver architectures are brie ?y summarized.

2.2 Heterodyne Receiver Architecture
In the heterodyne receiver architecture shown in Figure 2.1,an RF signal is downconverted to an intermediate frequency (IF)by the ?rst mixer.Then the signal is I/Q separated and downconverted to baseband by the second mixers.The ?rst band pass ?lter (BPF)located in front of the low noise ampli ?er (LNA)is for frequency band selection.The second BPF is for image rejection,and the third BPF and a low pass ?ler (LPF)are for channel selection.After low pass ?ltering for channel selection,the signal is sampled at baseband,which is called baseband sampling .As shown in Figure 2.1,the channel selection is done at IF as well as at baseband before ADC,which relaxes the Q required of each channel selection ?lter.The heterodyne architecture has been the most popular receiver architecture for a long time due to its good selectivity and sensitivity and can be found in many commercial RF transceivers. It relaxes the dynamic range of the baseband circuits.But,normally the IF band pass ?lter and the image reject (IR)?lter are external components,which makes this architecture not suitable for monolithic integration.
2.3 Direct Conversion Receiver Architecture
Fig.2.2 shows a direct conversion receiver (DCR)architecture which is also
known as homodyne or zero-IF receiver architecture.In practice,an RF band selec-
tion ?lter is placed between an antenna and an LNA,but it is not shown in the ?gure
for simplicity.The DCR downconverts the desired signal directly from RF to DC
and utilizes the baseband sampling.Thus,the DCR eliminates the need for discrete
image rejection (IR)as well as intermediate frequency (IF)?lters,which makes it
suitable for monolithic integration.In addition,as the desired signal band is down-
converted to the baseband at the early stage in the receiver chain,it is relatively easy
to design the baseband circuits for multistandard operation.That is,the channel
selection and automatic gain control (AGC)in DCRs are done at baseband using
on-chip low pass ?lters and variable gain ampli ?ers which can be made to work for
multiple wireless communication standards with low cost and less current consump-
tion.Since the information bearing signal and blockers are translated to baseband
together and channel selection is done at baseband,the DCR usually requires high
dynamic range baseband circuits.This architecture becomes the most popular archi-
tecture in implementing an integrated single-chip receivers,but the main problems
are DC o ?set problem and I/Q mismatch.for simplicity.The DCR downconverts the desired signal directly from RF to DC
and utilizes the baseband sampling.Thus,the DCR eliminates the need for discrete
image rejection (IR)as well as intermediate frequency (IF)?lters,which makes it
suitable for monolithic integration.In addition,as the desired signal band is down-
converted to the baseband at the early stage in the receiver chain,it is relatively easy
to design the baseband circuits for multistandard operation.That is,the channel
selection and automatic gain control (AGC)in DCRs are done at baseband using
on-chip low pass ?lters and variable gain ampli ?ers which can be made to work for
multiple wireless communication standards with low cost and less current consump-
tion.Since the information bearing signal and blockers are translated to baseband
together and channel selection is done at baseband,the DCR usually requires high
dynamic range baseband circuits.This architecture becomes the most popular archi-
tecture in implementing an integrated single-chip receivers,but the main problems
are DC o ?set problem and I/Q mismatch.

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