On the Direct Conversion Receiver

CX74017

101735A

Skyworks Solutions, Inc., Proprietary and Confidential

3

July 20, 2001

Preliminary Data Subject to Change

101735A 3_071801

RF

Σ

cos(

ω

sin(

ω

-f

LO

+f

LO

f

IF

0

-f

IF

f

IF

0

j

+f

IF

0

-f

IF

90

°

f

IF

0

-f

IF

+f

IF

0

-f

IF

Figure 3. Hartley Image-Reject Architecture

101735A 4_071801

RF

Σ

cos(

ω

sin(

ω

+

-

-f

1

+f

1

0

-f

2

f

2

0

j

cos(

ω

sin(

ω

0

0

0

+f

2

0

-f

2

0

2×

0

f

2

Figure 4. Weaver Image-Reject Architecture

Low IF Single Conversion Receiver

Low IF single conversion, see Figure 5, is an offspring of the

DCR, and is covered in the following paragraphs. Its main

purpose is to protect the receiver from all the direct current

(DC)-related obstacles that pertain to DCR, while retaining the

DCR’s benefit of the elimination of high-Q IF filters. As its name

indicates, instead of directly converting the signal to baseband,

the LO is slightly offset from the RF carrier, typically one to two

channels. The low IF means that the fractional bandwidth of the

IF bandpass filtering is large, making it possible to implement it

with low-Q components. The IF SAW, or crystal filter needed in

the high IF case, can be replaced with an active resistive

capacitor (RC) filter or other filter suitable for low frequency

operation that is also conducive to silicon integration. The low IF

signal may be translated to baseband through another mixer, or

preferably, in the digital domain following analog-to-digital (A/D)

conversion. Of course, this comes at the expense of faster and

higher resolution analog-to-digital converters (ADCs). If the IF

frequency is equal to only one or two channel widths, then it is

not possible to provide image-rejection at radio frequency (RF),

as the RF filter must be wide enough to pass all channels of the

system. In this case, all image rejection must come from the

quadrature downconversion to the low IF, which itself resembles

the Hartley architecture, once the baseband conversion is

added.

101735A 5_072001

Image-reject

filter

LNA

RF

Low IF

BPF

RF

Low IF

0f

f

BPF

Figure 5. Low IF Single-Conversion Receiver

Wideband IF with double conversion

This architecture, shown in Figure 6, is very similar to the

superheterodyne. In this case, the first mixer utilizes an LO that

is at a fixed frequency, and all RF band channels are translated

to IF, retaining their positions relative to one another. The

second mixer utilizes a tunable LO, thus selecting the desired

channel to be translated to baseband. A subsequent lowpass

filter suppresses adjacent channels.