CX74017

On the Direct Conversion Receiver

4

Skyworks Solutions, Inc., Proprietary and Confidential

101735A

Preliminary Data Subject to Change

July 20, 2001

101735A 6_071801

RF band-select

filter

LNA

Wideband filter

RF

IF

RF

DC

0f

f

Wideband IF

f

0

LPF

LPF

Image-reject

filter

Figure 6. Wideband IF with Double Conversion.

Direct Conversion Receivers

Direct conversion reception, also referred to as homodyne, or

zero-IF, shown in Figure 7, is the most natural solution to

receiving information vehicled by a carrier. However, it has only

been a decade or so that this type of reception has found

applications other than pagers, for example [13]. For it has

several qualities which make it very suitable for integration as

well as multi-band, multi-standard operation, but severe inherent

obstacles that had for long kept it in the shadow of the

superheterodyne.

RF band-select

filter

LNA

RF

DC

LPF

RF

f

DC

0f

LPF

101735A 7_071801

Figure 7. The Direct Conversion Receiver

First, the image problem is no longer present, since the IF is

zero and the image to the desired channel, for all but single-

sideband signals, is the channel itself. Then, only one LO is

needed, which means only one phase noise contribution. The

need for the bulky, off-chip filters is consequently removed.

Filtering now only occurs at low, that is, baseband, frequencies

with some amplification. This means less current consumption

than at higher frequencies (to drive device parasitics), fewer

components, lower cost, etc. Practically, however, strong out-of-

band interferers or blockers may need to be removed prior to

downconversion in order to avoid desensitizing the receiver by

saturating subsequent stages, as well as producing harmonics

and intermodulation terms, which then appear in baseband.

Such a filter may be placed after the LNA, for example.

DCR, however, brings its own set of issues. The following

paragraphs describe those in more detail.

DC offsets

In direct conversion, as the signal of interest is converted to

baseband very early in the receive chain, without any filtering

other than RF band-selection, various phenomena contribute to

the creation of DC signals, which are directly appearing as

interfering signals in the band of interest.

The LO may be conducted or radiated through an unintended

path to the mixer’s RF input port, thus effectively mixing with

itself, producing an unwanted DC component at the mixer

output, see Figure 8.

Worse still, this LO leakage may reach the LNA input, producing

an even stronger result. This effect presents a high barrier

against the integration of LO, mixer, and LNA on a single silicon

substrate, where numerous mechanisms can contribute to poor

isolation. These include substrate coupling, ground bounce,

bond wire radiation, and capacitive and magnetic coupling.