74017 View Datasheet(PDF) - Skyworks Solutions
Part Name
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74017 Datasheet PDF : 10 Pages
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CX74017
RF band-select
filter
RF
LNA
Image-reject
filter
Wideband filter
IF
On the Direct Conversion Receiver
LPF
LPF
RF
Wideband IF
DC
f
0
f
0
f
101735A 6_071801
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
RF
LNA
RF
LPF
DC
LPF
DC
f
0
f
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.
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Skyworks Solutions, Inc., Proprietary and Confidential
101735A
Preliminary Data Subject to Change
July 20, 2001
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