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| 型号： | 74017 |
| 厂商： | Skyworks Solutions Inc. |
| 英文描述： | On the Direct Conversion Receiver |
| 中文描述： | 在直接转换接收器 |
| 文件页数： | 3/10页 |
| 文件大小： | 315K |
| 代理商： | 74017 |
CX74017
On the Direct Conversion Receiver
2
Skyworks Solutions, Inc., Proprietary and Confidential
101735A
Preliminary Data Subject to Change
July 20, 2001
downconversion to IF, any unwanted activity at a frequency
spaced at fIF offset from fLO, on the opposite side of fLO from the
desired RF channel, produces a mixing product falling right into
the downconverted channel, at fIF. In practice, a RF bandpass
filter, usually a Surface Acoustic Wave (SAW) device, is utilized
to perform band selection ahead of the Low Noise Amplifier
(LNA), while a second filter follows the LNA to perform image
rejection. If these filters are identical, then, in fact, they share
the burden of the two functions. But some amount of image
rejection must particularly follow the LNA, for without it, the LNA
noise figure effectively doubles due to the mixing of amplified
image noise into the IF channel. Instead of the RF SAW filter,
other passive filtering technologies, such as dielectric or
ceramic, can also be featured. It can be seen from Figure 2 that
the higher the IF, the more relaxed the requirements on the
image reject filter cut-off frequency. Once at the IF, the
presence of an interfering signal in the vicinity of the channel
mandates sharp filtering around the channel; this is performed
after the first mixer by the channel select filter, which is also
often an IF SAW filter. Essentially, the exercise is that of a
carefully engineered balance among several variables:
Rejection provided by the various filters
Frequency planning
Linearity of the active stages
Dual IFs provide additional room to maneuver with filter
selectivity, but complicate the frequency planning somewhat.
The selectivity required of the two aforementioned filters, in
terms of fractional bandwidth, makes them unsuitable
candidates in the foreseeable future for integration. This is
because of low Qs of current silicon processes and the need to
be implemented by bulky, off-chip components. The IF channel
filter especially requires high-Q resonators for its
implementation: the higher the IF, the lesser the filter’s fractional
bandwidth, that is, its ratio of bandwidth to center frequency,
necessitating ever-higher Q. This high-Q requirement is most
commonly met by the use of a piezoelectric SAW and crystal
filters. This introduces additional constraints, as those filters
require often-inconvenient terminating impedances, and
matching may impinge on such issues as noise, gain, linearity,
and power dissipation of the adjoining active stages. The
narrower the fractional bandwidth, the more likely that the filter’s
passband shape will exhibit an extreme sensitivity to variations
in matching element values. Additionally, the specificity of the IF
filter to the signal bandwidth and hence the standard used,
makes superheterodyne receivers unsuitable for multi-standard
operation.
Nonetheless, superheterodyne is praised for its high selectivity
and sensitivity.
Image-Reject Receivers
Alternatively, by a smart use of trigonometric identities, the
image can be removed without the need of any post-LNA
image-reject filtering. This is the principle of image-reject
receivers [8] and [10]. The first is the Hartley architecture,
introduced in [11] in 1928, and shown in Figure 3. It uses two
mixers with their local oscillators in a quadrature phase
relationship. This separates the IF signal into in-phase (I) and
quadrature (Q) components. It then shifts the Q component by
90
° before recombining the two paths. This is where the desired
signal, present in both paths with identical polarities, is
reinforced, while the image, present in both paths with opposite
polarities, is cancelled out. The dual of the Hartley architecture,
known as the Weaver image-reject receiver [12], achieves the
relative phase shift of one path by 90 by the use of a second
LO enroute to another IF or to baseband, see Figure 4. The
same result is achieved.
However, the reliability of these receivers depends heavily on
the accuracy of the I/Q paths, that is, the gain and phase
imbalance between the two branches.
101735A 2_071901
f
RF
f
inter
f
im
f
LO
f
IF
f
IF
Interferer
Channel
Image
Image-
reject BPF
f
IF
Interferer
Channel
Channel-
select BPF
Figure 2. Image-Rejection and Selectivity in a Superheterodyne Receiver (High-Side LO Injection)
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