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參數(shù)資料
| 型號： | CX74017 |
| 廠商： | Skyworks Solutions Inc. |
| 英文描述： | On the Direct Conversion Receiver |
| 中文描述： | 在直接轉(zhuǎn)換接收器 |
| 文件頁數(shù)： | 6/10頁 |
| 文件大小： | 315K |
| 代理商： | CX74017 |
On the Direct Conversion Receiver
CX74017
101735A
Skyworks Solutions, Inc., Proprietary and Confidential
5
July 20, 2001
Preliminary Data Subject to Change
Conversely, a strong in-band interferer, once amplified by the
LNA, may find a path to the LO input port of the mixer, see
Figure 9, thus producing self-mixing, once again.
101735A 8_071801
LO leakage
Figure 8. LO Leakage
101735A 9_071801
Strong
Interferer
Figure 9. Strong In-Band Interferer, Amplified by the LNA
Some amount of LO power is conducted through the mixer and
LNA, due to their non-ideal reverse isolation, to the antenna.
The radiated power, appearing as an interferer to other
receivers in the corresponding band, may violate emissions
standards of the given system.
It is important to note that since the LO frequency is inside the
receive band, the front-end filters do nothing to suppress this LO
emission. Additionally, the radiated LO signal can then be
reflected by buildings or moving objects and re-captured by the
antenna, as shown in Figure 10. This effect, however, is not of
significant importance compared to the previously mentioned LO
self-mixing and blocker self-mixing.
LO or RF signal leakage to the opposite mixer port is not the
only way in which unwanted DC can be produced. Any stage
that exhibits even-order non-linearity also generates a DC
output. This is covered in more detail later.
101735A 10_071801
Re
r
a
d
i
a
t
e
d
Reflected,
Refracted
Figure 10. Emission, Reflection, and Refraction
of LO Signal via Antenna
Whether or not the DC product desensitizes the receiver
depends on the system type. Obviously, it is preferable to
alternate current (AC)-couple at the mixer output to eliminate the
DC. Some modulation schemes used in paging applications,
such as Frequency-Shift Keying (FSK), show little degradation if
low frequency spectrum components are filtered out, see
101735A 11_071801
f
0 f
c1 fc2
High pass
Figure 11. High-Pass Filtering of the Modulation Spectrum
However, other modulation schemes present a peak at DC, and
capacitive AC-coupling will infer significant information loss,
hence considerably degrading the BER. In TDMA systems such
as GSM, although there is no significant low frequency spectral
peak, it still becomes impossible to AC couple. This is because
of the conflicting requirements on an AC-coupling capacitor in a
Time-Division Multiple Access (TDMA) system: the capacitor
must be large enough to avoid causing a wide notch at DC, but
it must be small enough that all transients settle out upon
receiver power-up (every frame) before data reception begins.
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