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参数资料
| 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： | 7/10页 |
| 文件大小： | 315K |
| 代理商： | 74017 |
CX74017
On the Direct Conversion Receiver
6
Skyworks Solutions, Inc., Proprietary and Confidential
101735A
Preliminary Data Subject to Change
July 20, 2001
In TDMA receivers that cannot be AC coupled, the idle time slot,
that is, just before reception, can still be put to good use by
storing the value of the offset on a capacitor and then
subtracting it from the signal path during the burst. This is
exactly the same method which is normally used to correct DC
offsets occurring at the second mix of superheterodyne TDMA
receivers, where this mix goes to baseband. In that case the
only problem causing DC is LO self-mixing. In this method, the
DC value produced by the receiver is obtained in a pre-
measurement prior to the receive burst.
It is important when using this method, that the signal path prior
to the mixer be opened during the DC pre-measurement, to
prevent any large blocking signals from affecting the result.
Blocking signals, which can appear at any time, most often
induce variable or wandering offsets. The measurement-and-
subtraction process cannot correct these offsets, because the
blocking signals may appear during the measurement and not
during the burst, or vice-versa. For blocking-induced DC, the
most effective measures are the elimination of self-mixing paths
and the maximizing of linearity to prevent the DC to start. Failing
these, there is still the possibility of DC-correction after-the-fact
in the digital signal processing (DSP) occurring at baseband.
DSP techniques can be used to remove the DC offset in TDMA
systems, in a way that cannot be duplicated in the analog
domain: a full timeslot of the received signal can be buffered,
the mean of which is determined and then removed from each
data point of the signal. The resulting signal has zero mean. For
systems such as Global System for Mobile communications
(GSM), an unwanted result of this is that any DC that is part of
the signal is lost, but the typical effect of this is minimal.
Figure 12 illustrates the use of such a method for a typical GSM
receiver. This technique can be further refined by tracking the
mean over portions of the burst, allowing the detection of
sudden interferers or blockers and canceling their DC product
only where it occurs.
Careful layout can also improve isolation.
101735A 12_071801
Figure 12. BER Improvement with DSP-Based DC Offset
Cancellation
Non-Linearities
As mentioned previously, another DCR problem is non-linearity.
Just as with the superheterodyne receiver, the DCR exhibits
spurious responses. For the superheterodyne, these occur at
RF input frequencies where
()
( ) IF
LO
M
RF
N
=
±
,
while for the DCR they occur where
()
( ) IF
LO
M
RF
N
=
When a blocking signal’s carrier falls on one of these spurious
frequencies, the signal is translated to baseband with an
attendant shift in its bandwidth, dependent on the spurious
order.
However, more importantly, large blocking signals also cause
DC in the DCR, whether on a spurious frequency or not. The DC
is produced at the mixer output and amplified by the baseband
stages. It is due primarily to second order mixer non-linearity,
characterized by IP2 (second order intercept point), IM2 (second
order intermodulation.) It can be alleviated by extremely
well-balanced circuit design. However, only a short time ago, the
mixer and LNA used to require a single-ended design because
the antenna and a hypothetical preselect filter were usually
single-ended.
In most systems, third order intermodulation is important, as it
usually falls in-band, in the vicinity of the signals of interest, and
is characterized by IP3 (third order intercept point). In direct
conversion, the second order intermodulation becomes critical,
as it produces baseband signals, which now appear as
interfering signals in the down-converted desired signal. The
second order non-linearity is measured by the IP2. IP2 is
defined in the same manner as IP3 Figure 13.
Either a 2-tone, or 1-tone test can be performed, and the IP2 is
defined by extrapolating the low-frequency beat tone in the
former or the DC component in the latter, until it intercepts the
fundamental curve. To illustrate in the case of a single tone test,
the input signal is:
)
cos(
)
(
t
A
t
x
ω
=
Assuming a non-linearity modeled by a polynomial:
...
)
(
3
2
1
+
=
x
a
x
a
x
a
x
y
...
)
2
cos(
2
)
cos(
2
...
2
1
)
2
cos(
)
cos(
)
(
2
1
2
1
2
+
=
+
+
=
t
A
a
t
A
a
A
a
t
A
a
t
A
a
x
y
l
fundamenta
DC
ω
43
42
1
3
2
1
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