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| 型號： | IEEE 802.11 |
| 廠商： | Intersil Corporation |
| 英文描述： | () |
| 中文描述： | （） |
| 文件頁數(shù)： | 5/7頁 |
| 文件大?。?/td> | 69K |
| 代理商： | IEEE 802.11 |
2-5
Thefrontendnoisefloorwaspreviouslycalculatedfora20MHz
bandwidth as -101dBm, if the required gain is now added, this
noise floor becomes -81dBm as shown in Equation (7). Now
verify that this level is larger than the minimum signal power
required to limit the limiter chain and guarantee that the radio
limits on front end noise
81dBm
–
(
.
As will be explored next, the actual front end gain is 10.4dB
and the NF is 6.8dB for a front end output noise floor
increase of 17.2dB typical, not the desired 20dB. This results
in limiting, when no signal is present, on mainly front end
noise, but also some limiter broadband noise. This limiter
output, although still fully limited, when band filtered will have
a slight drop in baseband quadrature output voltage to the
HFA3824 ADCs as compared to totally limiting on front end
noise due to some of the limiter noise being out of band. The
net impact to the system is a small reduction in sensitivity
due to reduced ADC signal to full scale. If desired, additional
front end gain, or a reduced bandwidth limiter BPF could be
used to gain back performance.
Receive Chain Front End Cascade
Analysis
The typical receiver performance of the chip set is illustrated
in the level diagram analysis shown in Figure 4. Across the
top of the figure are descriptions of the blocks in the receive
chain. Each block has typical values for Insertion Loss (IL) or
Gain (G), Noise Figure (NF) and Output Third Order
Intercept Point (OIP3). The noise figure for a passive
element is equal to its insertion loss and its OIP3 is set to
100dBm for the purpose of calculation. The left side of the
figure lists the parameters to be calculated for each block
element; Gain, NF, IIP3 and typical signal level. The entry in
each space is the result of the parameter after the effect of
the chain element, with the exception of IIP3 where the entry
is for the block input. Cascaded parameters may be
determined anywhere along the chain, and so this type of
analysis can show where a units performance is not being
utilized as it should and therefore, has increased the system
cost over what it could be.
The receiver front end total gain is 10.4dB for a radio with
better than -90dBm sensitivity. This is due to the high gain of
the IF limiter stages. Minimizing front end gain is desirable
so that IIP3 may be maximized. The front end actually has a
significantly larger amount of gain than is indicated by the
total. A total of 21.2dB of gain is required, to overcome the
insertion loss of the receive filters, particularly the IF SAW
filter with an insertion loss of 10dB maximum.
The IF SAW filter is critical to the design because it provides
a large amount of ultimate rejection for adjacent channel
interferers and sharp band edges to select the desired
channel. This allows other channels to operate close by in
frequency without interference. Typically this 17MHz
bandwidth SAW filter provides adjacent channel rejection of
63dB at 25MHz of frequency offset.
The figure shows that noise figure is dominated by the first
LNA, the HFA3424, where it is set to 5.2dB. The rest of the
chain only contributes another 1.6dB for a total front end
noise figure of 6.8dB. Even a noise figure of 12dB for the
downconvert mixer only adds 0.3dB latter in the chain. The
reason for this dominance by the first gain stage can be seen
from the cascaded noise figure Equation (8). The noise
figure of subsequent stages are divided by the total gain of
prior stages. This fact makes the selection of the first LNA
important because the front end noise figure sets the
amount of front end SNR degradation and therefore,
contributes to receiver sensitivity.
98dBm
–
>
)
20MHz kTB
(
)
+ NF
LIM
+ BPF
Noise
+Lim
Margin
= Front End Noise
i.e.,
-101dBm + 7dB + 7dB + 6dB = -81dBm
Where:
(20MHz KTB) is Available Thermal Noise
NF
LIM
is Noise Figure of Limiter Chain
BPF
Noise
is Added Noise due to Limiter BPF
Bandwidth Wider than Front End
Lim
M
in
arg
Guarantee Limiter Jammer Rejection
is Front End Margin Required to
(EQ. 7)
NF
TOTAL
= NF
1
+
NF
-1
1
------------------
+
NF
-1
1
2
------------------
+ ...
Where NF and G (Gain) are Linear Number, i.e.,
Where NF = 10N10
(EQ. 8)
INPUT
POWER
FL1 TOKO
RF
FILTER
IL=2.0
HFA3925T/R
SWITCH
IL=1.2
OIP3=34
HFA3424
LNA
G=13dB
NF=2.0
OIP3=11.1
ATTEN
IL=5.0
HFA3624
LNA
G=15.6
NF=3.8
OIP3=15
FL2
MURATA
RF FILTER
IL=3.0
HFA3624
MIXER
G=3.0
NF=12.0
OIP3=4.0
FL3
TOYOCOM
IF SAW
FILTER
IL=10.0
HFA3724 IF
LIMITER
STRIP
G=0
NF=7.0
Gain
-2.0dB
-3.2dB
9.8dB
4.8dB
20.4dB
17.4dB
20.4dB
10.4dB
10.4dB
NF
2.0dB
3.2dB
5.2dB
5.5dB
6.0dB
6.0dB
6.3dB
6.4dB
6.8dB
IIP3
-16.8dBm
-18.8dBm
-20.0dBm
-6.9dBm
-11.9dBm
4.0dBm
1.0dBm
NA
NA
-90dBm
-92dBm
-93.2dBm
-80.2dBm
-85.2dBm
-69.6dBm
-72.6dBm
-69.6dBm
-79.6dBm
-79.6dBm
FIGURE 4. PRISM1 RECEIVER LEVEL DIAGRAM
Application Note 9810
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