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| 型號(hào)： | IEEE 802.11 |
| 廠商： | Intersil Corporation |
| 英文描述： | () |
| 中文描述： | （） |
| 文件頁數(shù)： | 6/7頁 |
| 文件大小： | 69K |
| 代理商： | IEEE 802.11 |
2-6
Where the NF calculation is dominated by the first element in
the chain, IP3 is typically dominated by the last element, the
element with the largest signal. In this case, the
downconverting mixer, with an OIP3 of 4dBm, is where the
largest signal occurs and is the dominating element for our
cascade analysis.
Third Order Intercept (IP3) is a measure of signal distortion,
and the equation for cascaded IP3 has the same form as
another distortion parameter 1dB compression (P1dB). The
equation for cascaded IIP3 is shown in Equation (9).
The total receiver front end third order intercept point of
-16.8dBm is important in the management of non-desired
jamming signals. Jammers can be high power out-of-band
signals that make it past the 2.4GHz ISM band pre-select filter
or especially in-band signals. In-band jammers can be either
intentional (i.e., other radios on the same or another channel)
or unintentional (i.e., microwave ovens).
As the power of jamming signals approach the receiver IIP3,
harmonics and mixing products of these jammers appear.
The risk here is if the jammer itself is not directly in the
desired signal channel, it is possible that some of the
distortion terms can be. Once the distortion product is in the
channel, it can no longer be filtered out and the signal must
be rejected through processing gain or limiter suppression.
Another effect from jamming signals related to the IIP3 is
receiver desensitizing. The receiver input 1dB compression
point may be approximated by subtracting 10dB from the
IIP3 resulting in -26.8dBm. A jamming signal larger than this
will begin to compress the receiver front end, reducing the
front end gain of the large jamming signal along with the
desired signal. This desensitizing of the receiver causes a
loss of sensitivity and limiter margin, even if the jammer is
later filtered out by the IF SAW filter.
A trade-off between noise figure and input intercept point
exists in any receiver. The attenuator after the first LNA is
used in this trade-off between high gain, for good sensitivity,
and lower gain for better IIP3.
The receiver front end circuits have been specially designed
to permit input signals as large as 0dBm with the receiver still
able to function. This is important because if the designer is
not careful, the design may have great sensitivity and range,
but when the two radios get close they will not work! The
signal is so large that it overdrives the input causing the input
amplifier or RF mixer to saturate and stop functioning.
Transmit Chain Front End Cascade
Analysis
The transmitter performance analysis is shown in the level
diagram in Figure 5. Here again, the function blocks are
described across the top, with the only difference being that
Output 1dB Compression Point (OP1dB) is listed for each
block. The parameter list, on the left side, includes Gain,
OP1dB, and Output Power (P
OUT
).
The large gain in the Power Amplifier, HFA3925, keeps the
signal level small for the whole chain before it. This helps
conserve supply current and the cost associated with
devices that need to handle large signals.
The output power at the RF filter is shown as 18dBm, with
the output of the Power Amplifier at 20dBm. The design has
been optimized for the best use of supply current and cost of
the PA. If the output power was lower, the supply current and
cost of the PA could be saved with smaller and cheaper
devices. If the output power was higher, the signal will be too
close to the P1dB and the signal will have excessive
distortion, causing regrowth of the side lobes. The
transmitted signal must suppress side lobes to -30dB to
meet the 802.11 spectral mask specification.
Therefore, the output power needs to be controlled very
carefully. The run to run variation in gain and insertion loss of
the elements in the transmit chain requires either a manual
power adjustment or an active power adjustment feedback
circuit. In this design, a manual potentiometer is used to
adjust output power and side lobe performance on each unit
during manufacturing. For purposes of this analysis, the
variable attenuator shows 0dB loss and the Modulator output
as -10.4dBm. This was done for illustration purposes only, in
actuality the Modulator output (HFA3724) has a relatively
large output (200mV
P-P
) that needs to be significantly
attenuated to the level indicated.
IIP3(Watts)
TOTAL
1
--------------
+
2
--------------
+
2
3
---------------
+ ...
--------------------------------------------------------------------------
=
Where IIP3 and G (Gain) are Linear Numbers.
(EQ. 9)
OUT
POWER
FL1 TOKO
RF FILTER
IL=2.0
HFA3925
PWR AMP
G=28dB
P1dB=24.5
FL7 TOKO
RF FILTER
IL=2.0
HFA3624
PRE AMP
G=12.3
P1dB=5.6
NF=5.7
FL6
MURATA
RF
FILTER
IL=3.0
HFA3624
MIXER
G=2.1
P1dB = -10.5
NF=14.5
Z
I
=1k
Z
O
=50
ATTN
IL=
VAR
FL5
TOYOCOM
IF SAW
FILTER
IL=10 MAX
IL=7.0(TYP)
Z
O
=270
HFA3724
MOD OUT
Z
O
=270
Gain
35.4dB
37.4dB
9.4dB
11.4dB
-0.9dB
2.1dB
-
-
-
OP1dB
19.2dBm
21.2dBm
-4.0dBm
-2.0dBm
-13.5dBm
-10.5dBm
-
-
-
P
OUT
18dBm
20dBm
-8dBm
-6dBm
-18.3dBm
-15.3dBm
-17.4dBm
-17.4dBm
-10.4dBm
FIGURE 5. PRISM1 TRANSMITTER LEVEL DIAGRAM
Application Note 9810
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