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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)： | 4/7頁 |
| 文件大?。?/td> | 69K |
| 代理商： | IEEE 802.11 |
2-4
This is calculated as follows: since the source and load
impedances are different (250
vs 500
) the input signal is
calculated in terms of voltage. Remember that the limiter is a
voltage gain device and so gain is independent of source
impedance. Substitute the result of Equation 1 into 2 and
calculate V
IN(P-P)
.
Calculate the input power with a 250W impedance by using
Equation (3) to get V
RMS
and then substitute into (4) with
R = 250
to get power in Watts. Equation (3) assumes a
sinewave crest factor for the
converted to dBm with Equation (5) to get -98dBm input
power at 250
source impedance.
term. Power in Watts is
The limiters have a noise bandwidth of over 500MHz and so
the cascaded limiters will fully limit on their own noise, if no
BPF is used between the stages. The thermal noise voltage
delivered from the 250W source to the limiters in a 500MHz
band is -87dBm, as calculated from Equation (6). This
thermal noise adds to the limiter noise figure (NF) of 7dB
resulting in an equivalent input noise power of -80dBm,
which is significantly higher than the -98dBm required for
limiting.
The RF front end 3dB bandwidth is 17MHz, with an estimated
noisebandwidthof20MHz,asdefinedbytheIFSAWfilter.This
makes the available thermal noise at the limiter input -101dBm
and with the 7dB limiter noise figure, is an equivalent -94dBm.
If the limiter BPF was also 20MHz, the front end would only
need to supply 7dB of noise floor gain to overcome the
limiter noise figure. This would result in a receiver that limits
in a 20MHz bandwidth from front end noise with no margin.
The 20MHz limiter BPF would require a second SAW filter
and therefore, is not cost effective or practical.
The alternative chosen to be implemented is a simple one
pole LC BPF with a bandwidth wide enough so that the
variability of fixed components do not result in the filter being
off frequency. The filter selected has a 3dB bandwidth of
50MHz, and an estimated noise bandwidth of 100MHz.
Using this method, the front end gain must be increased to
compensate for excess limiter bandwidth.
It is desired that the second limiter to be fully limited on front
end noise, as opposed to noise generated in the first limiter.
This requires that the front end noise floor must be greater
than the sum of the following; the limiter NF of 7dB, the ratio
of limiter BPF noise bandwidth to front end IF SAW
bandwidth (10log(100MHz/20MHz) or 7dB), and the amount
of limiting margin (6dB for -1dB limiting). The front end
output noise floor must therefore be greater than 20dB.
The limiting margin was measured on the HFA3724 IF
Mod/Demod, with good agreement to a theoretical estimate
based upon the hyperbolic tangent response of a bipolar
differential limiter. The measurement means that if a non-
desired jamming signal, noise in this case, is 6dB below the
level of the desired signal, the desired output will be
reduced 1dB.
Gain (V/V) = 10
-------------------------
(EQ. 1)
V
OUT
= Gain (V/V)
V
IN
(EQ. 2)
2
V
RMS
=
V
2
2
----------------
(EQ. 3)
Pwr(Watts) =V
2RMS
-------------------
(EQ. 4)
Pwr(dBm) = 10log(Pwr(Watts)
10
3
)
(EQ. 5)
P(Watts)
A
= kT
f
Where:
P(Watts)
A
= Available Noise Power
k = 1.38042 x 10
-23
Boltzmans Constant
T = 300 Degree Kelvin
f = 500MHz
(EQ. 6)
LIMITER
BPF
LIM1
LIM2
250
-SINGLE-ENDED
500
-DIFFERENTIAL
50
-SINGLE-ENDED
FRONT
END
BW
N
= 20MHz
NF = 6.8dB
GAIN = 10.4dB
BW
N
= 500MHz
NF = 7dB
GAIN = 42dB
BW
N
= 100MHz
NF = 3dB
IL = 3dB
BW
N
= 500MHz
NF = 7dB
GAIN = 42dB
WHERE BW
N
= NOISE BANDWIDTH
FIGURE 3. RECEIVE CHAIN GAIN DISTRIBUTION DIAGRAM
Application Note 9810
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