sm9_17

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Problem 9.17. The signal ( ) cos(400 )m t tπ= is transmitted via FM. There is an ideal 
band-pass filter passing 100 ≤ |f| ≤ 300 at the discriminator output. Calculate the post-
detection SNR given that kf = 1 kHz per volt, and the pre-detection SNR is 500. Use 
Carson’s rule to estimate the pre-detection bandwidth. 
 
Solution 
We begin by estimating the Carson’s rule bandwidth 
 ( )
( )
Hz2400
200)1(10002
2
=
+=
+= mfT fAkB
 
 
We are given that the pre-detection SNR is 500. From Section 9.7 this implies 
 
2400
1
2
500
2
0
2
0
2
N
A
BN
ASNR
c
T
cFM
pre
=
=
 
 
Re-arranging this equation, we obtain 
 
Hz102.1
2
6
0
2
×=
N
Ac 
 
The nuance in this problem is that the post-detection filter is not ideal with unity gain 
from 0 to W and zero for higher frequencies. Consequently, we must re-evaluate the post-
detection noise using Eq. (9.58) 
 
[ ]
7
2
0
33
2
0
300
100
2
100
300
2
2
0
106.2
3
2
100300
3
2
powernoisedetection-postAvg.
×=
−=
⎥⎦
⎤⎢⎣
⎡ += ∫∫−
−
c
c
c
A
N
A
N
dffdff
A
N
 
 
The post-detection SNR then becomes 
 
 
 
Continued on next slide 
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Problem 9.17 continued 
( )
( )( )
8.69230
106.2
5.01000102.13
106.22
3
106.22
3
SNR
7
2
6
7
2
0
2
7
0
22
FM
post
=
××=
×⎟⎟⎠
⎞
⎜⎜⎝
⎛=
×=
Pk
N
A
N
PkA
fc
fc
 
 
where we have used the fact that kf = 1000 Hz/V and P = 0.5 watts. In decibels, the post-
detection SNR is 48.4 dB. 
 
 
Excerpts from this work may be reproduced by instructors for distribution on a not-for-profit basis for testing or instructional purposes only 
to students enrolled in courses for which the textbook has been adopted. Any other reproduction or translation of this work beyond that 
permitted by Sections 107 or 108 of the 1976 United States Copyright Act without the permission of the copyright owner is unlawful.