Friday 21 June 2013
EXAMPLE ON SUPER HETERODYNE RECEIVER
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In a broad cast super
hetero dyne receiver having no RF amplifier, the load Q of the antenna coupling
circuit is 100. If the IF frequency is 455 KHz, determine
A) The image frequency and
its rejection ratio for tuning at 1.1 KHz station.
B) The image frequency and
its rejection ratio for tuning at 25 MHz station.
Solution
Given IF frequency is 455 KHz
Quality factor Q = 100
a) fS
= 1.1 KHz
Image
frequency fSi = fS
+ 2 fi
= 1100 +( 2 * 455 )
= 2010 KHz
ρ = (fSi/fS)
– (fS/fSi)
= (2010/1100) – (1100/2010)
=1.28
Image rejection ratio
α = √( 1+ Q2 ρ2 )
= √( 1+ 1002 1.282 )
= 128
b) fS
= 25 MHz
Image
frequency fSi = fS
+ 2 fi
= 25 MHz +( 2 * 455 )
= 25.91 MHz
ρ = (fSi/fS)
– (fS/fSi)
= (25.91 /25) – (25/25.91)
= 0.0715
Image rejection ratio
α = √( 1+ Q2 ρ2 )
= √( 1+ 1002 0.07152 )
= 5.22
Thus rejection is poor for
practical receiver in HF band
So, we can say that it is
not a major problem for standard AM receiver without an RF stage, but it causes
problem at high frequencies using same intermediate frequency.
We can eliminate this
problem by introducing RF stage or by increasing intermediate frequency
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