2. In Helsinki, the commercial AM broadcast stations operate in the 645 kHz to 1

Question

2. In Helsinki, the commercial AM broadcast stations operate in the 645 kHz to 1805 kHz band
with transmission bandwidth limited to 15 kHz for any station. The center frequency of the
intermediate frequency (IF) filter is 555 kHz. Each station is required to have 5 kHz guard
band.
(a) Determine the maximum number of stations that can be accommodated within the above
band?
(b) What are the possible range of image frequencies for the AM receiver that uses a downconverter?
Assume that the local oscillator frequency is higher than that of the incoming
carrier.
(c) If the receiver is tuned to receive a 1180 kHz AM signal using a down-converter, how would
the (front-end) RF and IF filters be set? Give the filter characteristics with appropriate values?

(d) What is the image frequency for the above setting in (c)?

Explanation / Answer

1) N*transmission bandwidth + (N-1)(guard band)= total available range
   where n is no of stattions
   So. N*15k + (N-1)5k = (1805-645)k
   20kN - 5k = 1160k
   N=1165/20=58.25
  So it can have maximum 58 stations.

2)Image Frequency IM= signal frequncy +2*IF
   so minimum value
645+ 2*555 = 1755kHz
and maximum value of Image frequency
   1805 + 2*555 = 2915khz.
3) if frequency is always same i.e 555kHz
   so IF=RF-signal frequency
   555=Rf-1180KHz
   so RF =1735 khz
4) image frequency = 1180=2*555=2290 kHZ
      

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