The figure is shows the first stage of a super heterodyne receiver. The desired input signal is at a frequency of 700MHz. The local oscillator (L. O) frequency is 1 GHz. The mixer is an ideal multiplier with a gain independent of frequency. A band-pass filter (BPF) is used to select the Intermediate Frequency (IF) output at 300MHz.
(a) What is the image frequency of the desired input?
(b) A Low Pass Filter (LPF) can be used before the mixer to reject the image frequency. If a perfect rejection (zero transmission) of the image is desired, what type of LPF should be employed?
(c) The input in the figure is corrupted by an undesired 750 MHz signal which has the same amplitude as that of the desired signal at 700MHz. Let the BPF be of second order. At the BPF output, the undesired signal should be 20 dB below the desired signal. Calculate the Q required for the BPF.
(a)
(b) Frequency response of ideal LPF
(c) Q=7
(c) Q=14
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