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Bandpass Signaling Principles and Circuits Chap. 4
inverted (i.e., the upper sideband on the RF input will become the lower sideband, etc., on the
IF output). If f LO 6 f c , the sidebands are not inverted.
The center frequency selected for the IF amplifier is chosen on the basis of three
considerations:
• The IF frequency should be such that a stable high-gain IF amplifier can be economically
attained.
• The IF frequency needs to be low enough so that, with practical circuit elements in the
IF filters, values of Q can be attained that will provide a steep attenuation characteristic
outside the bandwidth of the IF signal. This decreases the noise and minimizes the
interference from adjacent channels.
• The IF frequency needs to be high enough so that the receiver image response can be
made acceptably small.
The image response is the reception of an unwanted signal located at the image frequency due
to insufficient attenuation of the image signal by the RF amplifier filter. The image response
is best illustrated by an example.
Example 4–6 AM BROADCAST SUPERHETERODYNE RECEIVER
Assume that an AM broadcast band radio is tuned to receive a station at 850 kHz and that the LO
frequency is on the high side of the carrier frequency. If the IF frequency is 455 kHz, the LO
frequency will be 850 + 455 = 1,305 kHz. (See Fig. 4–30.) Furthermore, assume that other
signals are present at the RF input of the radio and, particularly, that there is a signal at 1,760 kHz;
this signal will be down-converted by the mixer to 1,760 - 1,305 = 455 kHz. That is, the undesired
(1,760-kHz) signal will be translated to 455 kHz and will be added at the mixer output to the
desired (850-kHz) signal, which was also down-converted to 455 kHz. This undesired signal that
has been converted to the IF band is called the image signal. If the gain of the RF amplifier is
down by, say, 25 dB at 1,760 kHz compared to the gain at 850 kHz, and if the undesired signal is
25 dB stronger at the receiver input than the desired signal, both signals will have the same level
when translated to the IF. In this case, the undesired signal will definitely interfere with the
desired signal in the detection process.
For down converters (i.e., f IF = | f c - f LO |), the image frequency is
f image = e f c + 2f IF, if f LO 7 f c (high-side injection)
f c - 2f IF, if f LO 6 f c (low-side injection)
(4–117a)
where f c is the desired RF frequency, f IF is the IF frequency, and f LO is the local oscillator
frequency. For up converters (i.e., f IF = f c + f LO ), the image frequency is
f image = f c + 2f LO
(4–117b)