Okay, here it is. To understand images, let's go back to the mixer stage before
the 1st IF. What's all that, you say? Well, this is about frequency mixing.

Within every scanner, there is a very weak radio transmitter. This is called the
local oscillator. When a scanner is programmed with a frequency, it is really
the local oscillator that is being programmed, but to a different frequency than
the one the user enters in. Imagine a scanner being programmed to receive a
frequency of 100.00 MHz. When the scanner gets 100.00 MHz entered in, the local
oscillator is tuned to 89.3 MHz. This difference in frequencies is precisely
10.7 MHz lower than the entered frequency. Some scanners use a difference of
10.8 MHz, since the local oscillator a bit different from the rest. The 89.3 MHz
signal is injected into the path of all the incoming frequencies at the MIXER
STAGE. One of these incoming frequencies is 100.00, the programmed frequency.
When the local oscillator mixes 89.3 MHz with 100.00 MHz, four frequencies are
the result: 100.00 MHz, 89.3 MHz, 189.3 MHz (100.00 MHz + 89.3 MHz), and 10.7
MHz (100.00 MHz - 89.3 MHz). The maker of the scanner pre-tunes the stages that
follow to give amplification only to 10.7 MHz and reject the others. The stages
that follow are called the INTERMEDIATE FREQUENCY (IF) stages. The IF stages get
their name because they are between the RADIO FREQUENCY (RF) and the audio
amplifying stages. To receive 500.00 MHz, the local oscillator would tune to
489.3 MHz, and the mixer would produce an IF of 10.7 MHz.

A dual conversion scanner is a scanner that converts a radio signal to 10.7 MHz
or 10.8 MHz (the 1st IF), and then to 455 KHz (2nd IF). Some scanners may use
other frequencies for IF, but the principle is the same. This conversion process
itself is what creates scanner images. That's how dual conversion scanners can
pick up cellular without any modification whatsoever, provided that they can
receive the 800 MHz band in the first place. Newer scanners today use triple
conversion, which places an additional IF stage before the 10.7 MHz IF stage.
Triple conversion scanners are very unlikely to pick up any images at all,
including cellular images.

Imagine a dual conversion, 800 MHz reception capable, scanner receiving a
cellular phone call. The cell phone call is on a frequency of 893.070 MHz. The
scanner display screen shows that it is receiving 914.470 MHz.The scanner is
supposed to have cellular reception blocked out. THIS IS WHY THIS HAPPENS: The
cellular reception bandwidth is between 869.04 MHz and 893.97 MHz. When the
display screen of a dual conversion, 800 MHz reception capable scanner shows
that it is receiving 914.470 MHz, the local oscillator is actually tuned to
903.770 MHz (the IF is 10.7 MHz). Mixing the two signals produces 10.7 MHz
(914.470 MHz minus 903.770 MHz equals 10.7 MHz). If a cellular phone signal is
strong enough to get through the filtering in the RF (RADIO FREQUENCY) stage, it
will also mix with the local oscillator signal to produce the same IF (903.770
MHz minus 893.070 equals 10.7 MHz). The difference between the frequency
displayed on the scanner and the frequency being heard is 21.4 MHz. That is the
image spread, which is exactly twice the 1st IF (10.7 times two equals 21.4
MHz). Got all that?

By:

Kepi Blanc kepiblanc@mailcity.com