Service manual

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Image Reception
Radios work by simple mathematics. For example, most tune to a frequency by mixing that frequency with another (local oscillator) frequency
which is slightly different. This mixing process primarily gives us the two original frequencies, their sum, and their difference. Well, the radio's
Intermediate Frequency (IF) filter normally passes either the sum or difference frequency, and this is then processed into the sound we hear.
Because nothing is perfect, certain "harmonics" will also get through if they are strong enough. For example, if a radio's IF is 10.7 MHz, we might
be able to tune to a frequency 21.4 MHz (2 x IF) above (or below, depending on the radio's design) a strong signal and hear it! This is more
evident in a
dual-conversion radio than a triple-conversion radio, because the triple-conversion radio's 1st intermediate frequency is quite high.
This causes the image to be so far off frequency that it is easy to effectively filter it out.
Just because a radio doesn't receive something which another does is not necessarily an indication of a problem. The one radio may simply not
be "tricked" into picking up an
image! This rejection of undesired signals is one reason that a triple-conversion receiver costs more than a similar
dual-conversion model. If you are more interested in finding more out about radios and radio operation, a good location to start looking is your
local public library. You might also wish to contact the
ARRL, as they are an excellent source of informative texts on the subject.
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Additional On-line Information
ARRL
Amateur Radio Relay League
http://www.arrl.org/
Strong Signals
©
(by Richard J. Wells, N2MCA)
http://www.strongsignals.net/
TrunkTracker™
(by
Uniden
®
)
http://www.trunktracker.com/
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Glossary
Base Station:
A scanner or other two-way radio which is non-mobile, often connected to a larger, outdoor antenna.
Birdie: A false/unwanted signal produced inside the scanner.
Channel: A memory location used to store a single frequency.
Conventional Scanning: Following conversations that are broadcast on manually tuned radios.
Crystal: A component which allows a crystal-controlled scanner to receive a particular frequency.
dB, decibel:
Unit used to express relative differences in noise level or signal strength.
Discone:
An antenna which has the horizontal elements connected to the shield and the cone connected to the coax center
conductor, so that the actual configuration is an upside down half-bow-tie.
Dual-conversion: A method using a receive circuit with two stages and two intermediate frequencies to extract the data signal from the
carrier.
Frequency: The number of cycles (Hertz) used as a carrier for a particular data signal.
Frequency Band: A particular frequency range used for a particular purpose.
Frequency Step: The increment between displayed frequencies on a digitally-tuned scanner.
Harmonic:
Describes a frequency which has a smaller amplitude and is a multiple of a larger frequency (for example, 480 MHz
is the second harmonic of 240 MHz and the third harmonic of 120 MHz).
Hyperscan: A faster scan method available on some scanners.
GHz (gigahertz):
A unit of frequency equal to 1000
MHz, 1,000,000 kHz or 1,000,000,000 Hz.
Hz (Hertz):
A unit of frequency equal to one cycle per second.