parnass@ihu1g.UUCP (Bob Parnass, AJ9S) (07/07/86)
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Why Can't My Scanner Scan Faster?
Bob Parnass, AJ9S
Scanner enthusiasts have tried to modify their radios
to scan and search faster. Some modifications are suc-
cessful, while others produce undesirable side effects.
Factors That Limit Scan Rate
There are limits to how fast radios can change fre-
quency (scan or search) and still perform well without
requiring modification of squelch circuitry. Most
AM/FM scanners use the logic level output from a
squelch circuit to determine whether a signal is
present or absent. This signal is used for the purpose
of stopping, or interrupting the scan operation. The
ability of the common squelch circuit to detect the
presence of a signal is dependent on at least 2 fac-
tors:
1. the strength of the signal
2. the duration of the signal
The first factor may be obvious to most readers, and is
often specified as the "squelch sensitivity". It is
the second factor that is important to our discussion.
If a signal is present only for an instant, the squelch
may not detect it, even if the signal is strong. In
general, the faster the scan rate, the less time the
squelch circuit has to detect the signal, and the
stronger it must be in order to stop the scan.
In practical terms, the squelch circuit time constant
imposes a limit on how fast a given radio can scan and
still be able to stop on signals of weak or moderate
strength.
Another limit is imposed by phase lock loop synthesizer
circuits, which require time to "lock up" on a fre-
quency.
Circuits That Determine Scan Rate
Most crystal controlled scanners contain a simple
oscillator (also referred to as a "clock") whose sole
function is to force the radio to switch frequencies.
Speeding up the scan rate in this type of scanner is
easy, usually just a matter of changing a resistor or
capacitor in the clock circuit.
Modern programmable scanners employ microprocessors to
control scanning, as well as other functions, like key-
board decoding, frequency limits checking, timing scan
delay, priority channel sampling, etc. The scan rate
in these sophisticated scanners is determined within
the microprocessor firmware.
Well intentioned hobbyists have attempted to speed up
these programmable radios by altering the speed at
which the microprocessor executes its internal instruc-
tions. Not only does this increase the scan rate, but
it can have several undesirable effects, like causing
loss of memory, increasing the power consumption and
temperature of the microprocessor chip, etc.
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Bob Parnass, Bell Telephone Laboratories - ihnp4!ihu1g!parnass - (312)979-5414