From: martin@datacomm.ucc.okstate.edu (Martin McCormick)
Subject: Re: Cellular scanning
Organization: Oklahoma State University, Stillwater, OK
Date: Tue, 13 Oct 1992 15:52:33 GMT


Since actually monitoring cellular frequencies is illegal, here is a
suggestion for those who want to increase the range of their scanners.
Just remember, what you do with this information is your responsibility.
	Any scanner which tunes the frequencies between 41 and 48MHZ has
the potential for expanded coverage.  This is the frequency range used by
the IF output of most television tuners.  Most TV tuners are considered to
operate with a 45MHZ output.  This means that the local oscillator of the 
tuner is45MHZ above the desired frequency.  This is called
"high-side injection."  The bottom line, here, is that a TV tuner makes a
good converter for a wide range of frequencies.  Several warnings are in
order, here.  While a TV tuner out of a television set is no more dangerous
than any other piece of electronic equipment, a tuner which is still installed
and powered by a television set is quite potentially dangerous.  Why?
Most solid-state televisions derive their power supply from the flyback
transformer.  The primary side of this transformer is fed via a rectifier
and switching transistor which are fed right off the AC power cord.  
by connecting anything at all to the TV tuner at any place other than the
antenna input, you run the risk of forming a circuit between what ever you
connected and the nearest ground such as the floor, another piece of electrical
equipment, etc.  Don't do that.  If you want to use a TV tuner as a converter,
find out the operating voltages it needs and buy or build a suitable supply
for this purpose.  The flyback transformers of televisions may be
autotransformers which have their secondaries in series with the primary.
This means that all parts of the TV chassis may have line voltage with
respect to earth.
	Now, for the operational aspects of using a TV tuner as a converter.
There are three types of TV tuner which are the most common.  The oldest
TV's have the turret type or some suitable variation on that theme.  These
tuners have a rotary knob and click once for each channel.  They usually
have a separate tuner for UHF mounted right next to the VHF unit.  On some
sets, the UHF knob turns smoothly like the tuner on a radio or it may have
detents or click-stops to help in finding channels.
	The other two types of tuners are electronic tuners.  The PLL or
phase-locked-loop type tuners use a digital frequency synthesizer to select
the proper TV channel.  They also have varicap diodes in their antenna
amplifier stages which are adjusted by the synthesizer to tune the antenna
port to the desired frequency or range of frequencies.
	The last kind of tuner is another electronic type tuner.  It uses
varicap diodes to tune the antenna port as well as the local oscillator.
	If you decide to play with an old TV tuner, don't expect the
performance of an Icom R9000.  TV tuners have some real drawbacks which
make them tempermental, at best and unacceptable, at worst as frequency
converters.  First of all, TV channels in the U.S. are 6MHZ wide and even
wider in other countries.  If the oscillator drifts or changes frequency,
slightly, the effect on a television picture is almost non-existant.  On a
narrow-band scanner, the effect is to cause the desired frequency to slide
right out of the pass-band.  The old rotary-knob-type tuners are really
unstable, both mechanically and electrically.  A slight bumb against the
fine tuning control or the channel selector means you will have to retune.
	The PLL or quartz-lock tuners are very stable, but have a problem
of their own.  Remember the first synthesized scanners and how much whine
and noise their synthesizers introduced into the signal?  Well, it's back
or, shall we say, it never left.  PLL TV tuners don't need to illiminate
that problem to deliver a good signal because the shift in frequency is
minor enough not to effect the picture and the sound is derived from mixing
the video and audio carriers.  Slight changes in the tuner frequency don't
effect the sound.  On a scanner, however, you'll hear any rapid changes
in frequency as additional modulation to the signal.
	Varicap tuners need a splendidly regulated power supply to even hope
to stay on frequency.
	As you can see, there is a lot to using a TV tuner as a down-converter,
but it can be done.  The only other safety-related warning is to be sure to
make sure there is no DC voltage at the 45MHZ IF output.  This output is usually
a phono jack or, what some call, an RCA plug.  I have seen tube-type tuners
that had 200 Volts at that jack.  To keep it out of your scanner, put a 
suitable value of disc capacitor in series with the center conductor of the
coax.  A 10PF capacitor of suitable voltage rating would do the trick.
Organization: Electrical & Computer Engineering, Carnegie Mellon, Pittsburgh, PA