I played around with my scanner for a while (Pro 2045) to see if it was feasible to modify it...it is very possible.
The Pro 2045, and all modern scanners with a wide frequency range use a PLL (Phase Lock Loop) to generate the local signal...back up...
Radio signal comes to the radio as elecromagnetic waves. The antenna detects either the electrical or magnetic part of the wave (loop antenna gets magnetic). In either case, it is converted to an electrical signal at the "feedpoint" of the antenna. This signal is first filtered to get rid of any undesired frequencies.
The "front end" (right after the antenna) filtering on mine was dynamic, as in, the frequency response of the filter would change so that only a small range/band of frequencies around could pass through ("band pass" filter). This poses the first problem.
The signal is then amplified and mixed with a signal from the LO ("local oscillator"). When sine waves mix, they add and subtract, depending on which part of the wave is mixed (peak or troff). If you have a sine wave with a frequency of 1000 cycles per second, and another with 1500 cycles per second, and mix them, the resulting signal will have sine waves with the difference of the waves (500 cycles per second), the original waves, and the sum of the waves (2500 cycles per second).
So, if you have a 900MHz signal, you can mix a 890MHz (could use 910MHz) signal with a "mixer" and get a 10MHz signal with all the original information still in it.
So, you have a signal from the antenna (lets say 900MHz), and you have an amplifier and demodulator that wants a 10MHz signal (this can be called the "IF", or Intermediate Frequency) because that's what they were designed for (these aren't usually dynamic). How do you get your 10MHz signal? Well, mix a 890MHz signal, generated by the LO, with it. This poses the second problem.
Say we want to recieve a 500MHz signal now. Well, if our IF is still 10MHz, we would mix a 490MHz or 510MHz signal with it.
***So, all that changes when your scanning through the frequencies is the LO AND the filter at the front end. So, all you have to do is change the LO .
On pretty much all modern (and old) radio equipment uses something called a PLL (Phase Loop Lock) to generate the LO signal. A PLL is just a thing that multiplies a "reference" frequency (usually from a crystal because of their very high stability) by some amount. This multiplication *is software controlled* by data pins on the PLL chip! What does this mean!? Well...if you wanted...you could take COMPLETE control of the PLL and the signals that it makes! What does THAT mean!? Since your radio always needs a constant IF (well, changes depending on band) you can recieve pretty much ANY frequency you want by setting the PLL to 10MHz (or whatever the IF is) above (or below) the frequency you want to recieve! Yeah!
Like I mentioned, the first problem is the front end. It is ALSO software controlled. The radio COULD be designed to block the cell and other frequencies by making it impossible to pass those frequencies through because of the values of the components that make the filter.. Why can't we control those you ask? Well...you can...it's just hard and requires a *very stable*, and usually very small DC voltage.
I injected a range of frequencies into my scanner to see what frequencies the bandpass filter passed...and there was a small null in the part of the cell area, but not that bad. The radio just had to be set to a frequency close to the desired freq. If I remember correctly, lower was better. So, this isn't that bad of a problem.
So...if you wanted...you could download the datasheet for the PLL in your scanner...wire up some wires from the parallel port on your computer (the PLL in mine only required 8 or so lines) and write up a little program in QBasic or something to control your scanner to a certain extent...at least enough to get "blocked" frequencies.
I haven't done it yet because of lack of time and I have no motivation to listen to peoples crappy conversations which I do much to often while shopping for groceries at the store. I'm more interested in the lower frequencies...which is more of a challenge (front end filter drops of at ~28M).
I drew up the front end filter schematics from the parts on the board...so if the front end filter was giving any trouble...it would be easy to "fix" it (just change a value).
Jeeze...didn't mean to write so much...
Written using the Dvorak keyboard layout.