Cell phone jammers are devices that create a temporary "dead zone" to all cell phone traffic in their immediate proximity. Cell phone Jammers are typically used by the police and military to control or disrupt communication during hostage situations and bomb threats. Traditionally, cell phone jammers were large antennas mounted on trailers that had to be pulled behind a truck. Cell Phone jammer now come in a variety of shapes and sizes including models that are about the size of a cell phone.

Circuitry of http://jammerfun.com/ cell phone jammers
The main electronic components of a jammer are:
• Voltage-controlled oscillator — Generates the radio signal that will interfere with the cell phone signal
• Tuning circuit — Controls the frequency at which the jammer broadcasts its signal by sending a particular voltage to the oscillator
• Noise generator — Produces random electronic output in a specified frequency range to jam the cell-phone network signal (part of the tuning circuit)
• RF amplification (gain stage)— Boosts the power of the radio frequency output to high enough levels to jam a signal