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Cellular Phone Detector


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abbas1707

Tue Apr 29 2008, 01:16AM

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Cellular Phone Detector
This is a device to detect the electromagnetic radiation from a transmitting cellular phone and also a phone that is in "receive" mode. It will only work on phones which transmit in the 825 MHz to 850 MHz band, and use an Intermediate Frequency (IF) of 83 MHz. In those particular phones, the Local Oscillator (LO) frequency has a range between 953 MHz and 978 MHz.

The device works by having a Maxim MAX4003 logarithmic RF detector fed via two selectable Band Pass Filters (BPF), which are "switched in" as needed. One BPF covers the 825 MHz to 850 MHz cellular uplink (handset transmit) band while the other covers the 953 MHz to 978 MHz LO leakage band. The particular frequencies for the LO band come about because most 800 MHz band cellular phones use a 83 MHz IF. This means that the LO frequency will be 83 MHz higher than the cellular phone's receive frequency, which is between 870 MHz and 895 MHz (45 MHz offset from the transmit). By monitoring both of these bands with a RF detector and LED display, a visual indication of any RF activity can be observed and even located.

Modern cellular phones are very well shielded, so the LO radiation leakage isn't very significant. A tuned Yagi (directional) antenna feeding a precision front-end band pass filter with a high-gain, low-noise receive pre-amplifier will all help to increase the detection range on "receive mode" cellular phones. The range with this particular setup is only around four inches.

Schematic