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.
- Cellular Phone Detector - Picture 1 Outside case overview. RF input (antenna) is the top N-connector on the left, the 10 segment LED display is shown. The switches control the power and filter selection.
- Cellular Phone Detector - Picture 2 Internal view. LED display is controled by a LM3914. The wires are removable via small pin connectors
- Cellular Phone Detector - Picture 3 Close up internal view. RF input is on the bottom. The two Murata 2-pole filters and Hittite MMIC RF switches are shown. These feed a WJ AG603-89 MMIC RF amplifier and the Maxim MAX4003 (far right). The DC output (0.3 to 1.5 Volts) of the MAX4003 runs to the LM3914 where it controls the LED display. A 74HC04 (small 14-pin IC) controls the Hittite MMIC switches via a toggle switch and some buffering.
- Cellular Phone Detector Operation - Picture 1 Detector, using a Ramsey Electronics LPY2 Yagi antenna, and a non-powered (off) cellular phone. 836 MHz BPF selected. The bottom two LED segments lit (background noise).
- Cellular Phone Detector Operation - Picture 2 Now the cellular phone is powered, but is not transmitting (receive mode). The 964 MHz BPF is selected and the antenna is near the phone. Three LED segments are lit. If the detector was using a high-gain (40+ dB) pre-amplifer, sharp BPF, and a better directional antenna, the range would be increased significatly.
- Cellular Phone Detector Operation - Picture 3 The cellular phone is now transmitting. 836 MHz BPF selected. Eight LED segments are lit.
- Cellular Phone Detector Operation - Picture 4 Example of the great range at which a transmitting cellular phone could be detected (about 8 feet). 836 MHz BPF selected. Nine LED segments are lit.
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