Today’s gadget is a spectrum analyzer for the 2.4 GHz Industrial, Scientific, and Medical (ISM) band.This band was originally set aside for non communications use by industrial type equipment: Microwave oven emissions, radar, etc. These bands are now allowed to be used for communications purposes by non licensed users, and today are used by cordless telephones, WiFi, Bluetooth, remote control toys, and other wireless computer peripherals.
Miguel Vallejo is a ham radio operator who was looking for a handheld spectrum analyzer for the ISM band. This type of project is made easier by components such as the Cypress Semiconductor CYWM6935, which is a complete RF transceiver with FCC certification. It is a PCB module including broadcast and receive antennas, ready to be incorporated into end user equipment. It uses an SPI interface, has an RF range of up to 50m, and a bit rate of up to 62.5k bits per second.
Miguel found many projects using this module as a spectrum analyzer, but all were tethered to a computer via a parallel or serial port. His idea for a standalone analyzer was to incorporate all of the components into a cell phone case, reusing the display and the keypad, but discarding the other internal components. Along the way he had to figure out how to interface to the display, get power from the battery, and correctly initialize the transceiver.
The brain of the analyzer is an ATMega microcontroller with software written in C. It runs fast enough to generate 6-7 display updates per second. The software has three modes essentially analogous to slow, fast, and infinite persistence on a digital storage scope. The WWW page for the project includes some nice information on the spectral signature of different devices that operate in this band.
Miguel discusses adding an on/off switch, but I think simply using the low power modes available in both the RF module and the ATMega8 would be good enough, and would interface to the existing keypad rather than requiring a switch to be added. These modes reduce consumption to a few micro amps. A good addition would be a Li-Poly charger circuit connected to the existing jack and using the existing power adapter.
Steve Ravet
Design News Gadgeteer
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