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02-22-2010

Handheld 2.4 GHz Spectrum Analyzer. Part 2

Miguel A. Vallejo

The software

The software is written in C using avÍ-gcc under Windows (WinAVR). LCD routines are based on Fandi Gunawan's Nokia 3310 routines and CYWM6935 routines are based on the code of Jason Hecker (see at the bottom for details). The program runs continuously and make around six or seven scans (from 2400 up to 2495 MHz) per second, displaying data in almost real time.

Handheld 2.4 GHz Spectrum Analyzer

There are three display modes implemented:

Fast: Display amplitude data directly from the module with no processing. This mode is suitable for searching analog signals (with carrier).

Slow: Update amplitude data on screen only if current signal is greater than the previous one, if not, displayed signal is decreased by one until it reach zero. This mode is useful to see digital signals, that usually comes in bursts. The fast and slow mode work much like the AGC setting in a communication receiver.

Exposure: In this mode, the signal is updated in the screen only if current value is grater than the previous one. In this way, running this mode for a few seconds or minutes, you can get a nice photo of the spectrum usage around you. The reset button just clears the display to start a new exposure.

In practice

The analyzer has become my best travel companion. It's very discrete: everyone think you are playing with a mobile phone!. You can see in a moment what are the used frecuencies / channels at your location. One of the most interesting things if to carry it in the pocket in exposure mode and walk sometime in your neighborhood. In this way you can find easily what are the free frecuencies or channels. With the time, I learned how to distinguish between different devices:

WI-FI signals: In exposure mode they have a nice rounded shape about 20-22 MHz wide at most.

Wireless FM cameras (and ATV signals): You can see clearly the carrier and the modulation. If the video signal have sound, you can see a pair of sidebands about 6 or 7 MHz from the main carrier.

Bluetooth devices: This devices are difficult to detect, except in exposure mode. They appear as random peaks all over the spectrum up to 2483 MHz.

Microwave owens: Just full scale noise. I noticed the center frecuency drifts with the owen load: If nothing is inside the owen, the center drift up several MHz. Usually they are centered around 2450 having a noise bandwidth of about 60 or more MHz. Of course, they are the main signals at breakfast, lunch and dinner time.

Other devices: There are many other signals in the band unidentified. Some of them are analogic, other one are digital, but usually a few MHz width. I suspect they are low rate devices, like wireless keyboards or such devices. I have also detect some places with strong carriers, most of then around 2440 MHz. I don't know what they are yet.

Out of band signals: The ISM band ends at 2483.5 MHz, but the analyzer scans up to 2495 MHz. I usually use this segment as a noise reference, but sometimes I had found signals in this area. Some of them seems to be spurs from ICM devices, but others are not...

The most used frecuency is always the segment 2450 - 2470 MHz, wifi channel 11. I suppose this is the common default channel in many many wireless routers and access points.

Improvements

If you have reached this, you would notice there are two missing things in the analyzer. The first one is an on / off switch. Where can I install a switch without damage the appearance? Originally it was placed at the top of the phone, but now there is located the module, and it is no good idea to place wires near its antenna. But I must admit I didn't miss an on/off switch. The analyzer consumes less than 15 mA, so it can work some days continuously with the typical 900 mAh battery from this phones.

The second missing thing is a build-in battery charger. It would be nice to implement a little Li-ion battery charger, but these kind of batteries must be charged very carefully. If I found an small Li-ion integrated circuit charger, I would try to install it inside, using the original charger jack, and of course, the original phone charger. Meanwhile I charge the batteries outside, in a spare Nokia 3330 I have lying around.

Documentation

Source files and makefile for this project can be found here. These datasheets will be also useful to you:

  • CYWM6935 datasheet can be found here.
  • CYWUSB6935 datasheet can be found here.
  • PCD8544 datasheet can be found here.

Also, I have taken ideas and some portions of code from these places:

I wish to thank you all for share your experiences with the rest of us.

ea4eoz.ure.es

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