SDR -- Software defined radio for those that don't know the acronym.
Some of you know I fly model aircraft. For the longest time, 72MHz FM radios were the standard and if someone turned on a radio on the same channel as you, goodby airplane. Then came 2.4GHz and spread spectrum radios that (theoretically) put and end to that. We have had some incidents at the field where I fly that have caused the lost of aircraft, even on 2.4GHz. Now, there are all sorts of issues with 2.4GHz that don't arise on 72Mhz -- see antenna fade for an example. Anyway, we started talking about doing a site survey to see just what really was whizzing through the ether where we are trying to fly.
Enter software defined radios. Essentially they do a direct analog to digital conversion of the IF signal or even of the RF signal for lower frequencies. High bandwidth, high performance radios are still pretty expensive. However, for $20 US or so, you can get a USB tuner that does a bang up job, within its limitations. Many DVB-T dongles for TV reception have the ability to do raw captures as well. I'm using on based on the Rafael Micro R820T chip. I get about 2.4Mhz of bandwidth in a range from 24-1776MHz. I have a 1998MHz downconverter in front of it, putting the 2.4GHz band right in the 400-500MHz range for the receiver. All for under $100US.
Below is a sprectrogram/waterfall plot of 2.4-2.5Ghz at my house.
In the upper left, you can see my wireless access point, on channel 1. I turn it off about 13:40. All the little blips you see from 13:40 to 13:55 are the various WiFi adapters throughout the house looking for access points, as well as bluetooth adapters. By 13:55 I have almost everything turned off. It is amazing how many little radios I have floating around. Just before 14:24, I took a couple of laptops out of sleep to power them down. The splash of energy at 14:28 is my wife warming a pita bread in the microwave. The blob that starts at 14:52 is my putting a 24oz cup of water in the microwave on high for four minutes. By about 14:40, I had turned off everything I could in the house and yet you will notice there were still blips into the -10db range now and again. I believe those are from the smart meter outside the house. The lower power blips here and there are likely from the neighbors houses.
This is the same data plotted as power vs frequency.
There are 100 scans in the above plot. It gives you a good idea just how noisy 2.4GHz can be.
Pretty cool stuff really. For the radio sauvy among you, you will notice the power scale goes from about -50db to around -2db. The ADC is only 8 bits so I get about 55db of dynamic range. The sensitivity is quite good. Something in -138dBm area, but only hanving 55dB of dynamic range is somewhat limiting when dealing with nearby wifi access points. Add to that the fact that my downconverter has a 37dB preamp in it and low power signals get lost in the noise. That is one of the reasons I was trying to get everything turned off -- to see what was down in the noise.