Month: March 2012

For the purposes of my project I am defining cell phone signal detection as a change in power level sensed at the frequency of the cell phone. I am not doing any decoding of signals and there is no jamming (although I fully support projects of this type), I am only detecting a very small signal.

From searching online, I notice a common frequency among US-based cell phones- the 850MHz band and the 1900MHz band. Each cell phone company has its own special frequency to broadcast on. For example, (I read this at RF Solutions) Sprint Nextel channels are scattered between 806 to 821, and then 851 to 866MHz. So Verizon, AT&T etc. won’t be on the Sprint bands. Have something to add to this? Please comment below.

As I’ve mentioned in the posts before this one, my circuit is based around the AD8363 from Analog Devices. I’ve learned in the past week or so that basing my circuit on this chip alone isn’t going to amplify the cell phone signal sufficiently (sensitivity) or do the frequency filtering for me (selectivity).

I need to filter out FM Radio (up to ~140MHz) and TV broadcasting stations (~470MHZ and up to 806MHz). Using the AD8363 requires some filtering on the input, because the chip itself is capable of sensing from 50Hz up to 2.6GHz. On the low end, I put a basic high pass filter in series between the antenna and the input. This cuts off lower frequency signals below about 700MHz.
Here’s a picture of a passive high pass filter. The R here represents the AD8363, which happens to be 50ohms. I know this because the datasheet told me so. The Vout is the filtered signal that goes into the RF detector. The RF detector has 2 RF inputs, one where the signal goes in and the other connected to ground.

The formula for the capacitor is Frequency in Hz= 1/(2 × π × 50 × C). What this capacitor will actually do is reduce the power going into the AD8363 below the calculated frequency, rendering the AD8363 useless at those frequencies.
One issue I have with this chip is that it will only detect to about -56dBm. The transmitted cell phone signal is actually about -60dBm or less. I also want my circuit to work from an 8′ distance so this requires an amplifier before the detector.

SIDE NOTE: DBm is means the power ratio in decibels (dB) of the measured power referenced to one milliwatt (mW). I keep looking at this great table on Wikipedia to get a sense of what that means in real life. My phone actually has a reading of its own received signal power and it is in the -88dBm range!

While visiting GigaHertz LLC last week, I had the opportunity to work with their spectrum analyzer. So I actually got to see what frequency my phone was working (about 840MHz) on as well as the transmitted power (-65dBm). Verizon phones apparently hop frequencies depending on location so it was fun to see what my phone was transmitting on in Pittsburgh.
I neglected to take any photos of the spectrum analyzer’s screen, but I found something at 911 Dispatch that illustrates the idea.


What you see with the spectrum analyzer is the frequency (x-axis) vs. amplitude (y-axis). This is exactly the tool needed to characterize the cell phone signal to be detected.

This has been a really long post, so if you got this far, thanks for reading. At some point, I’ll talk about the amplifier that was selected (recommended by GigaHertz) and some of the different antennas I’ve been testing with. Oh, and how to cut the upper frequency off before it reaches Wifi (2.4GHz).

Link to part 5 in this series
Link to part 4 in this series
Link to Diode Detector post
Link to part 3 in this series
Link to part 2 in this series
Link to part 1 in this series

This is a block diagram that shows the basics of the circuit:

This is a placeholder post, I am going to add to this brief post (graphs! math! antennas!) in the next day or so. So check back!

I did want to report that I got a small mention in MAKE blog which was SUPER COOL.

Last week, I visited GigaHertz LLC in Pittsburgh. They have been schooling me in the world of RF, and in many ways, encouraging me to keep going in a field that is occasionally kicking my ass. Luckily, I have no ego 😉

I’ve also been learning a ton from Ed Nisley Tuesday nights at an electronics group meeting. He knows a lot about RF and is one of the quoted “naysayers” in one of my previous posts. Turns out, he was right. My first PCB spin was embarrassingly noisy. But…moving forward, I’m waiting now for the next 2 spins to return.

I also have emailing a little with Noah Shibley, who did a similar project a few years ago. I really love how the Engineering-maker community is so willing to share information.

OK- more later…

20 days until Spring! This has been the suckiest winter ever for snowboarding, I haven’t gone even once. It snowed today and I just didn’t feel like it.
Instead I’ve been procrastinating with a Mindbands brainwave detector.

Last week I re-spun my cell phone signal detector board in two different versions, both to include smaller traces on the QFN (easier to reflow without shorts!). One of the versions has a power supply right on the board, the other uses an Arduino. The new spin moves the LEDs out of the way so that they aren’t switching on and off right next to the detector part of the circuit.

RF engineering is its own special brand of difficulty. At times I’m really discouraged, nothing seems to work and I don’t understand why.

So while I’m waiting for my boards to arrive in mid-March, I’m re-reading the RF detector chip documentation from Analog Devices. My circuit is based on the AD8363 and that will certainly change. The chip is fairly expensive and the range is too wide. I’ve been looking at some Mini Circuits chips because they’re cheaper and have a smaller range. I’d really like to detect up to 2GHz, and not have my circuit blinking about and confused because of random Wifi in the neighborhood.

You can think of an RF detector chip like a little power meter. It takes in RF signal on one end from the antenna’s output and gives out a DC voltage. The voltage is proportional to the received signal (Watts). Pretty cool. You can then take that DC voltage and “do stuff” with it.
I’m running a bar graph with it, so the higher the voltage received, the higher the LEDs on the bar graph light up.