If you're new to RF and things like signal strength measurements (dB, dBm, RSSI, etc.), the following whitepaper may be helpful: Converting Signal Strength Percentage to dBm Values

Decibals (dB) are used to indicate the ratio of one power level to another. An increase of 3dB doubles the power, so 6dB is equal to 4x the power, 9dB is equal to 8x, etc.

The specific formula is: dB = 10 * log(P1/P2), meaning that for a 4 times difference in power we get:

dB = 10 * log(4/1)
dB = 10 * 0.602
dB = 6.02

The following table shows some key values comparing decibels (dB) and their equivalent increase/decrease factor:

dBm is used to indicate transmit and receive sensitivity in milliwatts, and is the power ratio in decibels (dB) of the measured power referenced to one milliwatt (the m in dBm). 0dBm is equal to 1 milliwatt. The following two formulas govern the relationship between dBm and mW:

To convert dBm to mW: P(mW) = 10^[P(dBm)/10]

To convert mW to dBm: P(dBm) = log[P(mW)] * 10

For example, if we start with 13dBm, we get the following equivalent value in milliwatts:

mw = 10^[13/10]
mw = 10^1.3
mw = 19.953 (or roughly 20mW)

We can also convert 20 mW back to dBm as follows:

dBm = log[20] * 10
dBm = 1.301 * 10
dBm = 13.01

Some common value can be seen in the table below:

For more information, feel free to look at the Wikipedia pages for decibels and dBm.

While it's been rather quiet on the website lately, we've been busy working on a number of projects behind the scenes.  One of the key projects we've been working on for a while now (much longer than anyone expected!) is a set of wireless transceivers and devices in the 868/915MHz ISM band.  868 MHz (Europe) and 915 MHz (North America) offers an excellent compromise between transfer speed, signal penetration in an urban environment, and power consumption.  Unfortunately, most commercially available devices tend to use 2.4GHz, so there seemed to be an obvious value in developping a few ARM-based boards in the 800/900MHz band.

The first of these wireless boards was published this week, the LPC1114 802.15.4 Wireless Transceiver. This small board is based on the ARM Cortex-M0 LPC1114, and uses the AT86RF212 802.15.4 transceiver.  The board is intended to be run from batteries (specifically rechargeable Lithium-Polymer cells, though you could also use 3xAA NiMH or Alkaline cells or any 3.6-5.5V supply), and uses Chibi ... a lightweight 802.15.4 stack from Freaklabs specifically designed for the AT86RF212 and AT86RF230 (2.4GHz).

The boards include a microSD connector to read and persist files using a FAT32 file system, a basic voltage divider to check the battery voltage, a temperature sensor to calibrate temperature-sensitive sensor readings, and a handful of useful pins broken out (3xADC, UART, and I2C).  For more information, feel free to look at the project page though.

One feature worth mentionning on these boards, though, is that thanks to the latest update to the Chibi wireless stack -- and if you're interested in wireless at all you'll definately want to spend some time looking at Freaklabs website -- these boards can be used as an 868 or 915MHz sniffer for 802.15.4 traffic (6LowPAN, Zigbee/XBEE, etc.).  The latest version of Chibi (v0.91) includes support for something called PROMISCUOUS mode than make the transceiver transparently read any packet it sees over the air.  By using wsbridge, you can feed the 802.15.4 frames directly into wireshark (using any UART to USB adapter to connect the LPC1114 to the PC).  The SW is available directly from Freaklabs, but is also included in the '/tools' folder of the LPC1114 Code Base for convenience sake.  For more information see: Feeding the Shark - Turning the Freakduino into a Realtime Wireless Protocol Analyzer with Wireshark.

In the coming months, you can expect to see more wireless boards and projects combining different MCUs and transceivers, but given the number of revisions the LPC1114 board took (more than 10!), we'd rather keep quiet about any future plans and just publish the details when things are a bit closer to being ready.  In the mean time, take a look at some of the great stuff Akiba over at Freaklabs is putting out.  He's the real enabled behind all the wireless fun we're having lately!