More Wireless (5)

algae-interference1

Revisiting the nRF24:

The nRF24 device operates with the following power levels:

    • RF24_PA_MIN
        -18 dBm
    • RF24_PA_LOW 
        -12 dBm
    • RF24_PA_HIGH -6 dBm

There is one additional step in the power adjustment range, but it shouldn’t be used in the US without an amateur radio license (even then, it can only be used for particular things).

The dBm measurement is relative to 1 milliwatt, so this is seriously low power stuff.  The lowest setting is something like 0.0000158489 watts!  This compares to bluetooth or standard WiFi that runs about 100 milliwatts as a maximum power output from the PA.

WiFi uses digital modulation techniques, where both the bandwidth and amplitude are controlled to minimize interference. The bandwidth can vary from 5 MHz to 22 MHz in a WiFi channel.  Bluetooth just hops from channel to channel to minimize interference, but does not change the modulation itself.  Both things are known as “spread spectrum”  – where the digital modulation techniqe of WiFI is referred to as “DSS” and the channel hopping of Bluetooth is referred to as FHSS (frequency hopping spread spectrum).  The nRF24 style devices just camp out in any one of the Bluetooth channels, but do not use frequency hopping.

The rules are hard on devices that do not frequency hop.  The idea is that the hopping (or digital modulation techniqes) minimize interference enough to warrant the higher power. So, devices that do not use FHSS or DSS are subject to a -1.2 dBm maximum power output, which is about .78 milliwatts.  So, some people have been trying to make the nRF24 do frequency hopping, just like blue tooth.  Then, they say, they could purchase an add-on amplifier and run 100 milliwatts (actually, the top power allowed is variable, based on complicated sets of rules, and may be less than 100 milliwatts.  If you are of the sort, you can find this info in FCC rules PDF documents).

My thinking is, “Why?”  The whole idea of the the nRF24 is to do communication at very low power consumption levels.   My nRF24 unit draws as little as 11 mA of current, while my WiFi dongle draws 80 to 100 mA of current (constantly, since it polls all the time).  I guess they figure they could switch the higher power on when needed, but normal operation would be at the micropower levels.  I get 25 feet or so of range with these devices, with no special antennas, and a number of walls to penetrate.  That’s good enough for my stuff, but some other people are trying to use them for quad copters.  Hopefully not in my area!

RFM69W – another candidate for low power communications …

The RFM69W, by HopeRF, can run very low power (as low as the nRF24 at its lowest setting.  But, its highest setting would require an amateur radio licence (actually I have one of those 🙂 )   The highest setting on the RFM69W is 20 milliwatts, and on the RFM69HW is 100 milliwatts, without spread spectrum techniques (just single channel FSK, GFSK, or OOK).  But, people are working on this device, as well as the nRF24, to see if some spread spectrum technique can be created for it.

There is a nice project for contolling a RF69W directly from a Raspberry Pi or similar SoC/SBC.   It is at:

rdepablos.merlitec.com/mixed/rfm69-library-for-raspberry-pi

I haven’t tested it yet, but it seems like an easy way to work with the HopeRF board if you don’t want an Arduino style setup.

Note: the author does not have a recent, applicable background in circuit building, or battery related issues, so this is presented as the work of a hobbyist, and is not meant for duplication by others. Readers should look elsewhere for design advice and info.

The nrf24 is a product of http://www.nordicsemi.com, They are not affiliated with this author or website in any way. Note: The RFM69W module is a product of HopeRF, LTD.  They are at http://www.hoperf.com. This author and site is not affilicated with HopeRF, LTD in any way. Some of the modules in this series of posts are produced by LowPowerLab.com. This author and site has no affiliation with them. Bluetooth is a trademark of the Bluetooth Special Interest Group, and is not affiliated with this author or site in any way.