Hi Sergio, > So an important question is, how are you getting these 56 and 57 dB? Could you explain your setup? I can find difference up to 2 dB. A receiver is placed about 4 feet away from a sender. When the sender transmits at power level 3 (-25 dBm), the measured RSS at the receiver is -82 dBm on average (excluding noise), leading to an average attenuation of 57. While the average attenuation is 55 dB at power level 0 (0 dBm).
>In my experience, the levels specified by the CC2420 brochure were not very accurate. I'm just wondering if the signal attenuation difference of 1~2 dB using different tx power can be explained by measurement error (e.g., RSSI sampling precision of 1 dB and hardware variation) alone, so the theory that tx power does not affect attenuation actually holds. Do you know how many dBs can the actual tx power deviate from what the datasheet says, typically? Thank you very much and anyone else is welcome to share their perspective. -Xiaohui On Sat, Mar 23, 2013 at 1:45 AM, Sergio Donal <[email protected]> wrote: > Hi Xiaohui, > > Signal attenuation depends on the path of transmission from the > transmitter to the receiver. > > For instance, lets say that the power amplifier is able to transmit at 10 > dBm. > This 10 dBm get into the antenna, which may add some gain, lets say the > gain is 3dB, in some specific directions (but note that it may also loose > power in a different direction). > > At this point is important to note the difference between "absolute" units > (i.e., dBm which is like speaking of milli-watts, watts or horse-power) as > opposed to "relative" units (like dB, which says how many times your signal > is increased or reduced (i.e., 3 dB means that the power of the signal is > multiplied by 2, while -3dB means that the signal is attenuated by 2, or > multiplied by 0.5). > > So coming back to the scheme, in some specific direction the transmitter > radiates electromagnetic waves at 13dBm (10dBm +3dB you can add them up for > some reasons). > > The propagation channel typically attenuates the signal. And the amount of > attenuation depends on some factors like distance, frequency, weather > conditions, reflections in people moving around in the environment, etc. > But, since attenuation is given as a relative quantity, it should not be > influenced by the output power (at least in theory). > > Nevertheless, in practice, there may be many factors that could make > variations in your measurement of the attenuation. > > So an important question is, how are you getting these 56 and 57 dB? > Could you explain your setup? > > > Two years ago, I used a very precise setup (plugging the RF connector of > the mote into a vector-signal-analyzer in which I even was able to > demodulate the signal) and tested a bunch of radios... > In my experience, the levels specified by the CC2420 brochure were not > very accurate. > I don't find the table with the results, but I remember that the maximum > output power was never achieved and that the levels were not consistent > across different radios, especially the lower levels, which they were > poorly mapped. > > > Best!! > Sergio > > > > > > On Fri, Mar 22, 2013 at 9:33 PM, Xiaohui Liu <[email protected]> wrote: > >> Hi everyone, >> >> Does signal attenuation depend on transmission power? Based on my limited >> knowledge of physics, it does not. However, in measurement, noticeable >> difference is observed. >> >> For a sender and receiver, the measured signal attenuation of the link is >> *56* dB when the sender transmits at power level *3* while the signal >> attenuation for the same link is *57* dB at power level *31*. >> >> Another related question, according to CC2420 datasheet, the tx power is >> -25 dBm when power level is 3. Is the actual tx power equal to -25 dBm or >> there is some variation depending on the specific hardware? How large is >> the typical variation if any? >> >> Look forward to your explanation. Thanks in advance. >> >> -- >> -Xiaohui Liu >> TelosB >> TinyOS 2.1.2 >> www.cs.wayne.edu/xliu/ >> >> _______________________________________________ >> Tinyos-help mailing list >> [email protected] >> https://www.millennium.berkeley.edu/cgi-bin/mailman/listinfo/tinyos-help >> > > -- -Xiaohui Liu TelosB TinyOS 2.1.2 www.cs.wayne.edu/xliu/
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