Questions on output power
Guofeng
I have a couple of questions regarding output power of a transceiver
or power in general.
I know that 0 dBm is 1mW. But in "Telos: Enabling Ultra-Low Power
Wireless Research" by Polastre et al, it said, for example, the
transmission power of TR1000 radio at 0dBm is 36mW. I have seen
similar things elsewhere also. Why it is not 1mW?
>From the datasheet of cc1000, I noticed that its max output power is
10 and 5 dBm at 433MHz and 868MHz respectively. The max output power
of cc2420 is 0dBm. The aforementioned paper listed the transmission
power of cc1000 and cc2420 at 0dBm, which is 42mW and 35mW. So,
doesn't mean cc2420 is more power efficient but cc1000 can transmit
further?
Is it in general watt is a better unit to specify power consumption
compared to amper, because you don't need to consider voltage?
Does output power mean the output of transceiver circuit instead of
antenna? Powerless antennas are able to enhance the outgoing signal
strength in certain directions but reduce it in others because they do
not consume energy. Am I right on this?
Thanks.
Regards,
Guofeng
Ken
> Hi all,
>
> I have a couple of questions regarding output power of a transceiver
> or power in general.
I'm certainly not an expert, but I'll take a stab at this based on
what I've learned. -- Ken
>
> I know that 0 dBm is 1mW. But in "Telos: Enabling Ultra-Low Power
> Wireless Research" by Polastre et al, it said, for example, the
> transmission power of TR1000 radio at 0dBm is 36mW. I have seen
> similar things elsewhere also. Why it is not 1mW?
1 mW is the power output from the transceiver. 36 mW is the power
consumed by the board to generate the 1 mW of output power. For
example the Crossbow MPR/MIB User's Manual on page 22 shows that to
generate 0 dBm at 915 MHz requires 17 mA. Multiply by 3 V to get 51
mW, which is in the range quoted by the Polastre paper.
>
> >From the datasheet of cc1000, I noticed that its max output power is
>
> 10 and 5 dBm at 433MHz and 868MHz respectively. The max output power
> of cc2420 is 0dBm. The aforementioned paper listed the transmission
> power of cc1000 and cc2420 at 0dBm, which is 42mW and 35mW. So,
> doesn't mean cc2420 is more power efficient but cc1000 can transmit
> further?
Yes, in terms of output power, that's reasonable. However, be sure the
comparison is even-handed. For example the CC1000 typically uses 10 mA
to generate 0 dBm at 433 MHz, but uses 17 mA at 915 MHz.
I have uploaded a Crossbow presentation to this group's files:
Part_3_Power_Issues.pdf. Page 3 shows the current required to perform
various operations, like transmit, sleep, write to log.
The ultimate benefit to the user depends on their priorities. So
although generally a lower frequency means greater transmission range,
higher frequency generally means a greater data rate.
>
> Is it in general watt is a better unit to specify power consumption
> compared to amper, because you don't need to consider voltage?
I don't know, but this makes sense to me. The use of amps probably
also makes it easier to determine the input power required. For
example batteries are rated in amp-hrs. Of course battery power
available depends on how it's used as well.
>
> Does output power mean the output of transceiver circuit instead of
> antenna? Powerless antennas are able to enhance the outgoing signal
> strength in certain directions but reduce it in others because they do
> not consume energy. Am I right on this?
Yes, output power means what the transciever outputs, not what the
antenna outputs. For example a transceiver may output 0 dBm, and it
may use an antenna rated at 5 dBi. So the *effective* power is 5 dBm.
Antennas are measured relative to an isotropic antenna (the "i" in
"dBi"). For more explanation [1] makes sense to me.
[1] http://www.moonblinkwifi.com/dmystifying_the_db.cfm
>
> Thanks.
>
> Regards,
> Guofeng