Relation between # of RF carriers and composite power - RF Cafe
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Post subject: Relation between # of RF carriers and composite power
Posted: Tue Apr 10, 2007 11:51 am
Joined: Sat Sep 18, 2004
Is there a rule of thumb for calculating the composite
power as the number of RF carriers is doubled? I
have heard that it is approximately 2 dB increase
when the # of carriers is doubled. Say, equally
powered carriers at 4 dBm each. When the # of carriers
is 2 is the composite power = 4+2=6 dBm and when
# of carriers is 4, then is the composite power
= 6+2=8 dBm and like that
Also, is it protocol
dependent? For example, is the 2 dB rule true for
CDMA carriers only and not the GSM/EDGE based carriers?
The thing is that I can measure this in the field
but I would have to take down a live system just
for the measurements. If someone has already come
across this, that would greatly help.
Posted: Mon Apr 16, 2007 5:19 am
Joined: Fri Feb 17, 2006
Location: London UK
If the system were to be perfectly linear, then
doubling the number of carriers would add 3dB, so
2 x 4dBm carriers would produce 7dBm total power.
I guess the rule of thumb is warning of 2 mechanisms
in practice that reduce the power:
1) the combining
component will itself add some loss into the system
2)that due to intermodulation the
power at f1 and at f2 (the frequencies of the two
carriers) is reduced a tad (say another 0.5dB) because
some power from each carrier contributes to the
2*f1+f2, 2*f1-f2, 2*f2+f1 and 2*f2-f1 (among others)
unwanted products of adding the carriers in a non-linear
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