On Wed, Jul 14, 2010 at 9:30 AM, <address@hidden> wrote:
Hi,
I'm currently using USRP, GNU Radio and the file usrp_fft.py to find the
Signal-to-noise ratio, Noise- and Signal-power.
I use the ZigBee PHY from Thomas Schmid to generate the signals at 2.412GHz.
My questions are the following:
1. In usrp_fft.py are the axis denoted as "amplitude [db]" and of course
"frequency". Now in order to deduce the signal strength or the noise
strength respectively, I need the reference amplitude used to compute the db
value. In the code there is the setting "--ref-scale" and it's default value
is 13490.0. Now is this reference Amplitude 13490.0 mV? (look at units ->
milli-volts)
It sounds like you're trying to figure out SNR based on an FFT. If
that's the case, I don't recommend that. There are plenty of in-band
noise sources that would be counted as signal when, in fact, they are
noise. There are other methods for estimating SNR if you indeed want
to get that calculation.
I don't think the amplitude maps directly to milli-volts. The units
are literally the integer values after filtering. You need to take
into account any gains in the system before and after the digital
converter.
As a rudimentary example, if you had a converter which gave signed
14-bit numbers (+/- 8191), you may want to set the reference to be
84.28dB (6.02dB/bit * 14bits) and figure out how far down from full
scale input you are (this will be a negative number as 84.28dB should
be your top end). You can then add and subtract the overall system
gain from that point to figure out what power levels are at different
locations in the system. NOTE: This is something that requires you to
understand all the gains in the entire system.
Does that make sense to you?
2. How can I get from here to the signal-power and noise-power? Or maybe the
signal-strength isn't bad either... (I need those values to find the
transmitting power of the USRP, I guess it could be 1mW, but don't know how
to check that)
If you just want to find the transmitting power of the USRP, just
connect the antenna spigot to a 50Ohm terminated load, set your USRP
to output a vector of (1+0i) and measure the voltage. You now know V
and R as well as the fact it's a single carrier tone - calculating
power should be easy. You can then compare that in ratio to 1mW to
calculate dBm.
Calculating the exact noise power, on the other hand, ends up being a
much more difficult problem.
Thanks a lot for your help!
best,
Björn
Good luck.
Brian