[Discuss-gnuradio] calibrating USRP N210 power (was: Re: GR, USRP, and GPIB measurements)

[Marcus Müller]

Hello Gayathri,
On 25.08.2014 10:17, Gayathri Ramasubramanian wrote:
> Hi
> Thank you for your note.
> My questions here are just based on my previous measurements and your last
> mail. kindly clarify the same.
>
> 1) Is it possible that different USRPN210 devices with WBX boards have the
> different calibration factors. I set the channel gain to 0 and still get :
> [...]
> Your values seem to be different ,
> Hence the question.
Yes, that will be the case, as with any analog circuitry; amplifiers,
filters and such are produced with tolerances, and thus these values are
expected to change from device to device.
> Is this correct or am I doing something wrong. As my values seem to be
> almost 30 ~ 33 higher than ones you are getting from your tests. what could
> be causing this error/ discrepancy.
30dB *higher*? Are you sure you are inserting the same power (by the
way, -40dBm is quite some power and I would generally recommend using an
attenuator to avoid damage if the input power rises, as Lou already
warned about)? Are integrating over the same number of samples?
As a special note to your measurements: You'll sometimes see LO leakage,
which can contribute significantly to total power. To avoid that, you
could specify a uhd.tune_request(a,b) instead of a simple target
frequency. That would allow you to specify a digital tuning offset,
which would move the LO out of your measured bandwidth.

Generally, I wonder what you use these values for, afterwards. Because
they are only valid for the 250kHz bandwidth as limited by the
antialiasing filter in your USRP's FPGA; if you wanted to use this value
for higher sample rates, I'd expect there to be a high level of
proportionality, but I guess since your doing calibration now, it would
make sense to use a filter on the PC where you can control the noise
equivalent bandwidth yourself -- especially since I assume you're
measuring a single tone, and that would fit in the narrowest of bandpass
filters, which would avoid measuring the power of the input tone
including your noise floor over your complete sampling rate bandwidth.

If you were using a spectrum analyzer, that would actually shift a
analog filter through the spectrum and display the energy passing that
filter at every frequency, which would give you a display quite
different from mag squared for the full 250kHz bandwidth of 1s of input
signal samples.

Greetings,
Marcus