Recording Raw IQ Samples – Suspiciously low values?

I’m using the API to pull raw IQ samples and I couldn’t find the A/D output range in the manual.

I’m trying to troubleshoot some things and one thing I notice is that my IQ samples have suspiciously low values.

I’m putting in a ~2MHz wide QPSK signal at ~-30dBm, Spike gives me a nice beautiful constellation so I know my signal level must be good. My IQ samples range from -0.00662 to +0.006358. Is this correct? I was expecting something more like -1 to 1.

[stevecrm]

For reference these are my capture settings

`bb_api.bbConfigureCenterSpan(id, 70000000, 4.0e6); //center hz, span hz
bb_api.bbConfigureLevel(id, -20.0, bb_api.BB_AUTO_ATTEN);
bb_api.bbConfigureGain(id, bb_api.BB_AUTO_GAIN);
bb_api.bbConfigureIQ(id, bb_api.BB_MIN_DECIMATION, 4.0e6); //bandwidth hz

• This reply was modified 7 years, 1 month ago by stevecrm.

Steve,

The I/Q data is scaled to mW. To calculate the power of any given I/Q value in dBm, use the formula
dBm = 10*log10(I*I + Q*Q)

Let me know if you have follow up questions.
Regards,
Andrew

Probably it formula has error.
Correct:
dBm = 10*log10(mW)
Current (incorrect):
dBm = 10*log10(mW^2)
May by I/Q data is scaled to mW^0.5 ?

Maksym,
I and Q values are voltage rather than power, so you have to square them to get a power unit.

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