[USRP-users] Questions on USRP N200 data streaming

Marcus D. Leech mleech at ripnet.com
Wed Sep 17 17:55:09 EDT 2014


On 09/17/2014 05:32 PM, Marcus Müller via USRP-users wrote:
> The cause is: there is no voltage in digital domain, it's just numbers!
> The numbers correspond to voltages on the ADC, but it's not unless you
> calibrated your complete frontend till you can map a number to a voltage.
> Now, as I said in my previous mail, the UHD USRP source maps the ADC
> range to (-1;1)+ (-1;1)j if set to float complex output, but in complex
> int16 it uses the +- 2^15 range.
>
> Greetings,
> Marcus
Put another way--the numbers you get out the back of a chain of analog 
downconversion and ADC conversion, and decimation  and filtering
   are going to be *linearly proportional* to the voltage appearing at 
the antenna terminal.   You have to calibrate against a known source at
   your desired gain settting, frequency setting, and sample-rate, in 
order to understand what the proportionality constant is.

You may be used to expensive laboratory signal analysis equipment, where 
all this calibration has been taken-care of for you, at no small
   expense.

But in the case of SDR, all the bits and pieces (including the software 
bits in your signal flow-graph) are uncalibrated until you actually 
calibrate
   them.

-- 
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org





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