Re: [Discuss-gnuradio] USRP2 for my Radio Astronomy stuff

[Johnathan Corgan]

On Wed, Feb 11, 2009 at 11:25 AM, Marcus D. Leech <address@hidden> wrote:

>> The floating point source for the usrp1 has a range of [-32768.0
>> 32767.0], while for the usrp2 it is automatically scaled to [-1.0
>> 1.0].  This will affect your front end range scaling or software AGCs.
>>
> Oh.  Is that done on the host side?  And, well, why?  [no criticism
> here, just a little puzzled]

Yes, on the host, as we convert from the raw wire format into the
format of the particular source or sink block chosen.

We decided to standardize the range of the sample streams into and out
of the USRP2, such that regardless of daughterboard, decim/interp
rates, etc., the maximum and minimum values in the floating point
domain would be [-1.0 1.0].  This simplifies the rest of the flowgraph
design.

We're not there yet.  On the TX side, we're compensating for variable
interpolation gain, but on the RX side, changing decimation ratios
still affects the signal range.  Neither have we worked out the
differences among daughterboards.  Right now if you TX at 1.0 with the
BasicTX or LFTX, you get the full DAC range output.

Someday we'll have predictable RF power to flowgraph value
calculations for both TX and RX (at least for the linear portion of
their ranges), subject only to manufacturing tolerances.  Then a
calibration step by the user can store a board-specific adjustment
factor in the board's EEPROM and we can automatically take this into
account.

Someday.

Johnathan