On 11/3/06, Eric Blossom <address@hidden> wrote:
On Fri, Nov 03, 2006 at 06:58:41PM -0500, Dawei Shen wrote:
> Hi, Guys
>
> I am suddenly a little bit confused about the difference between
> usrp.source_s and usrp.source_c. Basically I have two questions:
>
> 1. Since the ADC only has 12 bits, does it mean the short integers entering
> the computer have their last four bits as zeros? Or are their first four
> digits are zero (the first bit should be the sign)?
The short integers entering the computer have been through the DDC and
have been processed as 16-bit ints. Without looking at the verilog
code I don't recall how they're adjusted before being clocked into the
DDC.
The 12 bit output form the ADC goes first through the adc_interface.v module (check the verilog code if you'd like). Inside that module, the most significant bit is propagated once to the left and 3 zero bits are shifted in from the right. The result is a 16 bit value. That value then goes through the rx_dcoffset.v module (I don't know what that does) before going to the DDCs in the rx_chain.v module.
I hope that helps.
Oussama.
> 2. Is anything interesting done when the short integers converted to float
> values? Or are they simply converted to a different type without changing
> the values?
At this point nothing besides the format conversion is done.
There have been some conversations about changing the _c format to be
normalized to +/- 1.0 but the details aren't worked out yet. We'll
probably define a new interface so that we don't break a bunch of
code. The normalization would also be a function of the kind of
daughterboard that's connected, and will probably factor in some kind
of two-tone IMD performance result.
Eric
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