Re: [Discuss-GnuRadio]: tx_chain.v module in usrp_std.v

[Matt Ettus]

Oussama Sekkat wrote:

> Hi,
> Thanks for your reply.
> So from what I understand, in the transmit side the digital up 
> converters are implemented in the AD9862 chip but the interpolation is 
> done in the FPGA.

On the transmit side we use our own CIC to interpolate from whatever 
ratio comes over the USB to 32 MS/s.  This is sent to the 9862 which 
then interpolates again by a factor of four to bring us to 128 MS/s.  
The upconverter is in the 9862 as you say.

> I believe in the receive side the down conversion is done in the FPGA. 

Yes, because the 9862 does not have a downconverter in the RX path.

> So  why can't we do the same for the transmit side and implement the 
> Digital Up Converters in the FPGA?

Not sure what you would gain from this -- You would use a lot more area 
in the FPGA.
Anyway, the bus going to the TX path on the 9862 is multiplexed (I then 
Q samples), and we run it at our normal clock rate of 64 MHz.  This 
means we can only get 32 MS/s complex across it.  If we did the 
upconversion in the FPGA, we would thus be limited to about a 12 MHz 
carrier.

> Also, the AD9862 chip contains interpolation filters as well. Could 
> those be used instead of the CIC interpolators on the FPGA? that way 
> both the up conversion and the interpolation would be done in the 
> AD9862 chip and more space would be available on the FPGA.

We use those interpolation filters already, but they only do a factor of 
4, so we still need a CIC in the FPGA.

> Oussama.

Matt


> On 7/11/06, *Eric Blossom* <address@hidden <mailto:address@hidden>> wrote:
>
>     On Mon, Jul 10, 2006 at 08:22:29PM -0700, Oussama Sekkat wrote:
>     > Hello everybody,
>     >
>     > I am still a beginner in the project so bare with me if my
>     questions have
>     > obvious answers.
>     > I was looking at the verilog code for usrp_std.v module. that
>     module
>     > containes the tx_chain.v module which uses a module called
>     phase_acc.
>
>     Actually, if you look at that line, you'll see that it's within an
>     `ifdef that's not enabled.  Take a look again ;)
>
>     > I am not sure what this later module does? It takes as inputs
>     (among others) a
>     > 7bit serial address and 32 bit serial data and outputs the 32bit
>     phase.
>     > Does anyone have any idea what that module does and what its
>     purpose is in
>     > the tx_chain?
>
>     If enabled, this code would implement digital upconversion in the
>     FPGA, instead of using the DUC in the AD9862.  N.B., this code hasn't
>     been tested in something like a couple of years and may have suffered
>     bit rot.  [Definitely.  The FPGA register definitions that would have
>     been required to use it have been removed...]
>
>     Eric
>
>
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