Re: [Discuss-gnuradio] Decimate by 2 possible?

[Brian Padalino]

On 10/22/07, Nirali Patel <address@hidden> wrote:
> Hi,
> I am using the TV_RX and USRP to capture the 64 Msps data from the A/D
> converter and send that to another downstream board for processing. I am
> connecting the signal from the ADC to the debug IO pins of the FPGA to get
> to the 16 io_rx[] pins of Basic RX. However, my board-to-board
> interconnection turned out to have a bandwidth limitation of 50 MHz so I can
> no longer hope to send 64 Msps raw data from the ADC over the interconnect
> bus. The next best thing would be to decimate by a factor of 2 and then
> route the 32 Msps data to the debug pins. I understand the the CIC has a
> decimation range of [4,5,6,…128] and the HBF decimates by a factor of 2. Is
> there a way to get an overall decimation of just 2 by possibly bypassing the
> CIC and using only the HBF? If this sounds like a dumb question then, what
> would be the best way to get 32 Msps over to the Basic RX card using what's
> already in the FPGA?
>
> Thanks in advance for any suggestions.

Is there a reason you want to play directly with 32Msps?  What is your
target symbol rate?  Do you need all that bandwidth, or do you just
need a portion of it?  Usually when there is a bandwidth limitation of
50MHz, and since a square wave is composed of all odd harmonics of the
fundamental, then 32MHz would really need a couple odd harmonics to
get through pretty reliably, shouldn't it?

Maybe it would be better if you decimated to 1x or 2x the symbol rate
and then processed that data on your other board?

You can change the halfband coefficients to make it a symbol matched
filter instead of a generic halfband filter if you'd like.

What do you think?  Is this more plausible of a scenario for you?

Brian