[time-nuts] Frequency multiplication

[Magnus Danielson]

On 05/02/11 04:33, jimlux wrote:
> On 2/4/11 1:18 PM, Magnus Danielson wrote:
>> On 02/02/11 19:47, Hal Murray wrote:
>>>
>>>> Bottom line - there's a lot to look into, and they are unlikely to
>>>> help you
>>>> out.
>>>
>>> There are a lot of FPGAs used in DSP applications where the clock to the
>>> front end ADC is critical. So I'd expect there would be some in-house
>>> knowledge about this area. It may be that all the help you will get is
>>> "Don't do that."
>>
>> You don't feed the ADC from the FPGA if you can avoid it.
>>
>
>
> especially if your ADC clock is a different frequency from the processor
> clock that's being used for most of the other logic on the FPGA. I'd
> give a ballpark estimate of 20-30 dB isolation between the two on a
> Virtex 2.

If you do it naively. You can do better, but it is not worth the time in 
most cases.

FPGAs is a lovely sea of logic. The 
MGT/GTP/whatever-high-speed-I/O-is-called-this-week has much better 
timing. FPGAs are perfect for taking care of the data and processing. 
Just do the timing sufficiently isolated from the FPGA. Once size 
doesn't fit all. FPGAs is there to fit the need of relative high speed 
and relative high integration of logic. It has taken more and more of 
the full custom market, but will not take the full ASIC or full-custom 
market.

Cheers,
Magnus