[time-nuts] Soundcard sampling Re: Picking a good HP 10811

[Bruce Griffiths]

Lux, James P wrote:
>> You will need to use high end sound cards like the M-Audio
>> AP192 for good performance.
>>     
>
> If you're using that particular card , I'd definitely build a new cable for it.  You can probably do better than 1/4" TRS phone plugs.
>
>   
I use XLR to TRS cables with it.
>
>   
>>>       
>> Dont forget to have an individual isolation amplifiers
>> between each LO port and the LO itself.
>> The easiest method is to use a DDS system to generate the
>> required offset LO frequency.
>> However bandpass filtering of the output is necessary and
>> care must be taken to select the DDS clock frequency to
>> ensure that the principal phase truncation spur as well as
>> other spurs are sufficiently far from the desired output that
>> they can easily be filtered out. Use the DDS tools on the
>> Analog Devices site to help with this.
>> The DDS clock itself needs to have relatively low noise
>> either divide 10MHz by 2 and then multiply by 9 to get 45MHz
>> which appears appropriate for the AD9850 series chips, or
>> build a low noise discrete 45MHz LO.
>>     
>
>
> You might want to look at the 99xx series DDSes. If nothing else, they run cooler than the 985x  series(AD9854, used in a variety of quadrature schemes, draws 1.2A @3.3V.. Yep, it gets mighty toasty warm, and needs serious heatsinking and a fan or it gets intermittent)
>
> The newer DDSes have lower power, faster clocks, lower spurs, more bits in the DAC, etc.  Although, I'm not sure how important all these things would be in this application.
>
> There are also some very low phase noise, low jitter XO available that can drive these things (not temperature compensated, though, so as the XO temp changes, so does the frequency)
>
>   

Using a low phase noise XO with a suitable frequency would probably be
preferable as there would be fewer potential problems with spurs.
With appropriate algorithms the drift in beat frequency with temperature
shouldn't be too much of a problem.

Bruce