[time-nuts] DDS module

Didier Juges shalimr9 at gmail.com
Mon Jul 22 00:15:44 EDT 2013


There is no "cleanup" PLL in the SG8002, the output spectrum looks like that of a cheap PLL-on-a-chip. I did not see any evidence of wide band spurs but the phase noise is terrible, as you would get when the VCO and it's tank components are on-chip, which is the only way they could achieve the price point.

My experience also shows that high level of radiated noise around the device (as when used inside a high power switching supply) causes it to malfunction, as if it was losing lock. In our applications, they have to be shielded.

Didier

Tim Shoppa <tshoppa at gmail.com> wrote:
>SG8002 is definitely a distributor programmable unit.
>
>It is a digital synthesizer in a can with no effort towards clean
>output.
>
>PLL? They would not bother to put a cleanup PLL in the can for the
>intended microprocessor application.
>
>Tim N3QE
>
>On 7/21/13, Didier Juges <shalimr9 at gmail.com> wrote:
>> I am familiar with the Epson Seiko SG8002 series and nowhere does it
>say
>> they are programmable. They are not user programmable for sure, only
>the
>> distributor has the tools to do it. The data sheet does not have any
>> information that would be a hint that they are PLL's.
>>
>> Http://WWW.KO4BB.com/Timing/MicroprocessorCrystals.php
>>
>> Didier
>>
>> Hal Murray <hmurray at megapathdsl.net> wrote:
>>>
>>>lists at rtty.us said:
>>>> A number of them are sigma delta PLL's and have some really nasty
>>>jitter and
>>>> spur issues. Anything that shows up listed as "user programmable" -
>>>watch
>>>> out.
>>>
>>>But "user programmable" won't find all the nasty ones.
>>>
>>>I haven't been looking recently.  There used to be some that were
>field
>>>
>>>programable.  The idea was that the distributor would do it so you
>>>didn't
>>>have to wait for the factory to grind the crystal to order if you
>>>wanted an
>>>oddball frequency.  They didn't mention the PLL in the data sheet. 
>You
>>>had
>>>to read between the lines.
>>>
>>>I assume the distributor stocked a handful of basic frequencies and
>>>there was
>>>a PC that told them which one to use and did the programming if you
>>>told it
>>>what frequency you wanted.
>>>
>>>It looks like a fun math problem.  How many frequencies do you need
>to
>>>stock
>>>in order to cover a given frequency range with a given PLL chip?  The
>>>result
>>>doesn't have to be a direct hit.  You can include some fudge as part
>of
>>>the
>>>overall accuracy spec.  (If you claim 100 PPM, but cut the crystal
>>>within 50
>>>PPM, then you have 50 PPM left for the PLL.)
>>>
>>>
>>>
>>>--
>>>These are my opinions.  I hate spam.
>>>
>>>
>>>
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>>
>> --
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>other
>> things.
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