[time-nuts] FEI5680A Rubidiums On Ebay
J.Bastemeijer at EWI.TUDelft.nl
Thu Sep 15 10:21:51 EDT 2005
The input frequency of a DDS doesn't really tell you the maximun
frequency. What's inmoprtant is the inside of the DDS (IC). A lot of
DDS-ICs nowadays have an internal reference multiplier. So the internal
frequency could be 20 times 8 (for example), so the internal DDS clock
is 160 MHz. Quoting Nyquist: pure theoretically 80 MHz would be the
limit and 10 MHz would be possible. However, the internal multiplier
will probably add a lot of noise.... Therefore, some manufacturers of
DDS-IC's e.g. Analog Devices, give you the option to disable this
By the way, did someone look into the type-number of the DDS? Or is it a
custom build "discrete" DDS?
Best regards, Jeroen
Mike Feher wrote:
>I am sure you really know this, but with a 20 MHz clock the maximum output
>of the DDS is probably no more than approximately 9 MHz. You just quoted the
>Nyquist limit, but in reality you do have to put a filter on the output to
>eliminate the spurious (aliasing) coming down from 20 MHz, the clock.
>Depending on the filter used you may even be able to get to 9 MHz or a
>little higher as that would still leave you 2 MHz for the filter's
>transition band, so the spurious at 11 MHz in that case would be
>sufficiently attenuated. Regards - Mike
>Mike B. Feher, N4FS
>89 Arnold Blvd.
>Howell, NJ, 07731
>From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
>Behalf Of Brian Kirby
>Sent: Wednesday, September 14, 2005 1:43 AM
>To: Discussion of precise time and frequency measurement
>Subject: [time-nuts] FEI5680A Rubidiums On Ebay
>In the past we had a bit of discussion on the FEI5680A rubidiums that
>are for sell on E-Bay.
>The units advertised by a certain seller were not the stock/standard
>units. The units the seller had were made for the cellular telephone
>industry and the pin outs were different from the literature that the
>seller pointed a link to, at the manufacturer's web site (These units
>were reversed engineered by another time-nut and myself to find the
>differences). The seller also pointed out the units were programmable,
>and pointed to a link for a different rubidium at Tom's site at
>http://www.leapsecond.com/museum/fei5650a/ as a programming example.
>The units we purchased were advertised by the seller to be programmable
>to 20 MHz.
>A couple of "us" time nuts have bought the units, and we found they did
>not program, as advertised by the seller. I specifically asked the
>seller about programming information and he claimed to have a friend
>that knew how. I asked for an introduction to the friend, and I never
>heard from the seller again.
>The DDS inside of this rubidium uses a 20 MHz clock, so in general, the
>DDS is limited to a maximum output frequency of 10 MHz. Also the DDS
>was not available to the user as a "optional" frequency output - the DDS
>was part of the internal servo multiplier loop.
>Tonight I was browsing E-bay, and I noticed a question from a user on
>these rubidiums and the response of the seller, which I have cut and pasted:
>Questions from other members : RUBIDIUM FE-5680A frequency standard 1pps
>output Item number: 7545757082
> Question & Answer Answered On
> Q: Hello, Before bidding for this product I want to know if you
>have capability in order to obtain the right 10 MHz frequency needed
>from such standard. In fact all the topics I have read on Internet for
>this product are saiding that each people who has buyed on Ebay this
>product has never obtained any frequency output. Best regards.
> A: Hello, Item was develop for 1 pps output. You can get some
>8.xxx Mhz from inside from the frequency multiplication stage, so
>whatever they said at the internet is ignorance of not knowing how to
>use it. If you programm the unit you can get 10 MHz nominal, I cannot
>supply any infor on the programming, however its available elsewhere. If
>you do not know how to programm the unit, just buy an Symm Lpro with 10
>Mhz nominal. What are your needs? Size? etc. thanks.
> End of the paste -----
>If the information is available elsewhere - we would sure like to know
>where it is ! By the way, this has been going on for over a year now
>(and its Sept 2005) !
>Brian - N4FMN
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>time-nuts at febo.com
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>time-nuts at febo.com
Ing. Jeroen Bastemeijer
Delft University of Technology
Department of Electrical Engineering
Electronic Instrumentation Laboratory
Mekelweg 4, Room 13.090
2628 CD Delft
E-mail: J.Bastemeijer at EWI.TUDelft.nl
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