[time-nuts] FEI5680A Rubidiums On Ebay

Mike Feher mfeher at eozinc.com
Thu Sep 15 12:12:13 EDT 2005

You are of course exactly correct in that some manufacturers now do have
multipliers inside of their DDS. And, as you pointed out it does increase
the phase noise on the clock by 20*log(N), where N is the multiplication
factor. On the other hand, the division at lower frequencies of the DDS (
not counting jitter, accumulator length errors, or other sources of noise
and crossover spurious) will reduce the phase noise by the same factor
20*log(M), where M is the ratio between the clock and the output frequency
of the DDS. I built my first DDS in the late 60's, using discrete parts. Of
course it only worked up to about 20 KHz output, but it was pretty neat.
What was a lot neater was my first digital filter in the early 70's. We had
to use serial arithmetic and the z^-1 s were shift registers. The first one
was an Elliptic (Cauer) and the results looked like a textbook filter
response. The neat thing was, even back then we could run computer
simulations using the same number of bits as our filters and the results
were identical. The good old days -     

Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Jeroen Bastemeijer
Sent: Thursday, September 15, 2005 10:22 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] FEI5680A Rubidiums On Ebay

Dear Time-nuts,

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:

>Brian -
>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
>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
>-----Original Message-----
>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
>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.
>    Sep-12-05
>    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
>time-nuts mailing list
>time-nuts at febo.com
>time-nuts mailing list
>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
The Netherlands

Phone: +
Fax: +
E-mail: J.Bastemeijer at EWI.TUDelft.nl

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