[time-nuts] FEI5680A Rubidiums On Ebay

Mike Feher mfeher at eozinc.com
Wed Sep 14 07:53:40 EDT 2005

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 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

-----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 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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