[time-nuts] Ebay FE-5680A Rb: Are they that good?

Bob Smither smither at c-c-i.com
Fri Dec 9 15:52:39 EST 2011

Bob Smither wrote:
> Chuck Forsberg WA7KGX N2469R wrote:
>> The price seems reasonable, esp. with free shipping.
>> Does anyone have experience with these?
>> Is the circuit board included?
> I could not resist at < $40.00 delivered.  Mine has been running for a 
> couple of weeks, but my measurements have been inconclusive (I suspect 
> my setup is far from ideal, especially the handling of the 10 MHz signal).

I rebuilt my FE-5680A setup with new power supplies and better routing of the 
10MHz output.

BTW - I noticed that mine has a 1PPS output (~1usec positive +5V pulse) on pin 6 
of the DB9 connector.

The results are much more consistent with the new setup.  I have a 1PPS that is 
derived from the FE-5680A (not the native 1PPS output on pin 6) that is compared 
to the 1 PPS from a GPS receiver.  The first 15 hours of data can be seen here:


 From the data, it looks like my FE-5680A is running about 1.7E-10 fast.  The 
data sheet specifies aging of < 2E-9/year so this seems reasonable.

I have been able to talk to it through the RS-232 port (pin 9 is output from 
FE-5680A, pin 8 is input to FE-5680A).  Using Cutecom on Linux with 9600 8N1 
results in the following dialog:

   2d 04 00 29 <- input command - request frequency offset
   2d 09 00 24 00 00 00 00 00 - reply

 From the reply I am assuming that my unit has no programmed offset (32 bits of 
zeros - last hex digit is the data checksum).

My documentation states that the full range of adjustment is:

   7f ff ff ff => +383 Hz
   80 00 00 00 => -383 Hz

To correct the 1.7E-10 offset I need to slow it down by .0017 Hz or a count of 

After this unit runs for 24 hours I will try to program in the correction.

More later ...

Bob Smither, PhD                                   Circuit Concepts, Inc.
The American Republic will endure, until politicians realize they can
bribe the people with their own money.
         -- Alexis de Tocqueville
Smither at C-C-I.Com  http://www.C-C-I.Com  281-331-2744(office)  -4616(fax)
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