[time-nuts] FE-5680A decoded - another piece of the puzzle

Skip Withrow skip.withrow at gmail.com
Sun Jan 29 23:22:12 EST 2012

Hello Nuts,

I've been trying to get the analog EFC to work on the FE-5650A (a very
close cousin of the FE-5680A).  On these oscillators it is normally on
pin-8 of the DB-9 (Option 12 and 30).  As one of the serial lines is on
this pin, I traced the jumper that is placed for the EFC option(s).
However, changing the voltage on the pin does not seem to move the
frequency (or at least not very much if it does). Anybody know the nominal
EFC sensitivity of the 5680?

On tracing things further, I have found that this input goes into the (4
channel) A/D associated with the microproccesor.  Changing the voltage
gives a value in message 5A that also changes a corresponding amount.  The
EFC bytes are bytes 5 & 6 of the 5A message, first byte is low second byte
is high.  Appears to be 12-bit value (0FFF max).

The question is - how do you enable the EFC?  Maybe it is a bit that has to
be set in one of the other messages. (But then how does the uP communicate
the EFC value to the physics package?)  It (more likely) may be that I have
not found the true C-field control circuitry yet and the A/D is just a

I recall someone implementing C-field control on a FE-5680A with the pot
disabled, but cannot find it now.  If someone can point me to that post I
sure would appreciate it.

Thanks in advance.

Skip Withrow

More information about the time-nuts mailing list