[time-nuts] Things your mother (or FEI) didn't tell you about FE series oscillators

Skip Withrow skip.withrow at gmail.com
Mon Nov 10 00:24:29 EST 2014

Hello time-nuts,

I have spent a lot of time recently with Frequency Electronics, Inc. (FEI)
oscillators including the FE-5680A, FE-5680B, FE-5650A, and FE405B and just
wanted to pass on some of my findings.  Not everything I am including here
is new, but figured I would include the informattion for the sake of

As already known, there are a bunch of different variations on some of the
oscillators.  Here is kind of a quick rundown:

1. The 'old' FE-5650A - This unit used a 50.255 MHz VCXO and placed the
synthesizer outside the physics PLL.  This unit could be programmed over a
wide range of frequencies and is great if you need an oddball output.  The
digital board in the unit has the two small pushbutton switches and the
nearby 5-pin connector for serial communications.

2. The 'new' FE-5650A - This unit has the same physical construction, but
the circuit functionalyity is quite different.  It uses a 60 MHz VCXO and
the DDS is inside the physics control loop.  The output is divided from the
60 MHz.  Many of these units that came from the telecom industry have 15MHz
output. Unit has serial communications on pins 8 & 9 of the DB-9 connector.

3. The 'old' FE-5680A - This unit has a different physical footprint than
the 5650A, but virtually the same circuit.  Again uses the 50.255 MHz VCXO
and has the programmable synthesizer that can be tuned over a wide range.
Circuit board has the same two push botton switches and 5-pin serial
connector.  Many of these units only have a 1pps output (they came off
Motorola cell boards).

4. The 'new' FE-5680A - This was the popular, inexpensive unit that was
available a year or two ago.  Unit uses the newer 60 MHz VCXO architecture
and has serial communications via pins 8 & 9 on the DB-9.

5. The FE-5680B - This appears to be a VERY close cousin to the new 5650A
and 5680A.  It is in the 5680 physical package, but has a compact DB-15
(think VGA) connector.  Unit only needs single +15V supply.

6. The FE-405B - This is a crystal oscillator that is the same physical
package as the 5650A.  It has a 15 MHz output.  The interface is the same
as the FE-5650A (except no LOCK signal), including the capability to
communicate over the serial interface to change its frequency.


Now for the new information.  Tuning the units with the 60 MHz VCXO has
been published (see the FEI document that is archived in several places)
and uses several commands to read and write a 32-bit command value.
However, only the tuning sensitivity for the FE-5680A has only been
published that I am aware of.

FE-5680A - The tuning sensitivity of the FE-5680A is 6.8126x10E-13 for the
LSB.  The command value runs the full range from 00 00 00 00 to FF FF FF
FF.  The value is 2's complement with positive values driving the frequency
higher and negative values lowering the frequency.  Larger values may
unlock the unit and the positive and negative values may be different
(depends on how well the VCXO is centered).

FE-5650A - The tuning sensitivity of the FE-5650A is 1.4900x10E-17 for the
LSB.  Sounds great, but wait, turns out it does not have any more
resolution than the 5680A.  It takes a 45,722 step of the command register
to step the DDS.  So, it retains the same 6.8126x10E-13 actual minimum step
of the oscillator.  Also, the minimum value of the control register is F0
00 00 01, maximum value is 0F FF FF FF.

FE-5680B - The tuning sensitivity of the FE-5680B is 3.7248x10E-16 for the
LSB.  Same situation as the 5650A above, but the are 1829 counts per DDS
step.  Minimum value is F0 00 00 01, maximum value is 0F FF FF FF, same as
the 5650A.

FE-405B - The tuning sensitivity of the FE-405B is in the neighborhood of
1.143x10E-14.  Minimum value is F7 33 33 34, and maximum value is 08 CC CC

So, if you know how far off frequency your oscillator is divide that by the
LSB sensitivity, convert the number to HEX, and load it to the oscillator
(with the right + or - sense).

Skip Withrow

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