[time-nuts] Ball/Efratom MFS-209 Rubidium GPSDO...

[Chuck Harris]

Ok,  time for a status report, and some modification information for
the Ball/Efratom MFS-205, 209, ... family.

First the status:

I have the unit working properly now.  I installed a nice 50db antenna
on top of the radon mitigation pipe for my house, and now my unit has
a nice view of the sky, and a good strong signal 24/7.  As a result, it
now regularly updates its frequency and time at the rate its setup prescribes.
The rubidium's control voltage seems to be slowly dropping with each
update, and each update is on the order of E-13, so that is probably normal.


Now the modification:

Tom Miller and I have been working on converting the 5MHz outputs, from the
nice 24 channels of isolation amplifiers, to 10 MHz outputs.

Tom identified the module marked "MBF", which is really a type "MBFD", as
the source of the 5MHz for distribution to the MBF modules, which are the
4 channel distribution amplifiers.

The MBF, and the MBFD modules really use the same PCB, only populated
differently to handle their different requirements.

The MBFD module takes the 10MHz signal from the master standard and sends
it to an internal daughter board that contains a level shifter, and a
74HC390 that is used as a divide by two.  The 5MHz output of the 'HC390
then goes through a 1K resistor, and back to the mother board where a
band pass filter cleans it up, selecting out just the 5MHz component.

The modification involves eliminating the 'HC390, and retuning the
band pass filter to 10MHz.  And then adjusting the isolation amplifier
gains, and the thresholds for the fault detectors.

1) Remove the daughter board from the MBFD module, and remove the
74HC390 chip... it isn't necessary, and it isn't a good idea to
leave its inputs floating...

2) Towards the middle of the daughter board beside the '390 chip is
a grouping of 4 two pin holes marked EFGH.  Remove the resistor
that connects to the "G" holes... save it... and all jumpers from the
EFGH holes.  The resistor should be a 1K.

3) Locate the 4 holes marked "E" and "F", and install the 1K resistor
to the pair of holes farthest from where the 'HC390 chip was mounted.

Ok, now the divide by 2 is eliminated, and the daughter board simply takes
the signal from the input buffer, and converts it to TTL levels, and
sends it back to the mother board.

Next, find the toroid coil mounted near the center of the mother board.
It is in the area that is underneath where the daughter board is mounted.

Locate a pair of capacitors, usually rectangular epoxy coated ceramic
type, that are in parallel, and quite close to the toroid.  These
capacitors resonate the toroid at 5MHz in the original circuit, and
were a combined value of 1043pf on my unit.  You need to pick a pair
of capacitors that will resonate the filter at 10MHz.  A ballpark
starting point would be 1/4 the value of the capacitors you find on
your unit.  Mine required 191pf... which is a bit less than 1/4.

To do this, it is best to power the board on the bench, and to solder
a couple of wires to the input.  The easiest way to power the board is
to clip lead 20V to the pair of test terminal posts (TP1 and TP2) on
the mother board. They go right to the main filter capacitor on the
board.... ground is negative.

The 10 MHz input connector is the top most coax connector on the MBFD
board's main connector.  0.5Vrms into 50 ohms is the desired input level.
I tack soldered a small coax to the board near the connector.  I connected
a scope set for 50 ohms to the output coax... everything on the MBFD
is laid out in an obvious fashion, so you should have no trouble locating
which output connector goes to which isolation stage.

There are 4 gain pots, and one fault threshold pot on the mother board.
The gain pots are grouped together one for each channel, and the
threshold pot is set away from the others.

Adjust the gain pots for 0.5Vrms output into 50 ohms.  Adjust the fault
threshold light so that it comes on when the output is less than 0.2Vrms.

I first set the gain of all the isolation amp stages to 0.5Vrms, and then
lowered the signal generator's signal until the scope showed 0.2Vrms,
which is about 1/2Vpp.  I then adjusted the fault pot to just come
on.

(as always, 0.5Vrms is 1.4Vp-p... If you happen to try and measure the
outputs without the 50ohm loading, they will be double the 50 ohm
value, or 2.8Vp-p)

When everything is adjusted properly, you should get 4 very low
distortion sine waves driving 50 ohms.

That is all that I have in my notes.

-Chuck Harris