[time-nuts] Stand-alone Use of the FE 5650A or 5680A DDS board

[Corby Dawson]

Hi, Everyone,

A couple years ago I used the DDS board out of an FE5680A in a project. 
Here are some details I discovered about using the board by itself.
The project was using the guts of an Efratom FRSC to run the physics
package
from an HP 5065A. (I called it a 5065A junior!) I needed to replace the
fixed 
.3125Mhz that mixes with the 5Mhz to provide the offset frequency with a 
.314962Mhz from the DDS.
The Efratoms 20Mhz clock provided the DDS clock.
I programmed the DDS for .629924Mhz, took the output thru an op-amp a
74HCT14 and a 74HCT74 to give me the TTL .314962Mhz. Once the system was 
working the two pushbuttons on the DDS board could be used for a fine 
adjustment. (LSB was approx. 2.5X10-13th) The board can be programmed 
via RS232 but only if the clock is at the original 50.255Mhz, which
equates 
to 9600 baud.
This was not a problem as I only needed to  program it once on the bench 
and then  use the C field for fine tuning.
My current project uses the same hardware but with a DDS clock frequency
of 5Mhz and a final output frequency of 5.7517195 Khz. This to substitute
for the built in synthesizer of a Hydrogen Maser for some
troubleshooting.
This requires a DDS output frequency of 92.027512Khz and replacing the 
HCT74 with an HCT393. Only 1/2 of the 393 is used for a divide by 16.
(this gives an LSB value of approx. 5X10-13th) Also 
I wanted to be able to vary the frequency via RS232 while installed. This

required the terminal program be able to be set to 955 baud. One other
bit of info is that at the low clock frequency the two pusbuttons
that inc/dec the LSB must be held down until the "heartbeat" LED on
the DDS board transitions from one state to another or the entry will
not take! This info will let you use 5 or 10Mhz as the DDS clock
frequency
and still be able to program it in realtime! (1910 baud for a 10Mhz clock

frequency) If anyone is interested I can provide a schematic for the 
opamp/divider circuits. The DDS board info is widely available on the
net.
At the frequencies I used I did not have to add the parallel capacitors
mentioned by some. Also note the DDS board I used is the one from the
1PPS
output units.

Hope this is useful!

Corby Dawson
____________________________________________________________
Moms Asked to Return to School
Grant Funding May Be Available to Those That Qualify.
http://thirdpartyoffers.juno.com/TGL3141/4ce71bb6f031745232m04duc