On 07/04/13 21:55, WB6BNQ wrote:
Hi Alec,
I am going to agree with Robert (G8RPI) on his assessment, particularly as the
dip switches are clearly visible in one of the pictures. Although I wonder why
Robert feels unfortunate about the [ two chip DDS arrangement ] ?
It matters not the number of chips that make up the DDS. Why ? Because the
actual physics package and its electronics has nothing to do with the DDS in
the older designs. That is, the signal output of the physics package is a
fixed frequency (around 50.255 MHz) that you could capture and feed to a more
modern DDS if necessary as replacement of the old one is probably not going to
happen. However, if it is working then once you get to a frequency you need
you probably not touch it again. So no loss there.
As for calibration, there is a hole on one side of the can that has a multi-turn pot that
adjusts the "C-field" current. The C-field is a very fine frequency
adjustment. You first adjust the C-field pot to its lowest setting (as in frequency),
then set the DDS to the closest frequency on downside of where you want to go and then
adjust the C-field pot to come up on to the frequency. The C-field pot is a really fine
adjustment, so we are talking very small movement that will require patience, a better
reference and at least a
oscilloscope to watch the drift rate over a very long time. A truly time
consuming experience.
However, if you leave that C-field pot alone, you will most probably be inside
1 part in 10 to the minus 9th.
The most recent designs have the DDS in the control loop for the physics
package. In the new internal design in order to change frequencies you need to
actually change the BASE crystal frequency, the DDS and the firmware to get a
new output frequency. So basically a real pain in the ass.
As for the foam that is sandwiched in between the boards, I agree with Robert
that it is for temperature stability in varying environments. If the unit is
kept in a normal room all the time then the foam is not a concern and could be
carefully cleaned off. There is some other components that have white stuff
that looks more like an RTV type substance used to hold down a component like
what is seen in photograph IMG_20130407_102937.jpg at the two toroids, That I
would leave alone.
Your finding the 12+ MHz just confirms that the design is very similar to that
described in the FTP file I provided. Make sure you record the switch settings
before you change them so you can return to a reference point.
Although you did not indicate, I assume you downloaded the FTP file ? Please
let me know so i can reduce the storage level as it is not my site although I
have use of it on a short term basis.
Thank you,
Bill....WB6BNQ
Bill,
Thanks for your input and sorry for the slow reply!
So, I've had a look at the datasheet and it looks like the output
frequency should be Fin*(tuning word)/2^32... I thought I'd check if
this matches up.
The tuning word was set to x4134111F (towards the inside of the board
seems to represent 1 and towards the outside, zero.
Assuming 50.255MHz going in that gives.... 12.799985 MHz output!
Perfect. Pretty much exactly what I'd measured. Alternatively I could
assume that the output is 12.8MHz exactly which sets the input frequency
to 50.2550586MHz - i wonder which assumption I should use when changing
the frequency? I'd like to adjust it to put 10MHz out, skip the 4000
series logic divider and remove/change the output filter.
Regards,
Alec
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