[time-nuts] Changing FE-5650A frequency?

[WB6BNQ]

Alexander Wright wrote:

> 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

Hi Alex,

Your alternate assumption is the correct one.  Working the formula backwards with the assumptions that the 800 KHz is spot on would then dictate that the DDS out is exactly 12.8 MHz with the final value of the physics package being

50,255,058.6495

based upon my HP-35s calculator.

Many construction and environmental factors affect the actual final frequency of the physics package.  Plus there is some small variance about the center frequency with which signal levels are still strong enough to allow functionality.  The C-field adjustment allow for tweaking within that small variance.  Fortunately, this ability allows for putting the Rb right on the assumed true frequency.

Provided you have a higher reference source (i.e., Cesium or a well tamed GPS arrangement), the way to properly adjust the Rubidium is to adjust the physics package C-field pot to it minimum frequency point, then set the DDS to the closest point just below the desired frequency and then re-adjust the C-field pot to come up onto the proper frequency.  This is a slow repetitive process requiring time and patience.  The degree of patience will directly correlate with the precision obtained.

However, if you are not dying to have absolute accuracy, but more interested in the stability provided by the Rb, then do not mess with the C-field pot.  If you do not have the means for the calibration, it would, most likely, be safe to assume that the Rb is within 1x10-9 if other means are used to make sure some gross error is not evident.

I read your blog and, besides the success, it looked like you were having fun.  Good luck,

Bill....WB6BNQ