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

[Robert Atkinson]

Hi Alec,
I'm an expert searcher ;-). I actually searched for FE-5650 and NSN (NATO Stock Number or National Stock Number to Americans) and then plugged the NSN into Webflis. There are lots of websites out there that trawl for part numbers and NSNs and then try to charge you for information that is freely available elsewere. Another good source of data is military specifications. Assist Quick search is good but the search engine is a bit fussy http://quicksearch.dla.mil/  try plugging crystal into the title keyword search.

Robert G8RPI.



________________________________
 From: Alexander Wright <alecjw at gmail.com>
To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
Sent: Saturday, 20 April 2013, 12:11
Subject: Re: [time-nuts] Changing FE-5650A frequency?
 

On 20/04/13 10:03, Robert Atkinson wrote:
> Hi Alec,
> Well done and thanks for the credit. I had not looked under the board for the crystal or spotted the divider.
> One comment. in your blog and ebay description you say that the unit runs on 15V. The "M" suffix 5650A is actually designed for a normal aircraft 28V DC supply (typical specification range 22-32V). It may run on 15V but it may be stressing the power supply circuit and / or under-running the oven. The external (28V) supply to the Marconi unit is just filtered, switched and connected to the FE-5650A without regulation. It appears that these units have options 08, 09, 22 and 25 rolled up into the "M" military suffix. By the way, these units can cost $10,000 or more each for specials see http://www.dlis.dla.mil/webflis/pub/pub_search.aspx
> 
> Robert G8RPI.
> 

Robert,
Good point, well observed!
The power-trends thing inside the package is an adjustable 3A switching regulator which seems to be set to nominally ~15.3V (91k setting resistor to ground). When i crank up the input voltage, the output voltage of the regulator settles to around 15.5v once the input voltage reaches around 19v. According the the datasheet the dropout voltage of those regulators is 4V, which means these frequency standards should be happy with any input from 19.5-38v.

Where did you find the info about pricing for these? I searched in that form for FE-5650A and couldn't find it. I tried searching the whole "stable oscillator" thing's NSN to no avail. I did expect these must have been very expensive when they were new though!


On 20/04/13 04:10, WB6BNQ wrote:> > 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
>
>
Bill,
I thought as much, thanks for confirming! I'll keep an eye out for any callibrated frequency reference I can borrow for 10 minutes to set this with, but otherwise I'll leave it as is.


On 20/04/13 09:37, GandalfG8 at aol.com wrote:> Hi Alec
> I see your web page and Ebay auction, are you able to supply one still
> complete and unmodified in its original enclosure?
> Regards
> Nigel
> GM8PZR

Nigel,
You mean just the FE-5650A enclosure or the whole "stable oscillator" thing? Do you want to ping me off list?

Regards,
Alec
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