[time-nuts] Jim Miller simple GPSDO

jimlux jimlux at earthlink.net
Fri Sep 16 15:57:08 EDT 2016

On 9/16/16 12:07 PM, Lars Walenius wrote:
> Interesting. Maybe not the optimum oscillator for a GPSDO.
> Lars
>> Från: Tim Shoppa<mailto:tshoppa at gmail.com>
> Skickat: den 14 september 2016 14:39
> There are special "wide-pull-range" VCXO's where a 10MHz unit will indeed
> have sensitivity of 600Hz/V or more. e.g.
> http://www5.epsondevice.com/en/products/vcxo_standard/vg4231ca.html
> I don't know exactly what Epson does inside that particular unit, but a
> trick to get wide pull range with discrete circuits is to put two or more
> crystals in parallel.
> Tim N3QE
>>> On Wed, Sep 14, 2016 at 4:40 AM, Lars Walenius <lars.walenius at hotmail.com>
> wrote:
>> The VCXO sensitivity given is strange as it indicates a far to wide span
>> so I guessed 30ppm and if it is higher it still needs the damping from
>> R2-C2.
>> For the OCXO I used the figures given. With 2Hz per volt and 8 Volt span
>> you have 16Hz of span. 16Hz divided with 10MHz is 1.6ppm (parts per
>> million) can also be said as 1.6us/s.

At JPL, we tried with middling success to build a DRO with two 
varactors: one loosely coupled with low tuning gain (for phase locking), 
one tightly coupled with high tuning gain (for coarse frequency selection).

The idea was to get away from "individually hand crafted" oscillators 
for deep space transponders.  Right now, you need to know your channel 
assignment a couple years in advance so you can get the DRO tuned for 
your channel (puck movement, tuning screw, etc), as well as getting the 
reference crystal cut/selected for the right base frequency to multiply up.

This raises a problem when you want to use a spare transponder from 
Mission A (which has already launched) for Mission B.  Aside from having 
multiple spacecraft at the same frequency, it raises a big issue if you 
have a dual string architecture.. you'd like the prime and redundant to 
have the same channel assignment.

We were able to do it, sort of, but we were still subject to all the 
ills of a DRO in a mechanical cavity.  Microphonics, for instance. Not 
an issue in space, which is very, very, very quiet, but a real pain on 
the ground when you're testing.

As it happens, NCO/DAC technology has advanced to the point where we can 
lock a wider band VCO well enough.  And NCOs let you be "crystal 
frequency independent".

Now we can buy a really high quality rock at 10.00000something MHz (or 
use a spare USO from another mission) and use it to lock a wider band VCO.

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