[time-nuts] Re: Commercial solution - 122.88 Mhz low noise source

[Bob kb8tq]

Hi

For a variety of reasons, you will have a loop bandwidth in the > 10 Hz 
to < 200 Hz region. The VCXO isn’t great close in and the GPSDO (even
after multiplication) should be as good / better. Even with some work,
the VCXO still will wander around due to temperature. Unless you go 
crazy, it also will wander a bit from supply. There’s also the chance of
vibration or shock. 

As mentioned in another post, component values on an properly designed
analog loop running one of the typical chips get pretty crazy at very low
loop bandwidths. You now get into questions about sensitivity of components
if you go with certain ceramics, leakage if you run an electrolytic,  and with 
size if you go with something else.

Bandwidth and damping ( or phase margin ) are two independent variables
when you design a PLL. You can have a narrowband bandwidth and a lot
of damping or the same bandwidth without much damping. It’s all in the 
design equations …. 

Bob

> On Apr 3, 2022, at 11:22 PM, Jeff Blaine <KeepWalking188 at ac0c.com> wrote:
> 
> It looks like the XO version is mounted on the PCB.  But there is a VCXO version which would run about +/- 35 PPM/3.3V typical and that's (amazingly) available.
> 
> Since this EFT is not an on-board capability, at the moment, I'm thinking to mount the VCXO - and associated goodies to slave it to the 10 Mhz GPSDO feed - in a separate enclosure that could allow the VCXO to be thermally insulated.  The temp swing vs. time will move very slowly and that would allow a very damped PLL loop to apply corrections.
> 
> I've not done anything with a PLL before but I don't envision this as being too complicated.  And it would be a pretty cool project.  Don't need true "phase" lock, but rather "frequency control" without degrading the noise characteristics of the VCXO.
> 
> 73/jeff/ac0c
> alpha-charlie-zero-charlie
> www.ac0c.com
> 
> 
> On 4/3/22 7:38 PM, Lux, Jim wrote:
>> On 4/3/22 5:09 PM, Bob kb8tq wrote:
>>> Hi
>>> 
>>> Does the Abricon have an EFC input? If so, rigging up a PLL to lock
>>> 122.88 to 10 MHz is probably the best option.
>>> 
>>> Bob
>> 
>> 
>> And what about moving off the board - so you don't have to chase the temperature swings.
>> 
>> 
>>> 
>>>> On Apr 3, 2022, at 7:57 PM, Jeff Blaine <KeepWalking188 at ac0c.com> wrote:
>>>> 
>>>> I've got a couple of Red Pitaya 122-16 SDR and want to discipline them to an external low noise GPSDO source.  Wanted to see if there were some easy solutions that fell into this category.
>>>> 
>>>> For the prior generation (QS1R), I had built a homebrew OCXO based on a custom low noise crystal from ICM and that worked great for the 125 Mhz application.  Unfortunately this Pitaya is a bit removed in Fc and ICM has been out of biz for several years now.
>>>> 
>>>> The RP's on-board oscillator is the respectable Abracon ABLNO 122.88 which runs about -115 dBc/hz at 100 Hz spacing and that's probably not a limitation for the unit.  Unfortunately the OSC is a bit too sensitive to external temp and wanders around quite a lot (+/- 5 PPM) compared to the homebrew OCXO - over the annual ambient temp range of (10-35C) (all datasheet referenced values). The Abracon unit is mounted on the board with the FPGA and sees additional temp range depending on other heat generation factors (FPGA loading).
>>>> 
>>>> I have a couple of commercial GPSDO on the bench now, so a pretty clean 10 Mhz GPSDO reference is available now.  And I've seen enough board discussion to know that a homebrew GPSDO solution is one of those "it's harder than it looks" things.  Hence the desire just to buy whatever is the silver bullet, as long as it does not require too much silver.  ha ha
>>>> 
>>>> Appreciate any suggestions.  TKs!
>>>> 
>>>> 73/jeff/ac0c
>>>> alpha-charlie-zero-charlie
>>>> www.ac0c.com
>>>> 
>>>> _______________________________________________
>>>> time-nuts mailing list -- time-nuts at lists.febo.com -- To unsubscribe send an email to time-nuts-leave at lists.febo.com
>>>> To unsubscribe, go to and follow the instructions there.
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts at lists.febo.com -- To unsubscribe send an email to time-nuts-leave at lists.febo.com
>>> To unsubscribe, go to and follow the instructions there.
>>> 
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at lists.febo.com -- To unsubscribe send an email to time-nuts-leave at lists.febo.com
>> To unsubscribe, go to and follow the instructions there.
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com -- To unsubscribe send an email to time-nuts-leave at lists.febo.com
> To unsubscribe, go to and follow the instructions there.