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

[glen english LIST]

Jeff one thing to be careful of with slower (lower bandwidth) PLLs and 
low VCO gains, is that you end up with loop filter capacitor values  
that is microfarads.

This will require film, tantalum etc caps that are not microphonic 
sensitive. this needs a bit of space. Ceramic caps make good microphones 
! Same considerations  for (large) power supply bypasses .. if your pcb 
is in foam in a box, not say bolted to an aluminium chassis in a 
vehicle, you may have an easier time.

-glen

On 4/04/2022 1:22 pm, Jeff Blaine 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
>>>>
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-- 
Glen English
RF Communications and Electronics Engineer

CORTEX RF

Pacific Media Technologies Pty Ltd trading as Cortex RF

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