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

[glen english LIST]

Hi Jeff

I know the device well (Red pittaya 122-16. Great ADC) . Have you 
thought of tweaking the supply volts to the on board oscillator (which 
isnt too bad at all) to shift it ? that sometimes works.

Crystek have a 122.88 MHz CVHD-950 series VCXO which you can disclipine 
and it is VERY good. The only twist is that its about 20ppm/volt and 
that is alot of variation so you need to take care with noise injection.

I've changed my  new SDR designs to using a ADF4356 in integer mode,  
and a dielectric resonator (393 MHz) on the output to kill wideband 
spurs. I'd suggest running the '4356 at 122.88 (I'm using the common and 
some sort of resonator on the output to kill wideband spurs if they are 
a problem for you.

I've been using a common footprint Connor Win D75A part with that 
familiar footprint.  but you really want a 16.384 or 12.288 / 24.576 to 
run integerer.  Taiten TSEAALJANF-16.384000   which is easy to 
discipline, worth a look also.

VK1XX

On 4/04/2022 9:57 am, Jeff Blaine 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

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