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 <[email protected]>
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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