[time-nuts] 125/122.88 MHz low noise ovenized source

[Jeff Blaine]

Nuts,

Wanted to give you an update on this.

Based on board comments, I decided to go with the home brew version of 
this project in lieu of using the commercial (Leo Bodnar, etc) solution.

The topology of the XO is:

1. W6PQL oven controller using SMT resistors, an op amp and a NTC 
termistor for control

2. A low noise VCXO from Abracon ABLNO/Crystek CVHD

3. 7809 feeds the oven side, with the ON Semi NCP163ASN330T1G low noise 
3.3V regulator feeding the XO and the frequency trim circuit.

4. Frequency trim is an LM4040 shunt regulator biased at 5 mA, feeding a 
trio of series connected resistors with the middle resistor being a 
10-turn trimpot.  A tap in that string feeds a series resistor / tant 
cap on the frequency trim to provide a RC LPF.

5. All of these components are mounted on the PCB save for the trimpot 
which is in the oven controller environment, but attached above the PCB 
by short jumpers.  So it's heat stabilized as are all the other items, 
but just not nailed down the the PCB (in the current iteration).

6. The LVCMOS output Fc is slightly affected by loading.  So there is a 
MC GALI-74 mounted between the XO & the 50 ohm output termination.  This 
provides a lot of isolation for the XO to varying loads and improves the 
frequency stability, plus the 74 is a native 50R driver.  The PCB has 
pi-network pads on both sides of the '74 to allow fiddling with the 
drive & output levels.

7. MC has a series of low cost easy to obtain LPF modules that are about 
1/2 or 3/4" square and I may add space on the PCB to stick one of 
those.  That would help push the 2nd & 3rd harmonics lower.  In the past 
I've used different MC filters with good results, as well as a discrete 
L/C shut that would be trimmed in final assembly to put the LC notch on 
the 2nd & 3rd harmonics - also with good results.  I don't know how 
sensitive the load (SDR) is to this but given the natural harmonic 
levels of the native Abracon, it may not be needed at all.

I've got a hacked combo of an old 'PQL PCB with a dead-bug mounted XO & 
EFT attached.  But once the design fiddling is done, I'm going to send 
it off and get some PCB made.

I would appreciate any advice the nuts list may have on these, 
especially the frequency trim circuitry.

This XO drives a Red Pitaya 122-16 in an SDR role that I use for CW & 
RTTY ham radio spot decoding.  So there is no hard requirement on purity 
or stability.  I know from prior experience that both the XO and the 
oven controller circuit are "good enough" in practical terms, but now 
that I'm going to clean it up and get some PCB cooked up, wanted to see 
if there are some areas of improvement that would make sense to do 
without going "full-nut."

73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com


On 4/5/22 3:12 AM, ghf at hoffmann-hochfrequenz.de wrote:
> Am 2022-04-05 3:34, schrieb ed breya:
>> I was curious and looked up this Red Pitaya SDR thing (which I had
>> never heard of - I would have guessed a poisonous snake, without the
>> internet), and the Abracom TCXO. It's getting a little confusing to
>> keep up with what's going on.
>>
>> First, does the installed TCXO have EFC capability, or not? Jeff
>> apparently says not, while Gerhard says yes. According to the
>> datasheet, the ABLNO can be an XO only (no suffix), or a VCXO ("V"
>> suffix). Jeff should be able to tell which, by inspection.
>
> The Abracon can, if you choose the right option.
> There is no Abracon at least on my older RedPitaya. There is just a
> 4*6 mm or so thingy on the board, on the microscope I can read "ExpressO"
> but I really don't care. My solution is the ECS xtal oven board,
> picture from my previous post. Attached is the phase noise of the 
> ECOC2522,
> I got it from ECS.
> The RP can be strapped for external sample clock and my board has a
> LVDS driver to feed it. When I lock my synthesized transverters to a
> precision reference, then it makes no sense to have the SDR floating
> around.
>
> The circuit of the RP is on their web site, only sligthly censored.
>
> Also interesting: The Koheron project, where they somehow synced 4 RP
> over their SATA connectors.
> < 
> https://github.com/Koheron/koheron-sdk/tree/master/examples/red-pitaya/cluster 
> Â  >
> < https://www.koheron.com/blog/2016/11/29/red-pitaya-cluster >
>
> < 
> https://www.koheron.com/blog/2018/07/08/phase-noise-measurement-alpha250 
> Â Â Â Â Â  >
> It seems that an external clock can do good.
>
> regards, Gerhard
>
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