[time-nuts] Re: Custom Quartz crystals

[Bob kb8tq]

Hi

If this is a more or less “from scratch” build:

1) You can get perfectly good 3rd overtone crystals at 60 MHz.
They will be easier to work with than a 5th. You will get more pull
range and the phase noise will be as good / better.

2) As Rick mentions, you want a series resonant ( no load cap ) 
crystal for this sort of thing. The same “tune range” stuff gets into
this part of it as well.

3) If you are really headed to 10’s of GHz, you might want to think
through the entire chain. In some cases a 120 MHz crystal is a
better bet than 60. Your far removed phase noise can / will be better
with 2X ( or 3X or maybe even 4X) the “starting” frequency. 

4) I would not want to do this sort of thing without a pretty good 
PCB layout being part of it. There are just to many ways for VHF
oscillators to get a bit nutty ….

5) Typically aging is not a “big deal” in a multiplier chain sort of setup.
Everything gets phase locked all the time. You simply need enough 
tune range to keep it running that way. I would not spend a lot of 
time “worrying” about aging. 

6) Oddly enough crystals do impact phase noise at a 100Hz offset
at 120 MHz. For a tidy sum, Croven can sell you some pretty good 
crystals. Not clear if 100 Hz matters or not though ….

7) At the typical ADEV sort of measurement Tau’s everything should
be dependent on your PLL design rather than the OCVCXO’s. Yes,
there can be crazy problems. If the phase noise is “good” then 
ADEV should be into the PLL arena. 

No this isn’t easy to guess at. There are way to many variables in
terms of what one might want to do. 

Of course, there’s also the “off the shelf” alternative of just going
with one of the many low noise VCXO’s out there:

https://www.crystek.com/crystal/spec-sheets/vcxo/CVSS-945.pdf <https://www.crystek.com/crystal/spec-sheets/vcxo/CVSS-945.pdf>

The unfortunate truth is that Digikey will sell you a 60 MHz example
for ~$20 ( if they had them in stock ….. ). Getting a “build to spec” 
crystal for that price, not so much. 


Bob

> On Aug 14, 2022, at 9:08 AM, Corby Dawson via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Thanks for the advice, I'll see if those companies can help.
> I am wanting to replicate the oscillator circuit  at
> http://www.wa1mba.org/PLLVCXO/stephensen.pdf
> but at 60Mhz. The article specifies...
> 
> "Y1 is a
> fifth-overtone, AT-cut crystal ground
> for operation in the series-resonant
> mode with a load capacitance of 30 pF"
> 
> I thought that a series resonant crystal did not have a load
> capacitance??
> 
> I'll send this info along to the Crystal manufacturer and see what they
> say.
> 
> I had bought an FE 2216A OCXO oscillator of eBay a couple years ago
> thinking of this same project however it has a 120Mhz output.
> I read that a lot of these higher frequency oscillators had a fundamental
> that was doubled.
> So I opened it up hoping to find a 60Mhz crystal.
> Once the potting was removed I could see a Minicircuits doubler on the
> bottom of two boards and scopeing it showed 60Mhz in!
> However the oscillator/oven board has a 5Mhz SC cut crystal and a hybrid
> module that gets it up to the 60Mhz! See PIX.
> 
> While trying to find a crystal I'll use this board to test out my scheme!
> 
> Cheers,
> 
> Corby<FE2216aOCXO.jpg>_______________________________________________
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