[time-nuts] Best way for generating 8994.03 MHz from 2899.00042272.....MHz?

Rick Karlquist richard at karlquist.com
Sat Aug 15 02:17:59 UTC 2009


Lux, Jim (337C) wrote:
> These days, you might want to consider using the GaAs VCOs from Hittite,
> rather than the DRO. DROs are SUCH a pain to build and tune, being a
> mechanical resonator in a cavity.  Everything you do seems to adversely
> affect the DRO.  The MMIC VCO is just a die (or a die in a package) and
> it's
> pretty much immune to external effects, since the resonator is built into
> the oscillator.
>
> We built some prototypes at JPL using the VCO and a GaAs divider, and the
> performance was better than DROs.
>
> If you were building a very narrow band PLL, where tunability of the DRO
> isn't needed over a wide range, the DRO might be a good solution, but
> still,
> you have microphonics, etc. (we used to demonstrate the latter by hooking
> up
> the output to a spectrum analyzer that has a FM demodulator, and talking
> to
> the DRO)

We've used both.  The DRO has much better open loop phase noise, and this
synthesizer is for a single frequency, so the DRO only needs enough
tuning to make up for drift.  I was assuming the original poster
would purchase a DRO.  I agree that they are not something you
want to build.  We had no trouble with the ones we purchased
from various vendors.  BTW, the 5071A cesium frequency standard
uses a DRO to help generate the 9192... MHz signal that
excites the CBT, as explained in my FCS paper in the early
90's.

I don't understand how you got worse phase noise with a DRO
than a VCO, unless you used a narrow loop bandwidth for the
DRO and a wide loop bandwidth for the VCO.  We try to get the
loop bandwidth up around 1 MHz for DRO's.  The wide bandwidth
also mitigates against microphonics.  One thing we found was
that you want to cover up the tuning screw hole so that microwaves
don't go out of the hole and bounce around and go back inside.
We had to put copper tape over it.

Perhaps the original poster should start with a VCO and see
if the phase noise is good enough, then go to a DRO if necessary.

Rick Karlquist N6RK




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