[time-nuts] Low phase noise VCO

John Miles jmiles at pop.net
Wed Feb 10 06:06:27 UTC 2010

> You're right, it's for a USRP. I just got annoyed with the
> constant frequency offset, so I'm rolling my own. Turns out there
> isn't much available for good off-the-shelf 64MHz VCXOs. The
> USRP2 has built-in support for 10MHz sync, but not having one,
> I'm left to what I do have. Can't injection lock the oscillator
> on board, as it's a self-contained square-wave clock. So it looks
> like I'm going to try my hand at a Butler VCXO.
> That said, I know that for PLLs, the maximum control loop
> bandwidth you can use is limited by the pullability of your
> oscillator: if you use a VCXO with very low Kv, you might end up
> with a maximum useful loop bandwidth of 10Hz.

Well, it's not *quite* that simple, because there are other sources of gain
in the loop that can make up for the low Kv.  The VCO's modulation bandwidth
is what ultimately calls a halt to the party.  If nothing else, as the loop
BW is increased, the pole formed by the VCO's tuning-port capacitance will
eventually destabilize the loop.

> No sense in using a
> 10544A to tune that! The phase noise performance would be pretty
> awful, since you can't tightly lock the reference oscillator to
> it.

You aren't generally trying to improve phase noise with a reflock circuit.
You're just trying to improve medium/long-term stability without making the
short-term noise worse, or (just as important) corrupting the reference with
wideband noise and spurs that might come in on the user's 10 MHz input.

There are always exceptions but in most cases you want a slow loop for
reference locking.

> So does it make more sense to use a high-Q LC oscillator
> instead, with a much higher Kv? I'd be able to phase-lock it much
> more tightly to the reference oscillator. Plus, a 64MHz LC
> oscillator is pretty easy to build.

You could do that, of course, but you would have to use a much wider loop
bandwidth.  It's rarely a good idea to use a crappy oscillator and a wide
loop in a design that calls for a decent oscillator and a narrow loop.

A SAW oscillator can work well in cases where you need more pullability than
a crystal provides.  This is unlikely to be one of those cases, though.

-- john, KE5FX

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