Re: Yo! RF dudes!

John Larkin <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

>> How long does the ceramic resonator take to come up to amplitude?

> It starts in a couple of ns.

Is that a normal oscillator without special starting circuitry? I
thought the unloaded Q was in the hundreds to thousands. How long
does it take to settle to full amplitude?

> The LC-based design we have now is fine, and has low power
> consumption compared to a ceramic resonator osc. It's been highly
> evolved over a decade or more. The oscillator starts in about 4 ns
> and gives a nearly perfect 50 MHz square wave clock out after
> that, and can be killed and restarted in about 40 ns.

That sounds a bit like the circuit I just posted. I didn't show the
stop waveform, but it dies pretty quickly.

> Jitter is single-digits picoseconds.

9 picoseconds rms at 50MHz using a smd inductor with a Q around 30?
That's pretty good. I'll have to see if I can top it.

That's the advantage of working at a single frequency. Most of my
stuff is octave bandwidth to allow binary division to any lower
frequency. The jitter is pretty bad in comparison, but it's a lot
better than DDS. I haven't tried the latest ADI chips, so the jitter
might have improved.

>> I've heard of people starting a crystal oscillator in one cycle.
>> I don't know how they do it without damaging the crystal, but it
>> might be worth looking at.

> HP did that in one of their old delay generators. It's basically
> terrible. For a lot of work you wind up with a bad crystal
> oscillator.

> The worst part is that you can't quench it very well for the next
> burst. The quartz keeps ringing for a very long time. You can damp
> an LC tank to near zero residual oscillation in a single cycle.

That's what mine does also. But I'd be careful about any residual
energy in the tank. A small amount could cause significant phase
error on restart if it happened to be 90 degrees out of phase with
the previous timing.

I ran into that problem on an old data separator for hard disk
drives. It was the first time an LC oscillator was used for this
application, and it solved a lot of problems with previous
multivibrator vco's. That is patent 3,810,234 on my web site at

http://www3.sympatico.ca/add.automation/patents.htm

The start waveform for the oscillator shows a 180 degree phase
difference. That turned out the be the easiest to handle.

> John

Regards,

Mike Monett
.