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Design >> Analog Design >> Fast Start Of a Crystal Oscillator
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Message started by loose-electron on May 23^rd, 2009, 2:54pm

Title: Fast Start Of a Crystal Oscillator
Post by loose-electron on May 23^rd, 2009, 2:54pm

OK, I just got involved in a transmitter where somebody tried to create a fast start "kicker" for a crystal oscillator.

My initial response was "that won't work reliably" and their problems with chips in the lab has supported that initial premise.

So, the question of the moment: What techniques would you use to fast start a crystal oscillator?

Not a fast start in a simulation (there are tricks for that using initial conditions) but a method of getting the oscillator up and running as quickly as possible, in terms of circuits, not simulator methods.

Title: Re: Fast Start Of a Crystal Oscillator
Post by ACWWong on May 24^th, 2009, 7:04am

For crystal oscillators that are pulled, setting the initial condition of the tune voltage for minimum capacitance(max neg gm) will give speed up. Other techniques we have used with success in the past include ALC (amplitude level control) where a larger current is used during the initial startup which is reduced as swing increases.

Title: Re: Fast Start Of a Crystal Oscillator
Post by loose-electron on May 24^th, 2009, 5:39pm

Those are good possibles, I expect to use a high initial gain with amplitude regulation at a low output amplitude. I am still looking for some more ideas here. Has anyone used an initial Q reduction by putting a resistor in parallel with the crystal? Lower Q circuit should start quicker I expect.

Any other ideas or experiences here?

Title: Re: Fast Start Of a Crystal Oscillator
Post by wave on May 25^th, 2009, 10:31am

The answer probably depends on your jitter tolerance to start with, and continuing onward...after whatever "speed up" method settles out.
The xtal Q limits your startup time and settling to a given jitter level.
Any kicks or AGC loop must settle before jitter is constant, or you might have a momentarily large amount of jitter.

You may consider a ring oscillator to jump start some non-critical circuits and switch over to a Crystal after a given count.

Wave

Title: Re: Fast Start Of a Crystal Oscillator
Post by loose-electron on Jun 3^rd, 2009, 6:51am

The ring oscillator exists elsewhere, this is for a RF mixer so it has to be crystal controlled.

Title: Re: Fast Start Of a Crystal Oscillator
Post by RobG on Jun 3^rd, 2009, 4:45pm

search "quick start oscillator" on google scholar and you will get a few hits. The one I was thinking of is by Blanchard. He uses an oscillator that is trimmed to about the correct frequency to drive the crystal itself until it was at the right amplitude. I was never crazy about this idea, but I think AMI (now ON semi) still uses it.

I have removed the tuning capacitor to minimize the startup time, but I had an issue. I think it was that the oscillator was sensitive to noise which was causing glitches and double edges which was causing my PLL to go ape which prevented the micro from loading in the correct trim. This did not impress the customer! But it was a cool idea.

It may have been something else on top of that... that was 7-8 years ago.

rg

Title: Re: Fast Start Of a Crystal Oscillator
Post by loose-electron on Jun 13^th, 2009, 10:24am

The kick start oscillator by AMI is a failed concept. It yields horribly, does not start reliably and the kicker oscillator does not align properly.

I have had the AMI/ON semi part in the lab and seen the failures with it.

Title: Re: Fast Start Of a Crystal Oscillator
Post by ywguo on Jun 15^th, 2009, 1:50am

Hi,

My simulation experience proves that the kick-start oscillator is a failed idea. It takes almost the same time as that without kick to start the crystal oscillator in SPICE simulations.

I guess it is due to the status/current through Ls, the inductor in the equivalent circuit of a crystal, is not near the expected value. In other word, the current through Ls is probably near zero at the imediate moment that the kick is stopped. If the kick is stopped when the current throught Ls is near its maximum value, perhaps the crystal oscillator will start faster.

Best regards,
Yawei

Title: Re: Fast Start Of a Crystal Oscillator
Post by ywguo on Jun 15^th, 2009, 1:58am

Hi Jerry,

Sometimes I use a lower Q model of crystal to speed the SPICE simulation. However, I don't think this idea works in real circuit because the key issue is to inject current/energy into the L/C tank fast. In real circuit, that is determined by Q value of the crystal. Although you lower the Q value by putting a resistor in parallel with the crystal, the pass-band of crystal itself does not vary. It is still very very narrow for a high Q crystal.

loose-electron wrote on May 24^th, 2009, 5:39pm:

Best regards,
Yawei

Title: Re: Fast Start Of a Crystal Oscillator
Post by RobG on Jun 15^th, 2009, 9:59am

loose-electron wrote on Jun 13^th, 2009, 10:24am:

Interesting. It never seemed like a real robust solution so I believe what you are saying.

Referring back to my comment about the problems I had when reducing the capacitance... I had removed all of the cap which allowed the parasitics to couple in noise. If you leave a small amount there you should be fine. So that seems like an obvious.

As you probably know, Vittoz's paper has a dynamic current source to start the osc up quickly. I think that paper also covers the start-up time issue w/ Q. There is another paper covering start-up requirements (Berkeley or Stanford?) which shows that it won't start-up if Q is too low. Maybe that is why lowering Q isn't done.

All these solutions have the issue of recovery from the transient when you revert to the steady state configuration. Good luck!

Title: Re: Fast Start Of a Crystal Oscillator
Post by loose-electron on Jun 18^th, 2009, 6:47pm

Yawei -- In simulation (and just in simulation!) putting initial conditions of a current on the inductor within the crystal model works perfectly to fast start a transient simulation. That's great to have when simulating and you don't want to suffer the huge simulation run time to see the energy build in the crystal.

IMHO - the best way to start a crystal quickly may be a system with a VCO, (think a current into a capacitor and a ramp slope to control the VCO) that you sweep across the band, and monitor the amplitude coming out of the crystal. When the amplitude spikes (Q = 10E5 here!) you are on frequency, and freeze the VCO.

That might work to quickly stimulate things.

Title: Re: Fast Start Of a Crystal Oscillator
Post by vivkr on Jun 19^th, 2009, 3:14am

loose-electron wrote on Jun 18^th, 2009, 6:47pm:

IMHO - the best way to start a crystal quickly may be a system with a VCO, (think a current into a capacitor and a ramp slope to control the VCO) that you sweep across the band, and monitor the amplitude coming out of the crystal. When the amplitude spikes (Q = 10E5 here!) you are on frequency, and freeze the VCO.

Hi Jerry,

While this sounds like a good way of atleast speeding things up a bit, won't it also suffer from the same fundamental issue that Yawei already pointed out? I imagine that the amplitude of the signal coming out of the crystal won't really change that fast as you sweep the slope of the VCO control voltage.

Basically, the high Q of the crystal means that it does not lose much energy quickly (or gain much energy), i.e. the crystal has enormous inertia. Whatever you do, it takes an almighty effort to force an amplitude change. I don't see how you can get around that.

In my opinion, you can only speed up things in the sense that you avoid using suboptimal methods which themselves would slow down the process of the crystal reaching its steady-state. So, using the amplitude regulation schemes like in Vittoz's paper may be a way of speeding things.

Best regards,

Vivek

Title: Re: Fast Start Of a Crystal Oscillator
Post by loose-electron on Jun 20^th, 2009, 12:35pm

Vivek -
I do agree with the inertia statement. Totally and completely!

You got two issues to deal with: high Q and system inertia - that said, if you are going to "kick the crystal" you have to determine the "kick frequency" using the properties of the crystal. Thus my thinking on the VCO and frequency sweeping.

If you design a system that uses a VCO driver at Vdd amplitude, and design the system such that the crystal oscillates (final amplitude) at (for the sake of argument) at vdd/10 peak, you might be able to develop an architecture that works somewhat.

Need to put the crystal within a gain control loop, provide output buffer amplifiers to get to a larger usable amplitude, and then you may have what you need.

Jerry

Title: Re: Fast Start Of a Crystal Oscillator
Post by ywguo on Jul 1^st, 2009, 9:53pm

Jerry,

Assume the kick-start works if the you find the "kick frequency", it would have been very complex to implement such an architecture. For a high Q crystal oscillator, the bandwidth is very narrow, say 100 Hz. The VCO tuning range must cover PTV variation. At the same time, the frequency sweeping must be slow to find the accurate "kick frequency". Perhaps it takes longer time to find the "kick frequency".

BTW, what's the spec for the start-up time?

Best regards,
Yawei