Oscillator buffer

Do you want an LC oscillator that starts instantly and coherently, with a digital clock output? We do that, with LCs at low frequencies, and coaxial ceramic resonators at 500 MHz or so. 1 GHz shouldn't be horribly difficult, except for the milliwatt constraint. It's just a matter of getting the initial conditions right.

In general you're going to have a hard time getting the oscillator frequency to stay constant as it starts: the oscillator's characteristics must change as it settles into steady-state, because by definition the loop gain goes from more than unity to exactly unity (on average) at that point.

Making the oscillator so that the active device loads the resonator as little as possible will go a long way to achieving this, but won't get you all the way.

You may need to just do some breadboarding: if you add supply bypassing to the oscillator and the jump goes away, then the problem was supply coupling. If you figure out how to shove a buffer amp in there and the jump goes away, then it was input impedance on the 74xxx.

Study a 4046 PLL chip for how to efficiently AC-couple to a CMOS inverter. ...Jim Thompson

"o pere o" schreef in bericht news:k80f2n$dds$ snipped-for-privacy@dont-email.me...

Why can't you use a power on reset circuit that keeps the clock signal low until the oscillator has stabilized?

petrus bitbyter

Thanks for the suggestion for improving the coupling. I will have a look at it. However, in the 4046 the VCO is giving a stable amplitude, so the effect of the load is constant. Without having looked at it, I am afraid that in the gradual transition from small signal to large signal, the input impedance of the inverter will change significantly...

Pere

Are you building some kind of synchronous oscillator? In our application an external signal influences the startup transient (think superreg. principle) and the information contained therein should be more or less preserved.

Pere

Unfortunately not in this application. The startup transient is the relevant fact here.

Pere

The response from an unloaded oscillator is already ok, even taking these facts into account. Loading it more or less directly with a CMOS gate is not ok.

Yes, I have to investigate both ways. In the meantime I wanted to hear if there were other facts that I had overlooked.

Pere

We use gated oscillators in our digital delay generators. When we gat a rrigger, we start a clock oscillator, and count ticks to get coarse delay. An analog ramp thing gives fine delay to interpolate down to picoseconds. Sometimes just the LC is good enough, for short delays. The coaxial resonator things are great for medium accuracy and delay. he best is to use a gated LC for the clock, but phase-lock it to a crystal oscillator to get longterm precision.

In our application

OK, that's different. It's an externally quenched superregen, I guess.

Why not use a grounded LC and a non-inverting gain element? Or you could use a tiny toroidal transformer, with a secondary winding for the base of a PNP transistor, to provide the gain.

One oscillator that I really like is an LVDS-CMOS converter chip that is both the feedback gain and the comparator, with the LC grounded. But that wouldn't work for your application, if I understand it.

I have used ceramic coaxial resonators to build oscillators at ~433 MHz, and they are quite stable. I have also seen that you make stuff on FPGAs. IIRC there are interesting techniques to achieve high timing precision making use only of digital resources (keywords: time to digital FPGA)...

I have used several oscillator topologies. At 800 MHz the tapped-C // L resonator plus emitter follower works ok, i.e. it is a non-inverting topology. But the problem is more or less the same: how to tap the signal out.

You mean something like the DS90C032? You mean sensing with the LVDS side and feeding back from the CMOS side to provide the gain? I guess that this would not work, as I need a wide linear part where the oscillator grows up more or less slowly...

Thanks for your inputs.

Pere

Does your buffer need to replicate the oscillator amplitude ramp-up, or can the buffer simply square it up? ...Jim Thompson

That stuff is interesting, but we haven't done it inside an FPGA. We don't see a big demand for TDCs these days, and a couple of people do it really well. No point trying to compete with someone established in a small market.

We did do a new TDC lately, at a customer's request, but we used our old analog technique, with the FPGA just doing clocked logic.

Yeah, if you want a clasic exponential oscillation buildup, you need a linear gain element, a transistor or a fast opamp or a MMIC. Given your power budget, a transistor might be best.

How about a powdered-iron toroid with a feedback winding for the transistor/fet drive? The LC could be grounded. A fet would be cool, to keep the Q up.

Or something like this....

I have heard, from either Rhea's oscillator book or Hayward's RF circuits book, that a Hartley oscillator like that one tends to be more prone to UHF parasitic oscillations than a Colpitts. If you redraw the oscillator as a grounded-source, you'll see that the Hartley is a high pass between drain and gate (leading to more loop gain at high frequencies), where a Colpitts is a low pass.

I have, like, zero experience with this as a problem, but given who's sourcing the comments it certainly makes sense.

I wouldn't call that one a Hartley; I'd call it a transformer-coupled oscillator (which may have a formal name of its own?)

The transformer coupling will be bad at any frequency except resonance, and the fet, into the tank, only has gain at resonance, so it should be OK. Of course it needs details.

The references that I've read basically say that if it's got coils fore and aft then it's a Hartley, regardless of whether those coils are coupled or not.

Yes, the FET only has gain at resonance. Each and every one of them, whether intended or not. With the Hartley oscillator, the higher-than- intended resonances get a better chance of hijacking the oscillator behavior.

Am 15.11.2012 09:25, schrieb o pere o:

Download the operating & service manual for the HP 5370 A/B time interval counter. It has startable oscillators that really work. Circuits included.

regards, Gerhard

OK, rising to this here challenge here, I open my trusty copy of Radio Engineering (Frederick Terman, 3rd edition, 1947, p 410.) He shows the three classic oscillators as "Hartley", "Colpitts", and "Tickler Feedback", the latter being transformer coupled. But he does show the resonating cap in the grid winding, not the plate.

They used ECL gate delay-line oscillators, varicap tuned.

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Nice thread, (wiki calls the tickler the Armstrong.) The only RF oscillator I know is used to drive a Rb discharge lamp. It's a Hartly basically copied from an efratom lamp circuit.

To Opere, I don't quite get your problem. To sense the circuit you're going to have to take a bit of energy out. This must change the Q and (thus) the resonant frequency. If it's a changing Q when you switch in your circuit.. (?) then you have to balance it out.

George H.