Oscillator Proposal

Oscillator Proposal...

Suppose I have a series RLC, one end grounded, the other end driven by a chip, how might I make that into an oscillator?

All wild ideas accepted... this is for a custom chip. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson
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Series RLC is harder than parallel, because to leading order there's no signal on the pin at resonance. (If it were parallel, you could do a single-ended version of the MC1648, which you may have heard of.) ;)

You need to arrange a negative resistance at the pin, which isn't hard to do, and use the AC current to drive the next stage.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Reply to
Phil Hobbs

Gee! That part number sure sounds familiar ;-)

I was fretting over not wanting a DC path to ground, but maybe that can be minimized. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

Well, if it's series, you can bias the pin anyplace you like. All sorts of RF parts do that, e.g. the AD9956 DDS/PLL chip, which I'm working with at the moment. I had to dig out the demo board schematic to figure out how to drive all the differential inputs...the datasheet is a vast wasteland.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
Principal Consultant 
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Phil Hobbs

Done it many times, as an industrially important process: drive it with a voltage and phase lock on current. Or for a general resistance source (rather than ideal voltage source), you'll assume drive voltage (internal clock) and measure the pin voltage to infer current. Under the restriction Rds(on) < sqrt(L/C), but not by too much, so that the signal is not too small, nor the Q too low.

This will likely restrict what range of L, C can be used, since you get Q

  • VCC volts on them and Rds(on) ~ 50 ohms for average HC scale CMOS. And I(pin) < 10mA or whatever, and probably quite a bit below that if it's a low power application.

Maybe not as bad if it's a low voltage CMOS process where you can get low resistance cheaply, but then you need another supply (but hey... maybe make it variable and add AGC?). Or maybe it's reasonable to build a low impedance analog output pin, so the amplitude is small (optionally variable), and low impedance.

You could also do what every crystal ever does: add a loading cap, so your circuit effectively spans a capacitive divider that's part of the resonant capacitor. Crystals having the unique distinction of extremely large inductance and extremely small capacitance (the motional equivalent components), so that the impedance is still rather high (~kohms); a low impedance resonator would need a respectively large loading cap.

If you need a "discrete"* style oscillator (a few transistors and resistors rather than an entire loop), it should be fruitful to look at the V-I transform of a traditional parallel resonant circuit.

*Funny way to put it, since it's going inside a chip...

Tim

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Seven Transistor Labs 
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Reply to
Tim Williams

Hmmm. RLC is a 'short' to GND How about make the circuitry so noisy that the RLC shorts EVERYTHING BUT the noise and the circuit just 'looks' like it oscillates? I mean narrowband noise is an oscillator output, right?

Reply to
RobertMacy

How many wires will the chip have? 1 or 2?

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John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 
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John Larkin

Only one for the tank. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

OK, you can't do a parallel LC?

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John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 
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John Larkin

I'm pondering. I need a high circulating current in the inductor. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

It could be the serial version of the Baxandall Class-D oscillator.

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Tony Williams and I discussed this here some years ago - before Tony died. Driving a tank circuit with a square wave via an inductor implies peak voltage that go outside the rails by pi/2.

You can cut the amplitude (and reduce the third harmonic content of the excitation current to zero with a perfect drive) by contriving to make the drive voltage a "modified sine wave" which is to say a three-level rectangular wave that sits at 0V for a third of the time - the drive is low for one third of the period, centred of one sixth of the period, high for one third of the period and again centred for the last sixth of the period.

Don Lancaster's "magic sines" take the idea further.

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Bill Sloman, Sydney
Reply to
Bill Sloman

1) Ground resistor, feedback is at the top of the resistor, chip drive at top of LCR; micro does the dirty work to make this stupidity work. 2) Try opamp, positive feedback output to NI via the R, with LC from NI to gnd; standard resistive FB OP to INV; fiddle on values.
Reply to
Robert Baer

ok, ok, putting noise through the narrowband passband was NOT a good idea for making a 'clean' oscillator source.

So use the fact that there's a lot of 'logic' potential inside a chip, but external connections cost a fortune...what's wrong with a set of two variable, switchable current sources? One source, one sink to GND driving the RLC node? The idea is to start out with very small current ramp up until reach voltage near rail, switch OFF and turn on he other current source, and head back down, near GND [or neg rail] reverse etc. Now increase the current and keep doing until the voltage turns around on its own BEFORE it reaches the voltage switch points. Make up an appropriate rule.

maybe reverse process starrt at high current and come down in current. But with enough stuff inside the chip, and coming up with appropriate rules, you should be able to make a nice clean, stable output based upon looking like a Function Generator output that is a bit independentof the Q of the external tank.

Reply to
RobertMacy

If there's a current reference available, this means the RLC external can be driven, ICL8038-style, as if it were just a timing capacitor. Next step: make it a '555 astable?

If the external component were L, or C, the oscillation would be easy.

Reply to
whit3rd

Hmmmm? Interesting idea. I'll give that a whirl. Thanks!

...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

Anyone have success with this? I have a similar need: a Tesla-coil secondary coil + top-load-toroid capacitance to ground. Want to drive the bottom end of the ~35 mH coil to induce self-oscillation at ~120 KHz, using IC(s) from a 15V Vcc. Purpose is to sustain low-power oscillation between high-power pulse-bursts (that product the sparks from the top-load by driving the secondary from a primary coil). Seems as if it's tough to do...

Ken Herrick, Oakland CA USA

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Reply to
Ken Herrick

[snip]

I dropped back to a two-pin approach. I could make a one-pin version work only under very controlled gain conditions which were a function of the R-L-C... not a good condition to impose on a chip. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

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