Oscillator Proposal

can you measure the drive current ?

rearrange something like this,

-Lasse

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

Yep, that's the general direction I've been heading, observe the drive current and switch at zero crossings... with some minimum current latch-out flip-flop to make sure it starts. ...Jim Thompson

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

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

Am 14.03.2015 um 18:16 schrieb Lasse Langwadt Christensen:

I cannot see the start of the thread???

That calls for the circuit that most VHF VCOs use.

Connect the base of a CC stage to the RLC. Emitter resistor is usually relatively large and loaded with some C. That creates the equivalent of a negative R and a small capacitor if one looks into the base. Oscillates like hell.

Drawback is the area for the capacitor or a pin for an external one.

regards, Gerhard

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.

Tim

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?

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

A short translates to high gain at resonance in a simple opamp with feedback. Circuit noise gets it all going.

Jim didn't mention which was wanted, square or sine etc, nor which most important properties were wanted. Which make it a very open question. Cos there's no shortage of ways.... eg pass mains thru neon, apply to your RLC. All depends what you want.

NT

Only one for the tank. ...Jim Thompson

OK, you can't do a parallel LC?

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

Won't someting along these lines work ?

Why ?

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

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.

Didn't say what frequency, assumed audio range. To keep under the interest of the world's regulatory agencies. less than 10kHz completely ignored and less than 150kHz, somewhat ignored.

OpAmp with NOISE into the non-inverting input. Rfdbk very high Z and a single external connection to the RLC to GND. Does that produce a 'narrowband'waveform that 'looks' like an oscillator's output? Hmmm. time to apply .tranoise and see what THAT simulation shows.

External connections: Vcc, GND, RLC, Output? if interest is only in the currents in L, then don't need Output.

On Sun, 15 Mar 2015 04:09:26 -0700, RobertMacy wrote:

yecch!!!! as promised I tried a .tranoise analysis and got the worst looking output of an oscillator I've ever seen!

The output waveform looks ratty beyond belief. However the FFT looks just like it should - Bandpass for noise.

Watch out for the wrap in the very long line, straighten that out to run the LTspice .asc [I know i has +/-V connection but I got lazy.]

SimpleOscillator.asc

Version 4 SHEET 1 27404 680 WIRE 1120 -640 1088 -640 WIRE 1136 -640 1120 -640 WIRE 1248 -640 1216 -640 WIRE 1280 -640 1248 -640 WIRE 1408 -640 1360 -640 WIRE 1552 -640 1408 -640 WIRE 1616 -592 1584 -592 WIRE 1712 -592 1680 -592 WIRE 1248 -576 1248 -640 WIRE 1376 -512 1120 -512 WIRE 1584 -512 1584 -592 WIRE 1584 -512 1376 -512 WIRE 1616 -512 1584 -512 WIRE 1712 -512 1712 -592 WIRE 1712 -512 1696 -512 WIRE 1744 -512 1712 -512 WIRE 1552 -480 1552 -640 WIRE 1376 -464 1376 -512 WIRE 1520 -464 1376 -464 WIRE 1744 -448 1744 -512 WIRE 1744 -448 1584 -448 WIRE 1808 -448 1744 -448 WIRE 1824 -448 1808 -448 WIRE 1120 -432 1120 -512 WIRE 1376 -432 1280 -432 WIRE 1520 -432 1376 -432 WIRE 1280 -400 1280 -432 WIRE 1376 -400 1376 -432 WIRE 1552 -368 1552 -416 WIRE 1600 -368 1552 -368 WIRE 1616 -368 1600 -368 WIRE 1744 -352 1744 -448 WIRE 1856 -352 1744 -352 WIRE 1120 -320 1120 -352 WIRE 1744 -320 1744 -352 WIRE 1856 -320 1856 -352 WIRE 1376 -304 1376 -320 WIRE 1120 -208 1120 -240 WIRE 1376 -208 1376 -224 WIRE 1120 -112 1120 -144 WIRE 1280 -112 1280 -336 WIRE 1376 -112 1376 -128 WIRE 1744 -112 1744 -240 WIRE 1856 -112 1856 -256 FLAG 1808 -448 out FLAG 1248 -576 0 FLAG 1120 -112 0 FLAG 1376 -112 0 FLAG 1744 -112 0 FLAG 1856 -112 0 FLAG 1408 -640 cc FLAG 1120 -640 ss FLAG 1600 -368 ss FLAG 1280 -112 0 SYMBOL Opamps\\LT1115 1552 -512 R0 SYMATTR InstName U1 SYMBOL Misc\\battery 1376 -640 R90 WINDOW 0 -32 56 VBottom 2 WINDOW 3 32 56 VTop 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName Vcc SYMATTR Value 9Vdc SYMBOL Misc\\battery 1232 -640 R90 WINDOW 0 -32 56 VBottom 2 WINDOW 3 32 56 VTop 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName Vss SYMATTR Value 9Vdc SYMBOL res 1360 -416 R0 SYMATTR InstName Rbias SYMATTR Value {R} SYMBOL res 1712 -528 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName Rfdbk SYMATTR Value {R} SYMBOL res 1104 -448 R0 SYMATTR InstName Rcoil SYMATTR Value 2 SYMBOL ind 1104 -336 R0 SYMATTR InstName Lres SYMATTR Value 1mH SYMBOL cap 1104 -208 R0 SYMATTR InstName Cres

SYMBOL res 1728 -336 R0 SYMATTR InstName Rload SYMATTR Value 10k SYMBOL cap 1840 -320 R0 SYMATTR InstName Cload SYMATTR Value 10pF SYMBOL voltage 1376 -320 R0 WINDOW 123 24 44 Left 2 WINDOW 39 0 0 Left 2 SYMATTR Value2 AC 1mV SYMATTR InstName Vnoise SYMATTR Value "" SYMBOL bv 1376 -224 R0 WINDOW 3 24 96 Invisible 2

SYMATTR Value V={0.00001*sqrt(12/5/dt)*(rand(time/dt)+rand(tstop/dt+1+2*int(time/dt))

SYMATTR InstName Bnoise01 SYMBOL cap 1616 -576 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName Cfdbk SYMATTR Value 1000pF SYMBOL cap 1264 -400 R0 SYMATTR InstName Cbias SYMATTR Value 1nF TEXT 544 -520 Left 2 !.options plotwinsize=0\n.param k=1.38e-23 T=300\n.param N=200000 dt=5uS TEXT 536 -280 Left 2 !.param fstart={1/N/dt} fstop={1/2/dt}\n.param tstart={0.1*N*dt} tstop={1.1*N*dt} TEXT 544 -360 Left 2 ;.tran 0 {tstop} {tstart} {dt/10}\n.save V(out) TEXT 544 -416 Left 2 ;.ac dec 100 1kHz 10MEG TEXT 1496 -328 Left 2 !.param R=20k TEXT 584 -184 Left 2 !.tran 0 100m0 {dt/10}