Sensing small inductances

Jeroen's sym does not reflect reality. How do you control the amplitude if an oscillator is running on negative resistance.

Ok so I figure you don't have time to measure free running oscillator frequency to very high precision but since the inductance is pretty much confined to a small range you might be able to use very simple old technique like the 3 voltmeter method. No phase sensitive detectors required.

Select R (an ordinary positive resistor) and frequency for best ranging. Controller will have to do some trig to extract L value (and r loss resistance value of the coil) but if your range is limited lookup tables may be enough.

piglet

Any stable oscillator, with any gain mechanism, has to have a nonlinear mechanism to set the oscillation amplitude. You can think of a negative resistance as being nonlinear on sinewave amplitude, or you can imagine it to be a theoretically perfect negative resistor in parallel with a tank, in parallel with some sort of hard or soft clipper that adds lossy shunt resistance as oscillation amplitude increases. Something has to servo the gain to exactly 1 at the operating amplitude. Barkhausen magically assumes the gain is

So it's difficult to use one negative resistance element to make two tanks oscillate; if the gain is enough for one to oscillate, it's not enough for the other, lower-Q one.

Transistors used to be expensive, so the original TouchTone phone managed to oscillate at two frequences simultaneously using one transistor.

Sneaky.

Hey Steve, is negative resistance the same as having a pole in the right half plane? Or are those two different things?

I wonder if you've ever built a Wien-bridge oscillator?

George h.

If you want a clean sine wave, you need a controllable gain element.

Jim Williams application notes on Wien Bridge oscillators used a FET as a c ontrolled resistor.

My current mirror variation on the Baxandall class-D oscillator used an asy mmetrical current mirror,and trimmed the asymmetry to get exactly the gain needed to sustain oscillation.

I've simulated a Wien bridge oscillator that used an AD734 multiplier to ad just the gain in the same sort of way.

A controllable gain element is intrinsically non-linear, but some at least can be set to give a linear (if variable) gain.

You've got to rectify the output of the oscillator to find out what amplitu de output it is producing, and filter the rectified signal before you use t o set the gain, but that's doable, if you wanted to be really cute , you co uld cancel out the residual ripple that would otherwise get into the gain-c ontrolling signal, or use a sample and hold scheme to clean it up.

litude, or you can imagine it to be a theoretically perfect negative resist or in parallel with a tank, in parallel with some sort of hard or soft clip per that adds lossy shunt resistance as oscillation amplitude increases.

You can think of lots of things, but most of design involves discarding the less productive thoughts.

Something has to servo the gain to exactly 1 at the

That's what feedback is for.

Irrelevant, I can fix it.. the fact remains that it does rail. (w/o 'fix') I guess an excuse could be that watching it rail, helps you understand the oscillator.

George H.

Huh, neat. (I have a hard time following what the transformers are doing.)

Are RV1 and RV2 the non-linear gain control do-thingies?

George H.

Yes. Each parallel LC tank sees the series-shared negative resistance but is paralleled by its own local varistor, so it oscillates but limits its own amplitude.

Seems weird now, but switch matrices and multi-winding transformers and varistors were cheaper than transistors back then.

Many oscillators inherently rail, for example common CMOS XOs. There is no other amplitude limiting mechanism. Nothing wrong with that.

Some oscillators self-rectify at the gate or base and bias themselves off at some not-hard-railed amplitude. I invented this one when I was a kid; it flew on the C5A.

The p-p amplitude is almost exactly 2x V+, and it has a tiny flat on the negative swing of the sine wave. At the negative swing peak, it steals its own base bias. It even has a near-zero amplitude tempco.

That seems plausible, thanks! yeah the original problem is that when DUT is in a regular tank, when you can get it to oscillate stability at all (at a frequency that's not say 150 MHz and kills the power budget on the oscillator and uP clock speed requirement, both), the deviation with respect to say a 1 nH change is too few Hz for the uP to distinguish reliably from noise. The oscillators love to drift.

On Aug 29, 2019, snipped-for-privacy@highlandsniptechnology.com wrote (in article):

OK. I agree that a one-port amp must present a negative effective resistance to the resonant part of the circuit to support oscillation. But not all oscillators are one-port.

One can also build an oscillator with an amplifier and a length of twisted-pair transmission line connecting input to output. Someone got a patent for putting a half-twist in the line, to get the 180 degree phase shift. Nice and small, fits into an IC.

Joe Gwinn

On 2019-08-30 03:45, Joseph Gwinn wrote: [Snip!]

I like to think of a laser as a distributed negative-loss transmission line for optical frequencies.

Jeroen Belleman

The idea of negative resistance does not fit for such oscillators, I agree. I've done that with ECL logic gates as the amplifier. Very nice for snappy stop/start behaviour. Gosh, that was in the late 1980's, over 30 years ago.

Jeroen Belleman

Not true.

In a cc Colpitts, you set the amplitude by changing the current through the emitter follower. This changes the energy delivered to the tank so it is equal to the loss in the tank. This is a perfectly linear operation.

However, this presupposes the oscillator already meets the Barkhausen criteria, which states the loop gain must be equal to or greater than 1, and the phase shift must be zero or multiples of 360 degrees.

So the oscillator is not running on negative resistance. It is running on Barkhausen. It will oscillate regardless of negative resistance, so negative resistance really does not exist.

False. You do not want to add any lossy shunt resistance to the tank. This reduces the Q and increases the phase noise.

There is no clipping in a properly designed oscillator. Clipping increases the noise, which is critical in precision oscillators. You also have to be able to set the amplitude correctly for crystal oscillators. If it is too low, the crystal may not start. If it is too high, the crystal may fracture.

False. Barkhausen requires the loop gain to be equal to or greater than 1 for oscillations to start.

You set the loop gain by adjusting the energy into the tank to be equal to the loss. This sets the loop gain to be exactly 1.

Bruce Griffiths and Ulrich Rohde have described feedback methods to reduce the noise in oscillators. These also happen to set the loop gain to 1, but at a slightly higher level. The oscillator must already meet the Barkhausen critera and be oscillating before the feedback can operate.

Your statements prove that oscillators do not run on negative resistance, and it has no effect on a running oscillator.

Therefore, it does not exist.

George Herold wrote:

I have no idea. I do know that negative resistance has no effect on the operation of an oscillator.

No, but I worked on plenty in NATO after WWII as an instrument repair specialist. They were pretty reliable compared to some of the other stuff.

Of course, you'd never use a Wien bridge these days due to the phase noise and drift. But if you'd like to try an ultra low distortion audio oscillator, try this one. Cheap and simple, and it will beat any other oscillator for total harmonic distortion. All you need is a ganged pot from an old stereo amplifier marked as R1 and R2 in the schematic.

Version 4 SHEET 1 1204 680 WIRE 784 -128 0 -128 WIRE 928 -128 784 -128 WIRE 1088 -128 928 -128 WIRE 1120 -128 1088 -128 WIRE 0 -96 0 -128 WIRE 976 -96 848 -96 WIRE 992 -96 976 -96 WIRE 848 -64 848 -96 WIRE 784 -48 784 -128 WIRE 816 -48 784 -48 WIRE 144 -32 80 -32 WIRE 928 -32 928 -128 WIRE 928 -32 880 -32 WIRE 240 -16 208 -16 WIRE 272 -16 240 -16 WIRE 448 -16 272 -16 WIRE 512 -16 448 -16 WIRE 624 -16 592 -16 WIRE 800 -16 624 -16 WIRE 816 -16 800 -16 WIRE 0 0 0 -32 WIRE 112 0 0 0 WIRE 144 0 112 0 WIRE 624 0 624 -16 WIRE 0 16 0 0 WIRE 240 16 240 -16 WIRE 848 16 848 0 WIRE 448 32 448 -16 WIRE 848 32 848 16 WIRE 992 32 992 -96 WIRE 624 80 624 64 WIRE 0 112 0 96 WIRE 80 112 80 -32 WIRE 176 112 80 112 WIRE 240 112 240 96 WIRE 240 112 176 112 WIRE 240 128 240 112 WIRE 848 128 848 112 WIRE 992 128 992 112 WIRE 448 144 448 96 WIRE 544 144 448 144 WIRE 576 144 544 144 WIRE 624 144 576 144 WIRE 544 160 544 144 WIRE 624 176 624 144 WIRE 240 224 240 208 WIRE 432 224 240 224 WIRE 448 224 432 224 WIRE 448 240 448 224 WIRE 544 240 544 224 WIRE 240 256 240 224 WIRE 544 304 496 304 WIRE 624 304 624 256 WIRE 624 304 544 304 WIRE 624 320 624 304 WIRE 240 352 240 336 WIRE 448 352 448 336 WIRE 624 416 624 400 FLAG 624 80 0 FLAG 0 112 0 FLAG 992 128 0 FLAG 848 128 0 FLAG 976 -96 +12 FLAG 176 -48 +12 FLAG 848 16 -12 FLAG 176 16 -12 FLAG 112 0 U1P FLAG 272 -16 U1O FLAG 176 112 U1N FLAG 800 -16 U2P FLAG 1088 -128 VOut FLAG 240 352 0 FLAG 448 352 0 FLAG 432 224 J1D FLAG 544 304 J1G FLAG 624 416 0 FLAG 544 240 0 FLAG 576 144 C3R6 SYMBOL res 608 -32 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value {RT} SYMBOL res 16 112 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R2 SYMATTR Value {RT} SYMBOL cap 608 0 R0 SYMATTR InstName C1 SYMATTR Value 1.5n SYMBOL cap -16 -96 R0 SYMATTR InstName C2 SYMATTR Value 1.5n SYMBOL res 224 0 R0 SYMATTR InstName R3 SYMATTR Value 12k SYMBOL res 224 112 R0 SYMATTR InstName R4 SYMATTR Value 11K SYMBOL voltage 848 16 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value -12v SYMBOL voltage 992 16 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value +12v SYMBOL opamps\\lt1001a 176 -80 R0 WINDOW 3 -32 -18 Left 2 SYMATTR InstName U1 SYMBOL opamps\\lt1001a 848 -96 R0 SYMATTR InstName U2 SYMBOL res 224 240 R0 SYMATTR InstName R5 SYMATTR Value 1K SYMBOL njf 496 240 M0 WINDOW 0 -11 31 Left 2 WINDOW 3 -29 93 Left 2 SYMATTR InstName J1 SYMATTR Value 2N4416 SYMBOL diode 432 96 M180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D1 SYMATTR Value 1N4148 SYMBOL res 608 160 R0 SYMATTR InstName R6 SYMATTR Value 470k SYMBOL res 608 304 R0 SYMATTR InstName R7 SYMATTR Value 470k SYMBOL cap 528 160 R0 SYMATTR InstName C3 SYMATTR Value 1u TEXT 296 -304 Left 2 !.tran 0 {ST} 0 {MT} startup TEXT 376 -336 Left 2 ;'Ultra Low Distortion Audio Oscillator TEXT 832 -256 Left 2 !.param RT = 2e4 TEXT 832 -304 Left 2 !.param ST = RT / 1.5e6 TEXT 832 -280 Left 2 !.param MT = ST / 10000 TEXT 288 -184 Left 2 ;Original Version:

LOL!

I already mentioned that regarding Pierce oscillators. You adjust the oscillator amplitude by changing the resistor between the CMOS output and the input to the tank.

Many Pierce crystal oscillators badly overdrive the crystal. Some don't even have a resistor. I assume they rely on weak drive capability of the CMOS.

I forgot to mention - Bruce Griffiths method is shown in file # 07.asc in Oscillators.zip

Ulrich Rohde's method was too simple to be much challenge for my fast start method.

The Pierce crystal oscillator is shown in files # 08.asc and # 09.asc

What determines the amplitude of the voltage in the tank?

If the gain element delivers constant output power regardless of input, it's nonlinear.

If it's linear, how does it know exactly how much gain to have to keep the amplitude constant?