A Lock-In Riddle

Have you actually modelled what happens when you apply a DC biassed

10mV rms sine wave across a 3mV step in diifferential conductance versus voltage?

Is there a capacitance associated with the changing conductance? You might be seeing a phase shift - your digital lock-in incorporates a phase-sensitive detector.

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Bill Sloman, Nijmegen

Yes. You get a cos(arcsin(U/Up))-shaped peak centered around the step. Up = 1.41*Umod(eff)

Not a changing one. Of course there's the parallel capacitance of the tunneling gap and then some.

robert

Clarification: "uniform conduction" should mean: constant current independent of voltage.

robert

I don't totally understand your system but....

a lock in amp is a PLL (phase locked loop) and a PLL is a narrowband device...

sounds like you are applying a sine wave excitation to a non-linear device and trying to measure the I-V characteristic....the non linearity will distort the sine wave and the distortion is the data you want to measure, but distortion is harmonics and the lock in amp being narrow band will reject harmonics...

so I don't know what you will get...

why use AC excitation to measure I/V, why not DC?

Mark

Some lock-ins will detect and measure higher harmonics - there are a couple of physical techniques that depend on finding the second harmonic content of the an output. I'm damned if I can remember what they are ...

Threre ws an improved GaAs crystal puller that was going to detect the contact angle between the melt and the growing crystal by modulating the heat input - and thus the height of the solid-liquid boundary - and detecting the second harmonic content in the "weight"of the crystal (which includes the surface tension force acting on the circumference of the crystal. Don't know if it ever worked.

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Bill Sloman, Nijmegen

Well, it's a synchronous rectifier (demodulator), not a PLL.

Right.

Well, the idea is to make the modulation so small that the system looks linear at that particular offset. Of course the step function I discussed is a gross violation of this premise; it was just a device to estimate the smallest possible feature width obtainable by this technique.

The differential conductance.

Because the signal is buried in lots of noise.

Mind you, this is a well-established technique. We've been using it for years. It's just that these tiny peaks have been showing up occasionally and we don't want to ignore them any more.

The question is:

If, using lock-in technique, I find features that are much narrower than the pp excitation voltage, one of the following statements must be true

1) The presumption that a lock-in amplifier cannot yield features that are narrower than about 2.5*Umod(eff) is false. (Of course it can detect narrower features, but they will result in broader peaks)

2) The impedance characteristics of the system being measured are not time-invariant but change in sync with Umod.

3) The lock-in amplifier introduces a measurement artefact.

I'm somewhat puzzled. If 1) holds, I'd really appreciate an explanation. Also some LIA manuals would have to be rewritten.

2) would be odd because this has been observed on different tunneling systems, both metallic and semiconductor.

3) Unlikely, because the peak is reproducibly seen over certain locations of the sample and not over others. Also it is always at exactly zero bias voltage, about which the AC-coupled LIA knows nothing.

--Daniel

That sounds like good physics.

and quadrature outputs from your experiment at the excitation frequency, twice the excitation frequency and three times the excitation frequency?

The quadrature outptus may tell you something about any reactive component in the response of your specimen, and the higher harmonics could say something about non-linear interactions - second harmonic about even order terms in a power law response, third harmonic about odd order terms (square law versus cube law componenets etc).

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Bill Sloman, Nijmegen

In article , wrote: [....]

Another thought along the same sort of lines:

You may have lossy capacitances.

If the insulation of the wiring etc does not make good capacitors, you may have a lossy capacitance which will cause an in-phase compoent in the current that passes throught it. This effect will change with frequency so you should be able to check for it.

Stray capacitances may be interacting with the experimental area resulting in what looks like a lossy capacitor across the terminals. When the lossiness of this capacitor changes, it changes the in-phase component of the current.

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kensmith@rahul.net   forging knowledge

Let me try to understnad better what you are doing,,,

applying a small AC excitation current to a DUT (device under test) and using a lock in amp (PLL based synchronous demodulator) to recover the low level AC signal and measure its amplitude and from this info you can deduce the ___DIFFERENTIAL ___ resistance of the DUT.... and at the same time you are applyting a larger DC current to the DUT to move the operating (bias) point around at which you which to measure the small differential resistnace... is this correct?

What if the DUT has a step function non-linearity or maybe an I/V transfer with hysteresis or other "memory effect" , at some DC bias points , would that cause the effect you are seeing?

Mark

While people are thinking about them: I need a lock-in for, frequencies from 50KHz to 750KHz. Are there any suggestions of ones to look at or avoid from the group.

Other characteristics I'd like:

(1) I'd prefer the two channel type that allows phase shifts to be seen directly.

(2) My experimental setup will be sensitive to EMI, basically from DC to 1MHz so low radiation in that band is desired. I already have to live with lost of 60Hz so I have designed around that problem. As a result, 60Hz and its harmonics aren't a huge problem to me.

(3) Because of (2) I worry about anything that needs a PC to control. I want to avoid switching power supplies as much as posible,

(4) My input signal may be frequency modulated. I need the lock-in to be able to follow the modulation. The FMing will be by about 3% at a rate up to about 10KHz.

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kensmith@rahul.net   forging knowledge

a voltage, actually

the current through the DUT (a tunnel junction)

Exactly.

.... and at the

A voltage

yes.

Then I should see a peak in the dI/dV(V) curve that is about as wide as the peak-peak amplitude of the modulation signal. What I'm seeing is a peak that has only one-tenth the width.

That's what I exclude because the effect has been seen on metal systems as well which don't exhibit such slow charging effects.

--Daniel

Lock-ins typically have both in them. They lock the PLL onto the reference or the signal its self and then use that to time the demodulator.

[....]

You don't have to assume that the system is linear. The non-linear effect adds even harmonics to your output. With only a little added trickiness, you can measure the "distortion" products this could give you better results with the same basic set up. Here's basically the idea. We can flesh it out if you want to do it.

(1) Find (or you are) someone who knows which end of a soldering iron is which.

(2) For your few hundred Hz, use a synth that can either take or give you a reference at, lets say, 10MHz.

(3) Lets say your "few 100Hz" is really 100Hz

(4) Your output is the 10MHz divided by (10MHz/100) = 100,000

100,000 = 2^5 * 5^5

Do the 5s first and then the 2s

(5) Add some clever circuit to the reset pin of the counters so that we can let them start at the right time in the cycle.

(6) All the divide by two stages give you signals that can be used on the lock-in to isolate the harmonic.

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kensmith@rahul.net   forging knowledge

I do not understand your reply... If the step function change in differential resistance is from say 0.1 Ohms to 1 Meg Ohm, would that not create a very large retrun signal.... I don't understand what you mean by "width". Also I do not understan, you are applying an excitation VOLTAGE and measuring an return VOLTAGE??? If you want to infer the differentiual tesistance, you need to have current in there someplace...no? i.e apply and excitation current and measure the return voltage...or apply an excitation voltage and measure a return current??? Perhaps you are measuremting the return current indirectly by measuring the voltage across a fixed reesistor that is in series with the DUT and excitation voltage???

You are saying this is a well established procedure and I am not questioning that, but if you would like us to help you, we need to understand the well established procedure better.

I don't know what you mean by "slow charging effect". I'm envisioining your DUT as some kind of a spark gap or something like a neon bilb that can break down and have all kinds of non-linear behavior???

So does the effect change if you change the excitation frequency up or down by say an order of magnitude?

Darwin comes home to roost.

I'm not *that* old; 1974. BTW, this was in NY, not NYC. It was in one of my hens to 104 degree temps. One black turken and one of the recommended ways of acquiring free marijuana is: "Be very very nice to everyone you meet." :-) Cheers!

Who are you, and why don't you make any sense?

Even your nonsense is pretty low-quality stuff.

John

[snip]

He's a broad-spectrum spammer. Just killfile on name, until he changes to a new one.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
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I love to cook with wine.      Sometimes I even put it in the food.

It took me about four of these posts to figure out that this is either a raving lunatic or a bot.

Cheers! Rich

In article , Rich Grise wrote: [....]

I remember when S C I E N T O L O G I S T S would do this because someone mensioned them in a news group. They did it to push the article off the servers by filling them up.

Maybe this is being done again.

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kensmith@rahul.net   forging knowledge