Author Topic: A simple measurement  (Read 3250 times)

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Offline akis

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A simple measurement
« on: May 14, 2012, 08:54:13 pm »
Hello, can anyone please run this test and tell me what they are seeing on their scopes?

As you can see on the attached picture, I have a battery powered oscillator producing +/- 5V at 200KHz. Actually even when powered by the bench PSU it makes no difference to the test.

I am using one probe 10X, or two probes 10X in the "add" setting. I try to take measurements between points (a) and (b), or (b) and (c). The probes have a capacitance so I expect to see a bit less than half the total voltage across a-b and b-c.

The very weird thing is that if I reverse the probes, eg probe X on (a) and probe Y on (b), or in the case where I am using a single probe, if I attach the earth lead on (b) and the tip on (a) rather than the other way round, then I get 5% of the voltage on the scope or nothing.

The little oscillator is not related to earth in anyway, or at least in any way I can imagine. It is a 3"x5" breadboard on a wooden benchtop, I cannot imagine that some parts of this small breadboard have more affinity to the earth than other parts. Also the frequency is just 200KHz, not in the 100s of MHz for such weird things to be happening.

But clearly this is what is happening - somehow the little oscillator establishes an earth path at 200KHz even though there is no "earth" anywhere near it (battery operated).

Or maybe my scope/probes are defenctive - or I am making some other mistake?


 

Offline MikeK

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Re: A simple measurement
« Reply #1 on: May 14, 2012, 09:36:50 pm »
You have two identical resistors, so they will drop equal voltages across them.  Which means that Vab=Vbc.  And resistors don't introduce any phase difference.
 

Offline bilko

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Re: A simple measurement
« Reply #2 on: May 14, 2012, 09:42:36 pm »
You've checked that there is no DC offset on the oscillator output?
 

Offline IanB

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Re: A simple measurement
« Reply #3 on: May 14, 2012, 09:58:37 pm »
You don't accidentally have your scope or probes set to 50 ohm input termination?
I'm not an EE--what am I doing here?
 

Offline bilko

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Re: A simple measurement
« Reply #4 on: May 14, 2012, 11:08:12 pm »
symptoms of a diode across the scope or probe, tin whiskers perhaps ?


From wiki http://en.wikipedia.org/wiki/Cat%27s-whisker_detector

Whisker

The "cat's whisker", a springy piece of thin metal wire, formed the metal side of the junction. Phosphor bronze wire of about 30 gauge was commonly used because it had the right amount of springiness. It was mounted on an adjustable arm with an insulated handle so that the entire exposed surface of the crystal could be probed from many directions to try to find the most sensitive spot. Cat's whiskers in simple detectors were straight or curved, but most professional cat's whiskers had a coiled section in the middle that served as a spring.[3] The crystal required just the right gentle pressure by the wire; too much pressure caused the device to conduct in both directions. Precision detectors often used a metal needle instead of a cat's whisker, mounted on a thumbscrew-operated leaf spring to adjust the pressure applied
« Last Edit: May 14, 2012, 11:13:00 pm by yachtronics »
 

Offline amspire

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Re: A simple measurement
« Reply #5 on: May 14, 2012, 11:54:54 pm »
There are a few things.

First, after reading your question, I was not sure exactly how you were connecting the earth leads. The earth leads can only be connected to one point on the circuit, so if you connected both earth leads to b, and one probe on a and one on c, then in add mode you should get zero volts out (as long as you are not overloading either channel). Also you do need to connect the earth leads of both probes to the circuit. Do not leave one probe with the earth lead disconnected.

If you have the two earth leads clipped on to different parts of the circuit (like b and c) all you are doing is shorting out b and c.

What could still cause some unbalance is the fact that all objects have a self capacitance which is related to the amount of charge needed to raise the voltage on an isolated conductor - like static charges on a comb. This is roughly equivalent to a capacitance to earth.

Depending on how your oscillator is built, it is likely that one side of the oscillator has a bigger self capacitance then the other.

A 2pF differential in self capacitance would be about 800K which would cause a noticeable unbalance. Whether the self capacitance is an issue or not depends totally on the area of the conductive surface attached to the common output of the oscillator so a small PCB will have a minute self capacitance, but if it is connected to a metal case directly or via capacitance, it could be significant.

Richard.
« Last Edit: May 15, 2012, 12:00:06 am by amspire »
 

Offline akis

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Re: A simple measurement
« Reply #6 on: May 15, 2012, 09:08:34 pm »
There are a few things.

First, after reading your question, I was not sure exactly how you were connecting the earth leads. The earth leads can only be connected to one point on the circuit, so if you connected both earth leads to b, and one probe on a and one on c, then in add mode you should get zero volts out (as long as you are not overloading either channel). Also you do need to connect the earth leads of both probes to the circuit. Do not leave one probe with the earth lead disconnected.

If you have the two earth leads clipped on to different parts of the circuit (like b and c) all you are doing is shorting out b and c.

What could still cause some unbalance is the fact that all objects have a self capacitance which is related to the amount of charge needed to raise the voltage on an isolated conductor - like static charges on a comb. This is roughly equivalent to a capacitance to earth.

Depending on how your oscillator is built, it is likely that one side of the oscillator has a bigger self capacitance then the other.

A 2pF differential in self capacitance would be about 800K which would cause a noticeable unbalance. Whether the self capacitance is an issue or not depends totally on the area of the conductive surface attached to the common output of the oscillator so a small PCB will have a minute self capacitance, but if it is connected to a metal case directly or via capacitance, it could be significant.

Richard.

Hi I have done more tests. This "simple" test is not easy, is not intuitive and is weird! Certainly you must know and understand your scope as well as all other parameters.

First the probes must be calibrated in the 10X setting using a square pulse. I found that the calibration is different at 10KHz than at 1KHz. I settled with 10KHz calibration.

Then the channels A and B must be adjusted so that in differential mode (ADD) the sum is 0 for the same source signal at the highest sensitivity (eg 5mV per division). If you do not do that then you will read different values when you reverse the probes.

Still there are readings I do not easily understand. I have taken them all down and will have to study more in case I discover some pattern or explanation.

The self capacitance bit is also something I have to understand.
 

Offline amspire

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Re: A simple measurement
« Reply #7 on: May 16, 2012, 12:01:24 am »
What you have is basically two legs of a bridge circuit. In bridge circuits, a common way to check the balance is to make a center tapped transformer so you can turn the unbalanced signal from the oscillator into a true balanced sinewave.

If you wind two secondary windings together onto a nice large ferrite toroid, you can get an extremely high amplitude and phase balance between the two windings so that the center tap is an extremely accurate mid voltage. If you have the two resistors mounted in a very symmetrical way, so that neither resistor is closer to another conductive surface, you can see the balance using a scope or meter with the earth lead on the transformer center tap, and the probe tip on the connection of the two resistors (or capacitors or whatever else you are matching).

If the resistors are not mounted in a symmetrical way, it is not hard to get an additional 1pF circuit coupling across one resistor compared to the other, and that is enough to wreck the balance.

In this sort of configuration, at balance, the effect of the probe resistance or capacitance is totally eliminated.

When you are matching parts, the purer the sinewave the better, otherwise instead of needing a balance at 10KHz, you end up needing a balance at 10kHz, 20KHz, 30KHz, 40KHz, .....

On the scope at least, it is usually easy to see when the primary harmonic is at zero.

Richard.
 

Offline vk6zgo

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Re: A simple measurement
« Reply #8 on: May 16, 2012, 07:04:39 am »
Hello, can anyone please run this test and tell me what they are seeing on their scopes?

As you can see on the attached picture, I have a battery powered oscillator producing +/- 5V at 200KHz. Actually even when powered by the bench PSU it makes no difference to the test.

I am using one probe 10X, or two probes 10X in the "add" setting. I try to take measurements between points (a) and (b), or (b) and (c). The probes have a capacitance so I expect to see a bit less than half the total voltage across a-b and b-c.

The very weird thing is that if I reverse the probes, eg probe X on (a) and probe Y on (b), or in the case where I am using a single probe, if I attach the earth lead on (b) and the tip on (a) rather than the other way round, then I get 5% of the voltage on the scope
 or nothing.

The little oscillator is not related to earth in anyway, or at least in any way I can imagine. It is a 3"x5" breadboard on a wooden benchtop, I cannot imagine that some parts of this small breadboard have more affinity to the earth than other parts. Also the frequency is just 200KHz, not in the 100s of MHz for such weird things to be happening.

But clearly this is what is happening - somehow the little oscillator establishes an earth path at 200KHz even though there is no "earth" anywhere near it (battery operated).

Or maybe my scope/probes are defenctive - or I am making some other mistake?

It looks to me that you are over-analysing this.

Forget about "add" for the moment,& switch your Oscilloscope so that it shows two separate traces for  Channel (1) & Channel (2).
Check that both channels give the same amplitude display by touching the probes to your calibrate output in turn.
(1)
Connect the earth clip of CH1 to "c"
Place the probe tip on "a" -examine the CH1 display amplitude.& write it down on a piece of paper.
Repeat the process with the probe tip on "b".
Compare the readings---- Vbc should be very close to Vac/2. --record the result.
Move the earth clip to "b",place the probe tip on "a" ,then "c"---Vab  should be  very close to = Vcb.

(2) Remove  the CH1 probe entirely & repeat all the above using CH2,again recording the readings.

(3)Compare the results from (1) & (2) ,they should be very nearly identical.

(4) Clip the earth leads of both probes to "b",then place CH1 probe tip on "a",& CH2 probe tip on "c".

The two traces should show  waveforms of very nearly the same amplitude,but 180 degrees out of phase.(In other words,the +ve 1/2 cycle of one lines up with the -ve 1/2 cycle of the other.)

Now swap CH1 probe tip to "c",& CH2 probe tip to "a"---the two waveforms should swap channels.

(5)Now,with the 'scope still in the same setting,remove all probe connections & in turn,connect CH1 probe with the earth clip on"a"
& the probe tip on "b",then reversed,with the earth clip on "b",& the probe tip on "a".
I think you will find the same waveform  amplitude in each case.--record the result
Do the same thing with points "b" & "c".

(6) Repeat (5) but with the other channel.

(7) Repeat the connections of (4),verify that you are seeing the same as you saw before--then switch to "ADD".
At this point you should see a flat line,or very close to one.
Thinking back to what you found in (4),why do you think this is?*

If you still get weird problems,delete the oscillator & just connect the voltage divider across the battery,switch your 'scope to "DC coupling",& repeat the steps above.

* From your other posting,it appears that your Oscilloscope inverts one channel in the "ADD" mode,so I may be wrong here--I don't have a 'scope in front of me.
If it does invert,then the two signals will then be in phase & will add.
I'll just have to go out the back room & fire up the Tek 7613! ;D

« Last Edit: May 16, 2012, 07:18:31 am by vk6zgo »
 


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