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!