Oscilloscope question.

[radiograham]

When observing various waveforms recently I was confused by them being inverted to what was on the circuit diagram. Am I right in saying that a scope with a single stage Y-amp inverts the waveform? I'm a bit reluctant to ask this question and show my doziness. i have read umpteen books and none seem to explain. The waveforms were the output from a video detector and the video amp, in a 405 line TV of course .Hoping this doesn't show my lack of technical know how too much.

Regards, Graham.

[Restoration73]

In the typical valve or transistor video amplifier, inversion occurs. Many scopes have a
switch to permit the waveform to be inverted when required. I'm assuming you are
using a x10 oscilloscope probe, otherwise circuit operation could be affected.
You will find that for positive or negative going video the simplest method is to reverse
the video demodulator diode via the standards switch of the tv in question.

[peter_scott]

The sense of the video signal at the CRT depends on whether it is being applied to the grid or to the cathode. If applied to the grid then peak white will be positive but if applied to the cathode then the video waveform will appear to be upside down.

Peter

[Amraduk]

Graham is saying that the waveform he's seeing on his scope is inverted when compared to the waveform shown on the circuit diagram. Without knowing more about his scope, I don't know if it's the scope that's inverting the waveform (differential Y amp, perhaps) but, generally, if the waveform on the circuit is positive going, then surely it should be positive going on the scope.

Regards,

Dave.

[barrymagrec]

Some scopes have an invert switch on one channel......

[ajgriff]

Originally Posted by radiograham

When observing various waveforms recently i was confused by them being inverted to what was on the circuit diagram.Am i right in saying that a scope with a single stage y-amp inverts the waveform? .......

Surely the simple answer to the question is 'no' unless the oscilloscope has an invert option which has been selected by the operator.

Alan

[John Caswell]

Unless it is a specialised 'scope, all 'scopes show a positive going voltage/waveform on the screen as going from bottom -ve to top +ve, otherwise this would make mockery of pretty well all waveform drawings as they are based on the standard X,Y cross 2 dimensional format where the intersection of X and Y is 0
Y up is +ve Y down is -ve X right is +ve and X left is -ve
I am sure someone will explain this better than I.

John

[Graham G3ZVT]

All the comments about grid vs cathode drive, differential amps and inverting switches are correct, but I've never seen a scope where by default, the convention was otherwise than up=+ve going.

Edit.
I've just had a thought, is this a dual standard TV you are working on?

[micheal]

If Radiogram was only slightly confused before he should now be completely
confused.

Mike

[Skywave]

Graham - here's a simple test that should help diagnose what is going on - and what isn't.

1. Set the Y-amplifier for 1v. / div.; select the input for d.c.; position the scope trace in the middle of the screen.
2. Do not use a X10 'scope probe: use a piece of co-ax with a suitable connector for your 'scope at one end and simple, exposed bare wires at its other end.
3. Find a nominal 1.5v. battery: identify which end is +ve. and which is -ve.
4. Connect the -ve. end of the battery to the co-ax outer.
5. Connect the +ve. end of battery to inner of coax.
6. As a result of step 5, the 'scope trace will move its position on the screen.
7. Which way did the trace move? If it moved upwards, (should be about 1½ divs.) that is correct. If it moved downwards, then something is wrong with the 'scope: check if the 'scope has an 'invert' switch for that channel: if it has, it might be set to 'invert'.

HTH,

Al.

[radiograham]

The scope is a very basic one actually a GEC miniscope, I wasn't servicing any particular TV just playing around with it. The TV was a 50's circuit by EN Bradley using a 3BP1 tube published in Practical Television Nov 52. This uses grid modulation of the CRT which needs a positive going output from the video amp. I've put a few photos on here of what my scope showed me. Graham.

[Radio Wrangler]

Yes, a single stage amplifier is most usually inverting, but an electrostatic deflection scope tube has a pair of Y plates, so the maker of the scope chooses which way round to connect the plates so that positive at the front panel terminals moves the spot upwards.

As for 'Z - modulation' the old way of a coupling capacitor to the grid of a scope tube would brighten the spot on positive swings, darken it on negative swings.

In a TV, video applied to a CRT grid goes positive for brighter. Video applied to a CRT cathode has to go negative for brighter.

UK 405 line transmissions employed positive AM modulation. RF signal was strongest for white, weakest for sync pulses/
UK 625 line transmissions employed negative AM mod. Sync pulses got the peak RF power, white was the lowest RF power.

The change was based on the idea that sync pulses needed to be good, otherwise the set had no idea where to write all of the rest of the picture, and that noise/snow showed up less on white areas, so black got a better signal to noise ratio than white.

Detectors in sets could either produce positive going or negative going voltages from the RF/IF signal. Depends on which way round the diode is connected. Negative going was popular because it made a negative-going AGC voltage that could be applied almost directly to RF/IF stages.

So probing around sets will produce a mixture of upside down and right way up video waveforms. You just get used to it.

David

[Skywave]

Graham - the OP - wrote:

Originally Posted by radiograham

When observing various waveforms recently I was confused by them being inverted to what was on the circuit diagram.

The phrase "various waveforms" are not stated to be applicable to only TV sets. Yet recent comments seem to imply that that is indeed the case. My earlier post 10 referred to general use of this 'scope, not in a particular application. So with that in mind, perhaps my earlier post is not that helpful: sorry.

Al.

[radiograham]

No problem Al, i did do that test you describe and it was ok. I try to be as concise as poss when posting a question but sometimes drone on and on till i froth at the mouth and fall over backwards. (Monty Python) about 1969/70.

[radiograham]

Originally Posted by Radio Wrangler

Yes, a single stage amplifier is most usually inverting, but an electrostatic deflection scope tube has a pair of Y plates, so the maker of the scope chooses which way round to connect the plates so that positive at the front panel terminals moves the spot upwards.

As for 'Z - modulation' the old way of a coupling capacitor to the grid of a scope tube would brighten the spot on positive swings, darken it on negative swings.

In a TV, video applied to a CRT grid goes positive for brighter. Video applied to a CRT cathode has to go negative for brighter.

UK 405 line transmissions employed positive AM modulation. RF signal was strongest for white, weakest for sync pulses/
UK 625 line transmissions employed negative AM mod. Sync pulses got the peak RF power, white was the lowest RF power.

The change was based on the idea that sync pulses needed to be good, otherwise the set had no idea where to write all of the rest of the picture, and that noise/snow showed up less on white areas, so black got a better signal to noise ratio than white.

Detectors in sets could either produce positive going or negative going voltages from the RF/IF signal. Depends on which way round the diode is connected. Negative going was popular because it made a negative-going AGC voltage that could be applied almost directly to RF/IF stages.

So probing around sets will produce a mixture of upside down and right way up video waveforms. You just get used to it.

David

Hi Thanks for the answer,i thought that was the case.I found it a bit confusing when checking the video w/form from the video amp which in this case needs to be positive going(grid modulation of the crt)and it was as you say upside down. Regards Graham.

[Skywave]

Originally Posted by radiograham

No problem Al, I did do that test you describe and it was ok.
I try to be as concise as poss when posting a question but sometimes drone on and on till I froth at the mouth and fall over backwards. (Monty Python) about 1969/70.

O.K. - I'm pleased that I was able to help.
As for your comment about your 'reactions', i.e. the Monty Python reference, your stated reactions are clearly preferable to stomping around your workshop doing a funny walk!

Al.