Oscilloscope blanking (z modulation) possible on elenco s-1325?

[AndrewBourhis]

Hi,
I have here an elenco s-1325 analog oscilloscope. I am hacking it into a display unit for one of my projects and have designed two raster scans for the x and y-axis (the oscilloscope is running in x-y mode). I now need to be able to modulate the intensity of the electron beam so as to form an image on the screen. In order to do so, many analog oscilloscopes have a direct line to plug a bnc connector right into the rear panel (such as in the s-1330 model). Unfortunately, my model does not have such an option. I was wondering if anyone out there knew of a proper way to blank the crt with an external signal with models that do not have such a channel readily available? Do I have to mess around with the intensity potentiometer?

Please let me know if you need any more information to help.

Thanks a bunch,
-Andrew

 

[Baluncore]

If you download the pdf manual from;
http://www.elenco.com/admin_data/pdffiles/s1325.pdf
You will see on pages 29 and 30 that there are two versions.
Only the delay XX5 version has a Z-axis input on the rear panel.
On page 19 is written confirmation.

 

[AndrewBourhis]

If you download the pdf manual from;
http://www.elenco.com/admin_data/pdffiles/s1325.pdf
You will see on pages 29 and 30 that there are two versions.
Only the delay XX5 version has a Z-axis input on the rear panel.
On page 19 is written confirmation.

I have the thing sitting here. It does not have a z-axis input on the rear panel. It is a s-1325 model which does not have this input option, much like the s-1330 does that you are describing. I have been through those manuals, I was wondering if it is possible for a workaround of some sort.

 

[NascentOxygen]

I believe you identified the possible workaround. See if you can trace the wiring, to find where both ends of the intensity pot connect to. It would be easy if one end went to a fixed DC level and the other to earth. You could add something like a FET variable resistor in there.

 

[Baluncore]

When used in the normal time sweep mode it will have to blank it's retrace. If you can identify the signal from the sweep generator that causes the blanking you are well on the way to controlling the Z-mod. If you look at the block diagram on page 62 you will see the line from main sweep generator to CRT circuit, (via Z-axis amp if present).

I'm not certain, but I suspect the cathode and grid of the CRT will be at –1kV or more so be very careful playing with the signals near the neck of the tube.

It would be much safer and simpler to intercept the Z-mod control signal closer to, or at the sweep generator.

 

[AndrewBourhis]

I believe you identified the possible workaround. See if you can trace the wiring, to find where both ends of the intensity pot connect to. It would be easy if one end went to a fixed DC level and the other to earth. You could add something like a FET variable resistor in there.

I've never used a FET as a resistor before. Do they function in a similar manner to a typical field effect transistor, like a MOSFET? Would I reference the signal ground to the oscilloscope's ground?

I'll look into this as a potential option, thanks a bunch.

EDIT: I've been thinking. Would using a FET as a resistor lead to a nonlinear change in resistance between source and drain? If so, this could skew my input signal.

 

[NascentOxygen]

EDIT: I've been thinking. Would using a FET as a resistor lead to a nonlinear change in resistance between source and drain? If so, this could skew my input signal.

You haven't yet indicated what you wish to do with the z control, apart from "blanking".

 

[Baluncore]

They must have some way of doing it now. Find out how they do it and draw the circuit that controls the cathode to grid1 voltage. Once you have that circuit it should be possible to design a modification. Deciding what component to use before you know what the requirements are is a distraction.
My guess is that something like an opto-isolator would be needed to isolate a ground referenced analogue signal from the negative EHT voltage on the CRT cathode-grid circuit. Do they use an opto-isolator now for their digital blanking signal ?

 

[AndrewBourhis]

They must have some way of doing it now. Find out how they do it and draw the circuit that controls the cathode to grid1 voltage. Once you have that circuit it should be possible to design a modification. Deciding what component to use before you know what the requirements are is a distraction.
My guess is that something like an opto-isolator would be needed to isolate a ground referenced analogue signal from the negative EHT voltage on the CRT cathode-grid circuit. Do they use an opto-isolator now for their digital blanking signal ?

I would be surprised if there was an opto-isolator, but I will look into it. I hadn't really considered tinkering with the z-axis amp directly because the voltage potential that you describe between the cathode and wehnalt is too large to handle directly, although if I can find such an isolator, I might just do that. If I were to find such an isolator, would it be as simple as finding the control voltage and replacing it with my signal?

The other option I am looking at is to simply replace the intensity potentiometer that is on my front panel with some sort of electrically-controlled resistor, such as the type that Nascentoxygen suggested. The problem with this is that it would result in an inverted image which might be an issue for me.

To Nascentoxygen, I plan to blank the intensity with a signal from an Everhart Thornley Detector in a SEM, so as the ETD outputs a signal, it should be fed to the brightness to form an image as the raster scans both on the oscilloscope x and y and on the sample being viewed.

 

[Baluncore]

If I were to find such an isolator, would it be as simple as finding the control voltage and replacing it with my signal?

Possibly yes.
It will depend on the bandwidth of the link and if it is designed to be linear or on/off.
Between the cathode and grid there will be a resistor that is part of a voltage divider chain. If a transistor is placed across that resistor it can be used to control brightness. That transistor could be the opto-detector of the isolator. By varying the opto-emitter current linearly, the proportional opto-detector current should give a grey scale brightness variation.
Inversion of the brightness will be easy to reverse if it is needed. That part of the circuit will be determined by the specification of the output signal available from the ETD.
What is your scan rate and bandwidth requirement for the Z-mod. Is it a few MHz like analogue TV ?

 

[NascentOxygen]

To Nascentoxygen, I plan to blank the intensity with a signal from an Everhart Thornley Detector in a SEM, so as the ETD outputs a signal, it should be fed to the brightness to form an image as the raster scans both on the oscilloscope x and y and on the sample being viewed.

Are you using the word "blank" correctly? If so, I don't understand your earlier concern with linearity.

I don't see an inherent problem with inversion, not at this stage, anyway.

 

[AndrewBourhis]

Possibly yes.
It will depend on the bandwidth of the link and if it is designed to be linear or on/off.
Between the cathode and grid there will be a resistor that is part of a voltage divider chain. If a transistor is placed across that resistor it can be used to control brightness. That transistor could be the opto-detector of the isolator. By varying the opto-emitter current linearly, the proportional opto-detector current should give a grey scale brightness variation.
Inversion of the brightness will be easy to reverse if it is needed. That part of the circuit will be determined by the specification of the output signal available from the ETD.
What is your scan rate and bandwidth requirement for the Z-mod. Is it a few MHz like analogue TV ?

Yup, the bandwidth is in that range. I suppose you are also correct about the ETD signal as well. I'll do some tinkering tomorrow with the oscilloscope to see if I can get it working. Thanks a bunch for the help.

Nascentoxygen, if the transistor is not in it's ohmic state, the voltage on the gate terminal will have a non-linear affect on the drain to source current, which would mean that the electric signal fed from the ETD would not be the same as the signal being sent to the z amp if I were to replace the potentiometer with a FET resistor that did not produce a linear relationship. The toll this would have on my display unit would be 3d resolution, potentially creating spots of high intensity in the image. Of course, this could probably be avoided with some sort of circuit that I am unaware of. I'll do some more digging based on these two suggestions. Thanks both of you.