SM0VPO Simple Basic Oscilloscope progress

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Re: SM0VPO Simple Basic Oscilloscope progress

Post by Ivan on Fri Mar 16, 2018 1:48 pm

Hi Harry,
you overestimate me!

VBR Ivan

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Re: SM0VPO Simple Basic Oscilloscope progress

Post by Admin on Thu Mar 15, 2018 6:21 pm

Hi Ivan,
> 1/ You should be able to regulate the treshold voltage to any value within the range, not zero volts only.
> 2/ You need to act on positive or negative going voltage only. You do not want triggering on both simultaneously.


I didn't go into too many details. Simply adding DC will shift the "zero-crossover" to any point in the waveform you want, and since the two D-Type flip-flops  are following the input waveform it is a simple matter to select either +ve or -ve going crossover pulses with a single gate. But I like that you are also thinking about this project.

If you don't mind, I would like to pass the circuit by you before I build, since you have so much knowledge and experience. I want to keep this thing simple, but the trigger is the most important part of any oscilloscope.

Right now I will spend the next few weeks getting the basic scope built into a nice case. Maj-Lis is here in Sweden, so my hobby time is somewhat limited :-(

BR Harry - SM0VPO.

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Re: SM0VPO Simple Basic Oscilloscope progress

Post by Ivan on Thu Mar 15, 2018 7:04 am

Hi Harry,
you wrote:"you get a pulse at every zero-crossover of the input waveform. This can be used as the basis of a decent scope trigger."
But:
1/ You should be able to regulate the treshold voltage to any value within the range, not zero volts only.
2/ You need to act on positive or negative going voltage only. You do not want triggering on both simultaneously.

BR from Ivan

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Re: SM0VPO Simple Basic Oscilloscope progress

Post by Admin on Tue Mar 13, 2018 5:42 pm

Hi Zsolt,

> What is the advantage of using valves ? 
There are several advantage of using valves.
1 - The vertical deflection is DC-coupled using a single ECC81
2 - I can stuff 100s of volts into it without damage, perfect for PSUs and such
3 - I like valves, they are cool Cool and I have a lot of them  

When I add the basic trigger I need to do a bit more thinking. I will add a BC547 to shunt the zener. This will cause the two transistors to conduct, and hold off the sweep until triggered. Then the sweep will hold off the trigger, until the sweep is finished. I had thought about using a little OpAmp logic, but a little thought is needed.

Right now I just want the basic scope so I can get on with other projects.

> Maybe using a relaxation oscillator with a neon lamp would be more "stylish"?
Yes, Ivan, it would be a lot more elegant, but if the charging voltage is above about 200-V then the tube can stop oscillating. Also, it is more difficult to use a trigger circuit when high voltages are involved. I don't want to buy any more transistors, and this method uses only 10-V.

I also thought about totally replacing the sweep/trigger circuit sometime in the future. With this method all I need is a 5-V sawtooth.

> Oh and i'm very curious about the trigger circuit you intend to use.
A long time ago I built an FSK modem for RTTY, using just EPROMS. To do this I squared off the incoming audio with a schmitt-trigger and used this to clock two cascaded D-Type flip-flop. With an EXOR gate connected to the to Data output you get a pulse at every zero-crossover of the input waveform. This can be used as the basis of a decent scope trigger.

> will it go up to 100 ..1000 Mhz ?
No!!  Very Happy The rats-nest on the bench is more or less flat to 3.5MHz, but there are a lot of long wires. I anticipate it will easily cover 10MHz, as long as the impedance of the ECC81 and the stray capacitance is controlled. But a lot of that will come down to construction.

At the moment I am 3D printing some new formers for mounting the CRT heater transformer, the 1200-V capacitors and the inner tube support. But it is not a project I want to rush. The day it is finished I will be doing some more projects from the list, then looking at how I can improve this basic scope.

Thank you both for your comments. I really appreciate it. Very best regards from Harry - SM0VPO

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Re: SM0VPO Simple Basic Oscilloscope progress

Post by Ivan on Tue Mar 13, 2018 7:17 am

Hi Zsolt,
the two complementary transistors connected this way are a functional replacemement of a unijunction transistor (UJT), which is very rare now. It enables making very simple relaxation oscillators in the AF band. Maybe using a relaxation oscillator with a neon lamp would be more "stylish"?

BR from Ivan

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Re: SM0VPO Simple Basic Oscilloscope progress

Post by zsolt on Mon Mar 12, 2018 3:19 pm

The supply is clever, saves a lot of manufacture  Cool
 i also tried to build one with transistors , i found a B10S.. something tube,  it had 10 cm diameter and green spot . After building it i couldn't regulate the spot , unfortunately the 3'd electrode from K was broken . I could hear the pieces inside the tube . So i bought o cheep Chinese scope  in the end  Very Happy
What is the advantage of using valves ? will it go up to 100 ..1000 Mhz ?
How is the time base oscillator functioning ? Is the selected capacitor charging and when voltage reaches 10 V the 2 transistors suddenly discharge it ?
Oh and i'm very curious about the trigger circuit you intend to use . Is the purpose of that circuit to keep the starting of the oscillogram on the same point?

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SM0VPO Simple Basic Oscilloscope progress

Post by Admin on Sun Mar 11, 2018 6:21 pm

Hi Guys,
I promised to post the results of my simple oscilloscope project. I can now tell you that it is working fine, but without trigger. But the TB generator does have a suitable point to place a BC547 to shunt the timing regulator.

The circuit sort of grew a little as the horisontal deflection need a fair few 100's of volts, and I had to use push-pull to keep it linear. The TB generator uses just 17-V DC. Although not perfect, it is still more linear than I had hoped for. No need to have a 35-V DC supply :-)  I was also able to get away with using 6x Red LEDs as I do not have any 10-V zeners.

I was forced to use a separate transformer for the CRT heater, due to the 1200-V isolation needed. But that is a mini 5-W transformer from a small battery charger.

The 1200-V (-600-V / +600-V) started to sag a little with the re-wound 10-W transformer, so I used two more "meaty" transformers (25 Watts). The 1200-V PSU is surprisingly stable. There should be 305-V DC across every one of the capacitors, so all six caps are 47uf at 450-V DC.

Anyway, here is the "finished" (provisional) circuit-diagramus-confusicus. There will no-doubt be some minor component changes when it all gets put into a box. I will also try a selection of tubes for (new/used) to be sure the component values are not on the edge of the working range.

All comments are welcome. I apologise for having as many as 9 knobs on the final circuit.
But you will soon see this project on http://www.sm0vpo.com

BR Harry - SM0VPO


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