Build an Oscilloscope PART 1. Vertical Input circuit.

Nice. 👍 I'm looking forward to your series. I'm curious about the opamp comparator. I think opamps have rather low bandwidth, even in open loop configuration. Would that negatively impact this trigger circuit?

Thank you for the comment. The Opamp has no feedback. This is being used in linear fashion and setup for Satiration Mode. This is because once the condition is met it makes a transition which will be used for the second stage. (PART 2 will be posted in a few hours). All this circuit is doing is capturing a portion of the input waveform that meets the potentiometer (which represents the set point of the trigger voltage). So as long as the trigger set point is set within the input waveform peak to peak voltage, this will make the comparator change state in that area. This output goes through a RC differentiator (High Pass filter) to only show a transient pulse between the triggered setpoint. I decided to do this series as YouTube does not have a full explanation that is clearly done. So this is basically a beginning of what will be a full video start to finish with all math and theory involved.

Yes there is a limit for bandwidth which would be limited to the comparators part number. My Fundamentals of Pulse and Digital Circuits 3rd Edition show this trigger circuit is used on older HP 1220/12222 models. Im sure newer units use CPUs.

I got it. Thanks. I'm wondering if adding some hysteresis to the opamp (positive feedback) is a good idea. A noisy input signal then wouldn't cause false triggers. What do you think?

First and foremost. I love this app but this comment window is way to small to use on a phone effectively. I have accidently hit the send button multiple times while trying to reply.. mid sentence. What a pain.. Based on the Eref is held at a constant voltage and the ei is alternating with the input the only time this would make a transition would be on the rising or falling edge (based on the 2 switch position) that meets the eref condition. So basically let's say you have 10Vpp sinewave input to the [+] and your trigger setpoint (potentiometer) is set to +5V to the -. Both switches are in the up postion so this setup makes this a rising edge trigger to activate when ei is at or above +5V. So what happens is the opamp will change state at each +5V rising edge which would be at every 1 cycle. Let's look at the math to see if it will trigger anytime below the trigger setpoint. I dont think hysteresis in this setup is issue. Let's say this opamp has gain of 100K If ouput is based on difference of each input then ed=(e1-e2) [conditions are e1=+,e2=-] so let's say sinewave starts at 0 crossing and is rising and we pause everything just as the e1 (input sinewave) is at 3V, but remember e2 is at the eref voltage of 5V that is set to by the pot. So we have e1=3V and e2= 5V e1-e2= -2V x 100K which would be opamp saturated to (-VCC+1.5V) it would stay at this negative ouput the entire time the input waveform was below rising edge of +5V. Once it gets to >5.0000V then the condition is met and the comparator changes state. This state change on other side of RC is a positive edge pulse. Another great question. Hysteresis, We need to find Vut and Vlt (Voltage upper and lower threshold) so if the op amp is +/-12V Vcc that gives us 12V-1.5V. +/-10.5V rail to rail ouput. So now we have 10.5V/100K (gain) that's 105uV. So now adding hysteresis our set point of 5V would trigger on either 5.000105V or 4.999895V. Thats 210uV hysteresis. But you couldn't set this up as feedback to the +input, because this input alternates as Eref depending on the selection of rising or falling edge switches. Basically we want the opamp to make a transition as fast as possible (limited to slew rate) to +/- VCC when it meets the trigger point. Otherwise until it meets the setpoint is is being held hard in whatever state it's in. This opamp is saturated to VCC or -VCC. once the crossing point is reached it sets the output to transition. So noise won't effect the input unless the noise is larger than +5V. Because thats the set threshold of eref. If you provide me your email I can send you some screenshots of textbook related to the trigger circuit and the theory of operation. Pretty neat stuff. You can delete the comment with email, once I send you the info so its not broadcasted for years in here..lol

Thanks for the explanation. I didn't realize my question had such a long answer! You may want to try using EC on an internet browser on a PC. I've never tried it, but I think you might be able to type comments easier.