This has to do with that clunker HP 1725 a we got kicking around. If you kn ow the history, we got it with a makeshift vertical output amp for the uno btainium vertical output IC. Well we got the unobtainuim obtained out of an other unit. In fact I changed the whole board, figuring it would be tighter in alignment. Unfortunately someone had already screwed with the front end alignment. I should shoot that person's fingers off with a .45 but have no idea how to find him.
At any rate, got time on my hands and got back into aligning it, and seem t o be coming along really. Then I run into a problem. This is about a 1 mHz square wave on it :
Now it's pretty hard for a fault in the scope to do that unless it has some thing to do with the delay line I would think. Or maybe lytics...
Here's the setup :
Better shots of the waveform :
^ That is with the amplitude rduced.
^ That is at X10 mag on the time base.
Now the ringing is worse at a lower setting from the generator. It is a Wav etek 103. That is the 50 ohm output which is wiper fed to the output level pot. Seems a bit unlikely it would affect the ringing like that but...
Here's the but ; I looked at the BNC to BNC cable and noticed it uses RG59U wire. That is 75 ohms not 50. Is that causing all this ringing ? Other thi ng funny is a longer one of the same type of cable does the same thing, I w ould think the ringing should be different no ?
Anyway, I need to make sure the ringing is not in the scope, so I am rthink ing of getting a couple of ultra high speed diodes and soldering them right to the BNC jack on the scope to assyuure I am getting a square wave. One l ittle problem with that is this is best done at the max setting for vertica l gain, which is I think 0.5 mV/div. or something.
I think, ideally, this should be done at maximum vertical gain to remove th e attenuator from the picture, so to speak. Therefore, should I maybe go wi th a couple of Schottys, or what ? What I want to do is to make absolutely sure the scope is getting a square wave. Once I am sure the ringing is not in the scope I can move on.
The 50 ohm output on the Wavetek goes up to at least five volts. If running into the 50 ohms on the HP, say I use a 100 ohm resistor and two back to b ack diodes, what diodes would clamp this just right - be high speed and jus t clamp, leaving a perfect square wave right at the input jack of the HP ?
Or should I approch this a different way ? Maybe use an even higher voltaag e, clip it and then break it down resistively ? Whatever happens I really h ave to get that cable out of the picture.
All I can figure is with the BNCs on RG59U, they were surplus from a cable TV company or something. Maybe.
Any ideas ?