What am I seeing on this waveform?

[Blindog]

I recently repaired a circuit trainer designed for digital circuits. It has a fuction generator built in, so I thought I would start using it to learn more about my scope, and apply signals to my amp builds to observe the signal in the different stages, etc.

I dialed up a 1khz sine wave hooked the scope probe to the signal generator output, and if you look at my pic there is a weird spike at the peak of the wave.

Anybody have an idea what would be causing this?

Thanks!

Mark

[VacuumVoodoo]

That's because these trainers use a triangle/square wave function generator. They Low Pass filter the triangle to get an acceptable sine approximation. The spike on top of your waveform is what's left of the triangle and is a tell tale sign that what I wrote is so.

[Cliff Schecht]

That reminds me of the little *blip* that happens as the triangle circuit peaks and changes polarity in this circuit: http://www.radanpro.com/Radan2400/Oscil ... 720sch.gif

It happens even when the pot is adjusted correctly. This circuit works OK but does have some crap in the peak transitions (which is OK for musical instrument purposes, not really audible, but I wouldn't accept them in my bench function gen). FWIW the heart of most synthesizer VCO's is a ramp generator, all of the other waveforms are derived from the ramp. This makes for a "simplified" voltage control scheme but you can have some nonlinearities to contend with if you try to get all of your waveshaping with too few active components. The square/triangle wave generator develops relatively low distortion (and low jitter) signals and is easy to control the frequency and amplitude via pots. Since voltage controlled stuff isn't really necessary for a bench supply, you could rig up this simple oscillator to get the job done. Sine waves can be made with a couple of caps and an op amp, so really you can get an entire function generator out of a quad op amp if you get a bit clever.

[Blindog]

Thank you both for the input gentlemen. I just wanted to be sure there wasn't something else going on with this piece of gear. The PT bolts had come loose and it shorted against the power supply board and fried the bridge rectifier.

So this wave would do for poking around amps, eh? I wonder if there's any way to further smooth the wave? I think I have a schematic around here somewhere, I'll post if I can find it!

Thanks again mates!

Mark

[Stevem]

I was just last month that I was recaping my Sunn Sorado that I have not touched in years and in its output wave form I saw a similar thing to that, but not 100% the same, and this was always there since I got the amp some 8 years ago.
I was guessing that it may or may not be the OT ringing and for now was not too concernded with it.
While in the amp I also added a 470 ohm resistor and a diode in each of the UL screen feed legs after reading a article on UL output stages.
Afterward when I retested the amp that what a though ringing was gone, anybody got any clues as to why?

[martin manning]

I retested the amp that what a though ringing was gone, anybody got any clues as to why?

I'm guessing you had an oscillation in the screen circuit that was riding the sine wave for a moment, and adding the screen R stopped it.