LFO shapes and "proper" scope use

[Mark Hammer]

I finally decided to pull out the scope I bought last summer and put it to use.  Nothing special but 15mhz dual-trace is more than enough for stompbox troubleshooting.  I won't profess to having any skill in using them, but I figure if I'm seeing an LFO waveform on the screen, I'm doing something right.  I wouldn't trust my judgment of the signal amplitude, but at least I'm seeing something.  Always interesting to see the waveform of fuzzes and octave-up units as the signal dies away (one of the probes I have for the scope ends in a moulded 1/4" phone plug so I can just plug pedals in directly).

Currently, I'm looking at the LFO output of modulation effects, and specifically the one-op-amp LFO found in the Phase 90.  In one case, I get something that looks almost like a sine wave, but has a bit of a spike at the bottom of the waveform.  In another, I get something that looks more like an attack/release transient generator than an LFO.  It rises exponentially, but then has a "cliff" and falls exponentially, such that it looks a bit like an ocean wave sweeping to shore towards the right.  So neither of the circuits I looked at yielded true sine or triangle waves.  The circuits sound almost right, but not perfect.

Two questions:
1) Is the P90 LFO supposed to generate a true sine or triangle output?
2) If it is, are there particular components to tinker with to improve the waveform shape?
3) Is what I see potentially a function of tapping the LFO signal at the wrong point on the board, or setting up the scope wrong?

I have a couple of Ross phaser boards that use the LM13600 for LFO that I'm not entirely pleased with the feel of the sweep, so they will be next on the block once the P90 LFOs are optimized.

[EBK]

Does your scope probe have a compensation trimmer built into it?

https://www.quora.com/What-is-oscilloscope-probe-compensation-What-is-it-needed-for

[slacker]

The ocean wave shape is what you get from a P90 style LFO rather than a true triangle, so it looks like you're probing in the right place.
Going by this schematic http://www.home-wrecker.com/ross_phaser_orange2.png you want to probe across C16. The output of the LFO is the shape made by charging and discharging the cap through R37 and the rate pot, so you get an exponential rise and fall. It's basically like alternating between these two circuits http://www.electronics-tutorials.ws/rc/rc_1.html, http://www.electronics-tutorials.ws/rc/rc_2.html

[Electron Tornado]

2) If it is, are there particular components to tinker with to improve the waveform shape?
3) Is what I see potentially a function of tapping the LFO signal at the wrong point on the board, or setting up the scope wrong?

Mark, some factory schematics for modulation pedals had info on tesing and adjusting waveforms. You might try looking at some of them. I think it was mostly Boss that had that info.

[anotherjim]

Scope channel has to be DC coupled to see an accurate low frequency slope. If AC coupled, it's a high pass filter and distorts non-sine waves. It can be useful to see what your LFO looks like with AC coupling, so you can leave the scope on AC for general low level signal hunting where the DC level would otherwise throw your trace off the screen.

Dual amp LFO's where one is Schmitt trigger and the other an integrator (cap in feedback) produce a proper buffered triangle from the integrator, although you can get a spike at the triangle peaks caused by the Schmitt trigger switching transient. In the single op-amp type, you only have the Schmitt trigger using the amp and the integrator is being done directly by a cap, hence the exponential sloped wave.

[PRR]

A good Sine is not easy.

Whatever, the wave should be "smooth".

I feel Tremolo should be Sine-like; it should linger near the extremes.

In a Phaser, the real effect happens when "moving". A triangle may be appropriate.

The exponential triangle may have some pleasure on the ear (up and down are not the same) and may save a part.

Sine oscillators of low cost will often control amplitude by clipping, often on one side. This can give a glitch. If the glitch is in the low-gain side of a tremolo, it may not be noticed.

> I wouldn't trust my judgment of the signal amplitude

Keep a 1.5V battery handy. Remember where the DC setting is on the 'scope. Read the battery. The jump is 1.5V. Most guitar will be under 1.5V peak to peak. Many LFOs will be several times bigger. A 9V battery may be your ruler for bigger waves.

[Mark Hammer]

Scope channel has to be DC coupled to see an accurate low frequency slope. If AC coupled, it's a high pass filter and distorts non-sine waves. It can be useful to see what your LFO looks like with AC coupling, so you can leave the scope on AC for general low level signal hunting where the DC level would otherwise throw your trace off the screen.

Dual amp LFO's where one is Schmitt trigger and the other an integrator (cap in feedback) produce a proper buffered triangle from the integrator, although you can get a spike at the triangle peaks caused by the Schmitt trigger switching transient. In the single op-amp type, you only have the Schmitt trigger using the amp and the integrator is being done directly by a cap, hence the exponential sloped wave.

<insert epiphany here> I had it on AC, assuming that since it was a changing voltage, that AC would be appropriate.  I'll try it on DC when I get home from work.  Thanks.

[PRR]

> assuming that since it was a changing voltage, that AC would be appropriate.

Where is the line in the sand between AC and DC?

There isn't any. Geo-scientists treat 0.1Hz signals as AC. Wall-supply "DC" power has varying components past 600Hz.

Most 'scopes on AC mode start to fall below 20Hz. Most trem-things run nearer 10Hz. You are on the edge. Using "DC" will preserve the slowest changes better.

If there was a tiny AC on a big DC, like the signal at V1a of a guitar amp, then you might wish to discount the big DC so you could see the little AC better. But in an LFO the signal is typically a very large fraction of the supply voltage, so reading AC and DC together (on the "DC" mode) is no problem.

[blackieNYC]

Good point.  DC setting for LFOs. Otherwise the scope can lead you down a very strange path.

[Mark Hammer]

Took a bit of twiddling, and setting the LFO rate and scope sweep  juuuuuuuussst right, but setting it to DC, instead of AC brought up a nice triangle, just like Jim said.  And setting it back to AC yielded the "ocean wave".

Ya learn something new every day.  Thanks, gents.

[EBK]

Mark, you know my life is incomplete until you tell me whether you compensated your probes. 

[Mark Hammer]

How would I have compensated them?  FWIW, the scope shows "1M/35pf" on the inputs.  I'm not well-versed enough in such matters to know what the implications of that are.

[Digital Larry]

Normally there is a square wave test point out on the scope and a trim pot on the probe somewhere.  You tweak it for the squarest looking result, the idea being to flatten the high frequency response.

[EBK]

How would I have compensated them?  FWIW, the scope shows "1M/35pf" on the inputs.  I'm not well-versed enough in such matters to know what the implications of that are.

Maybe this will help a little.  Your 1M/35pF is on the right hand side of this figure.  In simplest terms, you adjust c_comp (with a screwdriver) until a known square wave at the tip of your probe looks like a square wave on your screen.


Edit: Yeah, what Larry already said. 

[Mark Hammer]

Nothing to adjust on either of my probes.  The scopeis a Philips PM-3207.  There is what appears to be a contact point just below the power switch that is labelled "probe adj".  Is that what you are referring to?

What range of capacitance what the adjusting part be?

[EBK]

"probe adj" is likely your scope's built-in test signal.  Go ahead and connect your probe tip to it and see if you have a nice square wave.

[Mark Hammer]

Nuthin that I can see.  May need some twiddling, and it's getting kinda late.  I'll check tomorrow.

[PRR]

> Good point.  AC setting for LFOs.

"DC" gets my vote.

> compensated your probes.

Irrelevant where Mark is sitting. 1X probes don't compensate. 10V probe compensation affects the response in and above the top of the audio band, NOT down in the LFO zone where Mark is today. Even his glitching is mostly below the compensation point. And he may be using the 1X probe, no compensation. Or some more modern scheme, perhaps with buffer in the probe.

[PRR]

[blackieNYC]

Typo!  DC setting for LFOs. I remember it well - whacked and weak waveform, yet I gave up on the waveform and finished the circuit and it sounded pretty good!