LFO shapes and "proper" scope use

[R.G.]

Deciding whether something is "AC" or "DC" on a scope is tough. You are, in effect, trying to guess what the designer of the input preamplifier for the scope was thinking, and whether his boss was standing over his shoulder, and about what. The cap that blocks DC on a scope preamp has to be of high quality, and is expensive, at least the way HP and Tek used to do it. When in doubt, use DC.

There's a subtle problem lurking in here. LFOs don't exist in a vacuum. The circuit that is being modulated has some transfer function of volts of LFO to [units of something] of effect. That's not necessarily linear. It too has quirks of not exactly following the exact voltage of the LFO or having the same audible change in (whatever) per small change in LFO voltage at all points on the modulation.

And it can "saturate", not following the LFO waveform if it's over X or less than Y. There's a lot of fussiness in analog synth circuits to make sure that voltage controlled things are either accurately linear in following a control voltage, or accurately exponential in frequency-control situations.

Unless it's in a well-designed synth module, you can't depend on the audible effect accurately following the waveform of an LFO.

LFO circuits don't exist by themselves. They have to take into account the needs of what they control, and (for instance) whether there is a fixed DC level needed by the Controll-ed One, or if the LFO must expand only UP from some floor DC level, or only down from some ceiling DC level. Tremolo circuits show all three of these quirks, and you can sometimes tell whether a trem does tremolo up, tremolo down, or tremo up and down by listening. It gets hairy if you're driving a variable-C or variable-L resonance, where you control the frequency linearly by the square of the component value, and when what you want is neither linear nor square-law, but exponential.

[anotherjim]

One or two x10 probes ought to be on your shopping list Mark. Those are the ones that need calibrating which will need doing when checking BBD clocks and other fast stuff. I have switchable x1/x10 probes which add another chance for error if the switch gets accidentally knocked.

As even the cheapest DMM might have 10M loading, and we can see even that causing a lower voltage reading on a 1M op-amp input, your 1M scope input is going to halve the voltage and seriously offset the op-amp * , since the scope 1M is to ground not the op-amp Vref. x10 probes up the load on the circuit to 10M, but you and/or the scope need to know that the voltage reading will also be x10 lower than with the x1 probe.

* Caveat, the scope input may not be grounded (is floating) to AC supply safety ground and the circuit under test may not be either. Where the scope AND circuit are grounded, I find it better to not connect the probe ground to the circuit ground or else you have a ground loop which can inject all sorts of noise on the scope trace.

If either scope or circuit are not grounded, then a ground needs connecting between them. It is actually better to use a separate wire - a banana jack or binding post on the scope to a croc' clip or use the screen of an unused BNC connection. That leaves your test probe free to move around more and reduces ground return effects in the probe.

[Mark Hammer]

Interesting.  Thanks for that, Paul.  I'm also glad I paid twenty 2016 dollars for it, instead of seven hundred and ninety-five 1979 dollars for it.

I guess with the money saved, I'll pop by Active Electronics this weekend and see what they have in the way of probes. 

[R.G.]

What he said.

I got into the habit of using two scope probes, even if only one was needed, so that one of them carried ground and I could use the other for probing voltages. This introduces noise for situations where the ground is noisy and the ground point is "far away" from the actual signal being tested. You see signal plus noise, which is what the probe is actually seeing.

This won't be a problem on pedals, likely. For low noise measurements, especially of fast signals, the probe really needs to be two-headed, with the probe ground going to the nearest ground point to what's being measured.

I have fond memories of connecting the scope ground to the Wrong Thing while working on a switching power supply design and watching in horror as the insulation melted off the wire on the ground probe. Seems that the scope was carefully third-wire grounded, the probe ground was tied to that, and the point I was "grounding" with the scope probe was also tied to the AC line - but not to AC sefety ground. Ooooops... 

Just saw your post as I hit "submit". Marlin P Jones sells decent(ish) 10:1 probes for ~~US$20, and I see them at various replacement parts places for a similar price. Tek used to nick you for $100 in 1970s dollars for similar stuff.

[Kipper4]

Yep been there Jim.
Excellant thread.

[Mark Hammer]

Is something like this suitable?  https://www.banggood.com/DANIU-P6100-Oscilloscope-100MHz-PKCATI-BNC-Clip-Probes-Clip-Cable-p-1157611.html?rmmds=category

[anotherjim]

"Banggood"? When the sniggers abate, look very much like the probes I bought from Rapid. Generic Chinee OEM I expect.

I've had other cheap ones where the probe spring cap flies off at every opportunity or fails to make contact with the actual tip underneath, so I don't know how those ones are for quality.

Note the little coloured ID rings you get with them (yellow shown on that catalogue page) - very useful with more than one scope channel on the go.

Also note the voltage rating which is typical. Only x10 is up to poking around a tube amp B+, that is if the scope input is good for it too.

[EBK]

Also keep in mind that, with Banggood, you are ordering directly from China.  I recently ordered a laser engraver through them, which took about 3 weeks to arrive.  Also, occasionally, imported goods get mysteriously stuck in Customs or require that you pay an import duty to receive them.

[PRR]

Mark, that 'scope seems to be "auto range". This does make reading actual voltage swing trickier, because the too-smart beast is changing the gain "for you". I did not see an Operator's Manual. I dunno if there's LEDs to tell you what gain it picked. A hasty peek at the input stage crossed my eyes, and I would not be *sure* that ordinary probes will work right. If you got the manual with it, take it to the throne room for a good read.
_____________

> ordering directly from China

His link suggests he gets it from a Canadian warehouse.

However it is also quoting 7-20 days, which does smell a bit like Air China over the pole and into the Customs pit.

[EBK]

His link suggests he gets it from a Canadian warehouse.

"CN Direct" means it is shipped from Banggood's warehouse in China.

[Mark Hammer]

I've ordered from them before.  It was fine.  Just slow.  I gather it came on the proverbial boat.

[highwater]

Mark, that 'scope seems to be "auto range". This does make reading actual voltage swing trickier, because the too-smart beast is changing the gain "for you".

Looks to me like the automatic level is for *triggering*, not the display. The picture Mark posted shows fairly typical V/div settings for both channels.

[anotherjim]

Yeh, Auto trigger means it's always sweeping the X axis rather than waiting for an edge trigger input  (+ or - going from A or B).
This is when you always get a display but waveforms have unstable position since they are rarely in synch with the timebase sweep.
Normal single channel probing use that channel as trigger source with +trigger for a stable display, but only with signal present.
If tracing through an audio path, use one channel as trigger source connected to circuit input  (chB usually) and the other to trace through the following stages. Display both channels (A+B) to see differences from input wave including phase shift/inversion.
ADD mode is the sum of A & B voltage in one trace (check for balance between differential outputs, they should perfectly cancel if exact 180deg phase and equal amplitude) . Invert chB with ADD and you have a difference amp input. Diff amp is excellent when you cannot use a common ground between scope and circuit as both probe tips only are used to provide ground isolated + and - signal connections. With x10 probes, this is one of the safest ways to work (safe for the equipment, not you).

X Via A on the timebase switch removes any horizontal sweep and chA voltage determines horizontal position. With signal on both channels, the CRO can draw shapes according to the differences between the 2 signals...
https://en.wikipedia.org/wiki/Lissajous_curve.