PicoScope help for scope n00b

[MrStab]

Hi guys,

As per recommendations here, i scored a Picoscope 2204a for my birthday last week (from my fiancée, because i implanted a clone in her belly and she says i'm not allowed anything cool after this). I need help using it, and i figure it may be better to ask here than the official Picoscope forums as i only ever intend to use the scope with guitar pedals.

I've managed to get the signal from the AWG output to go back into the A input when a pedal is bypassed, as i can see it in the PicoScope 6 software. So i've managed to connect two wires. Win.

Unfortunately, the test signal disappears when the pedal is activated. Nothing in Scope or Spectrum modes. I have a feeling the phrases "coupling caps", "pulldown resistors", "offset" and "coupling settings" are important here, but i have absolutely no clue where to begin in the context of stompboxes. i've tried changing the coupling option in the software, adding offsets etc., but no success.

i've tried a parametric EQ and Phase 45 so far, both of which pass signal to a guitar amp or soundcard input, and both of which have all the typical wrappings of de-popped, true-bypass pedals. garden-variety stuff. i'm using an isolated PSU, with a metal chassis that is safety-earthed on the same line as the PC, but the ground clips from the probes are only connected to the isolated ground.

i'm even open to being scorned and told to read something in the manual properly. i don't know what's general o-scope ignorance and what's P-scope ignorance. help!

cheers!

[MrStab]

i realised the waveform i was getting wasn't at all as strong as it is in the Beginner's Guide, and it turned out there was just 1 strand of ground wire i hadn't twisted properly, which was shorting the signal. seems to work now. i am easily the worst contributor to this forum. lol

any tips nonetheless? i've figured out ranges and whatnot in the software, but i'm still unsure about things like what coupling approach is best. triggering is something i'll need to get my head round, too.

any other circuit types we use round here that i can use this for besides audio and LFO? or does the 20MHz max limit it to those?

[bluebunny]

i implanted a clone in her belly

Way to go, congratulations, and other assorted felicitations!   

Now, what was the question?... 

[highwater]

...but i'm still unsure about things like what coupling approach is best. triggering is something i'll need to get my head round, too.

For 9v pedals, AC- vs. DC-coupling isn't very important. If you use DC-coupling, you can also see/measure bias voltages without having to switch modes or drag-out a DMM. If you use AC-coupling, you don't need to adjust display and/or trigger settings depending on where in the circuit you probe. AC-coupled mode works down to 1Hz - a slow-enough LFO, or an envelope detector/generator, might not measure correctly unless you use DC-coupling.

For other types of work, especially if you're dealing with higher voltages (for instance, a tube amp), it might be important. From the specs, though, it sounds like the +/-100V limit applies regardless of coupling mode.

For our purposes, triggering is mostly about getting a stable display that shows what you want to see. A rising-edge trigger at 0v AC is a good starting point. From there, playing around with the settings is probably a better way to learn than reading about it.

any other circuit types we use round here that i can use this for besides audio and LFO? or does the 20MHz max limit it to those?

BBD clocks. Flip-flops in Boss-style electronic-bypass pedals.

As far as DIY-able stompbox circuits go, there is hardly anything that operates past 20MHz, and that which does (PT2399 for instance) is unlikely to have an externally-accessible clock. When 20MHz isn't enough, you're probably either in the realm of radio-frequency work, or wishing you had a logic analyser instead of a 'scope.

[MrStab]

Way to go, congratulations, and other assorted felicitations!   

Now, what was the question?... 

thanks, dude! the original question was "y no wurk?", and the answer turned out to be "it's Grant". i got too hasty twisting together strands on a guitar lead so i wouldn't have to hold the probes.

If you use DC-coupling, you can also see/measure bias voltages without having to switch modes or drag-out a DMM. If you use AC-coupling, you don't need to adjust display and/or trigger settings depending on where in the circuit you probe. AC-coupled mode works down to 1Hz - a slow-enough LFO, or an envelope detector/generator, might not measure correctly unless you use DC-coupling.

thanks HW, that first part about the varying biases could come in handy and you may have saved me a future headache with the LFO thing. i noticed that in AC coupling mode, the waveform shifts position slightly. makes a bit more sense now.

Flip-flops in Boss-style electronic-bypass pedals.

ah, cool - i didn't think about that. It'll be cool to see the CV transitions and general transistor behaviour with my own eyes for once. From a quick look at a datasheet, it seems i might just be able to see some CMOS switching at work, too.

just a coupla other questions: i'm trying to teach myself how to actually measure SNR, and i'd like to verify that the preset measurement in PicoScope isn't total nonsense. is it reasonably accurate? i'm testing with a 1V, 1KHz sine on the input - should i be using something else? i can't find much info about standardisation here, so i should be testing with 420Hz for all i know. hopefully my own sums will be similar to the program's reading in the end.

(re. SNR: any good resources or guides are welcome. This was recommended to me on a thread a few months back and it's the only comprehensible resource i've read so far).

lastly: would buying BNC cables, cutting the ends off and twisting them to 1/4" phono connectors be advisable? maybe even preferable to crocodile-clipping probes, from a capacitance and interference standpoint? assuming i only need them in 1x mode, ofc.

cheers!

[anotherjim]

Congratulations.

That 20Mhz upper limit is for accuracy. The scope can often still show that something is happening and that may be all you need to know. Time base has to be suitable, or aliasing effects can display a completely bogus frequency. When probing shows an unexpected signal, sweep the full timebase range to be sure of what you see.
Some examples here...
http://jahonen.kapsi.fi/Electronics/Scope%20Sampling/aliasing.html

I often find I need 2 channels on the same test point. One DC coupled to show bias point and the other other AC coupled to see small signal. If signal is very small, then vertical range needs to be low and the DC level can throw it off the screen.

Watch out for probe issues.
Is the probe ground connected and causing ground loop problems? Is the scope already grounded same as the test subject?
If a switchable x1/x10 probe it can be easy to miss what it's setting. I leave mine on x10 but it can be easy to knock it on x1 by mistake.
If the scope can display voltage measurements, does it know that you have x1 or x10 probe?
If x10 probe, is it's calibration trimmer set properly?