Help me understand an oscilloscope display - diyAudio
Go Back   Home > Forums > Design & Build > Equipment & Tools

Equipment & Tools From test equipment to hand tools

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 28th August 2010, 04:25 AM   #1
diyAudio Member
 
Join Date: Dec 2006
Default Help me understand an oscilloscope display

I'm not entirely sure this is the right forum, sorry if it's not.

I've been working on some circuit experiments, seeing exactly what I expected on the o-scope out of my breadboarded oscillator, fuzz & filter circuits. Today I hooked the output of my keyboard (Dave Smith Instruments' PEK) to the o-scope to get an idea of what voltage swing I should expect from a synth output.

What I saw was a funky waveform of 60Hz (so clearly related to power supply, right?) that was about 160Vpp. This was with no detectable output level, metered or audible, when connected to a mixer. When I brought the output level of the synth up to some nonzero level, nothing on the o-scope changed. Nor did anything change when I changed the oscillator shape. This remained when I turned the synth off, and only changed when I unplugged the cord from the output or unplugged the synth's (external) power supply.

Obviously I'm missing something I should know. Can anyone enlighten me?
  Reply With Quote
Old 28th August 2010, 04:42 AM   #2
star882 is offline star882  United States
diyAudio Member
 
star882's Avatar
 
Join Date: Mar 2007
Make sure the ground is connected properly. You were probably picking up capacitive leakage.
__________________
"Fully on MOSFET = closed switch, Fully off MOSFET = open switch, Half on MOSFET = poor imitation of Tiffany Yep." - also applies to IGBTs!
  Reply With Quote
Old 28th August 2010, 06:35 AM   #3
infinia is offline infinia  United States
diyAudio Member
 
infinia's Avatar
 
Join Date: May 2005
Location: Californication
Quote:
Originally Posted by veracohr View Post
What I saw was a funky waveform of 60Hz (so clearly related to power supply, right?) that was about 160Vpp. Obviously I'm missing something I should know. Can anyone enlighten me?
yikes that's a lot of voltage.
60Hz usually indicates a measurement system ground loop. PS stuff is usually fullwave so 120 Hz is the norm. When looking at power supply stuff change the scope trig. source to <Line>.

Always use a earth / safety 3 prong plug at the scope, never 'float it' or defeat the 3rd wire. When you connect the probe gnd., that DUT point then becomes the only gnd connection.
__________________
like four million tons of hydrogen exploding on the sun
like the whisper of the termites building castles in the dust
  Reply With Quote
Old 28th August 2010, 06:37 AM   #4
diyAudio Member
 
Join Date: Dec 2006
Er...what ground? The synth's power supply is ungrounded, and the o-scope is grounded to the best of my ability (I don't know how it is beyond the wall outlet).

Edit: this was in response to star882's post.
  Reply With Quote
Old 28th August 2010, 06:42 AM   #5
infinia is offline infinia  United States
diyAudio Member
 
infinia's Avatar
 
Join Date: May 2005
Location: Californication
Quote:
Originally Posted by veracohr View Post
Er...what ground? The synth's power supply is ungrounded, and the o-scope is grounded to the best of my ability (I don't know how it is beyond the wall outlet).
good one gnd, keep searching, the 160V is coming from somewhere. Can you get a stable display and trigger off it?
__________________
like four million tons of hydrogen exploding on the sun
like the whisper of the termites building castles in the dust
  Reply With Quote
Old 28th August 2010, 06:45 AM   #6
diyAudio Member
 
Join Date: Dec 2006
Quote:
Originally Posted by infinia View Post
yikes that's a lot of voltage.
Exactly what I thought! But that's what it said.

Quote:
60Hz usually indicates a measurement system ground loop. PS stuff is usually fullwave so 120 Hz is the norm. When looking at power supply stuff change the scope trig. source to <Line>.

Always use a earth / safety 3 prong plug at the scope, never 'float it' or defeat the 3rd wire. When you connect the probe gnd., that DUT point then becomes the only gnd connection.
I triggered off the channel that was connected. I'll try Line next time. I did, however, try both the tip and sleeve connections of the TS connector, and both were the same.

The scope is grounded, through a grounded power strip. As far as the probe ground, perhaps I'm missing that. The only time I've connected a ground from the probe is when doing the scope's self-calibration, as described in the manual. Should I be connecting the probe ground to the ground connection of the synth output?
  Reply With Quote
Old 28th August 2010, 06:46 AM   #7
diyAudio Member
 
Join Date: Dec 2006
Quote:
Originally Posted by infinia View Post
good one gnd, keep searching, the 160V is coming from somewhere. Can you get a stable display and trigger off it?
It's triggered off the channel that's connected to the signal. The display is completely stable.
  Reply With Quote
Old 28th August 2010, 06:50 AM   #8
infinia is offline infinia  United States
diyAudio Member
 
infinia's Avatar
 
Join Date: May 2005
Location: Californication
Quote:
The only time I've connected a ground from the probe is when doing the scope's self-calibration, as described in the manual. Should I be connecting the probe ground to the ground connection of the synth output?
Reverse of that

Yes Every connection needs a return, so always use a gnd from the probe. Except maybe at the scope itself, ie at the cal. port.

Think about this for a second
So without using a probe gnd the scopes return path is only thru a long convoluted path > the pwr line.
__________________
like four million tons of hydrogen exploding on the sun
like the whisper of the termites building castles in the dust

Last edited by infinia; 28th August 2010 at 06:58 AM.
  Reply With Quote
Old 28th August 2010, 07:06 AM   #9
infinia is offline infinia  United States
diyAudio Member
 
infinia's Avatar
 
Join Date: May 2005
Location: Californication
Quote:
Originally Posted by infinia View Post
yikes that's a lot of voltage.
60Hz usually indicates a measurement system ground loop.


(...)

When you connect the probe gnd., that DUT point then becomes the only gnd connection.
__________________
like four million tons of hydrogen exploding on the sun
like the whisper of the termites building castles in the dust
  Reply With Quote
Old 28th August 2010, 05:12 PM   #10
diyAudio Member
 
Join Date: Dec 2006
Awesome, thank you! I get it now - the reason I didn't have a problem not using a probe ground on the breadboarded circuits is that both the scope and the circuits' power supply are grounded.

Now all makes sense in the world.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
I Don't Understand. Patrick Bateman Multi-Way 60 26th April 2012 03:47 AM
Try to understand have' Tubes / Valves 1 8th July 2006 11:27 AM
i don't understand theChris Analogue Source 6 12th November 2002 04:08 AM


New To Site? Need Help?

All times are GMT. The time now is 03:38 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2