New Member - Hello & Oscilloscope Probing Boards With Differing Power Supplies

[Barry A. Waters]

Folks,

Making my first post on the forum so do let me know if I've placed it in the wrong place. I'm a retired Real Time / Asynchronous Software Engineer (DEC PDP, VAX and various PC based systems running Unix, Linux, MS Windows etc.) that worked a good portion of my 'Glory Years' with a company that also developed Hardware so I had the chance to explore a bit with TTL and CMOS circuit design as a hobby.

I left the hobby around 1995 but by then had gone the Radio Shack / Forrest M. Mims III route, and with grand advice from the hardware development folks I worked with, have etched some PCBs of simplistic design and produced more than a few breadboard IC projects that did neat things like voice recording / playback and such. Those experiments were all Low Voltage (5V ~ 12V) DC powered stuff and did not include AC usage.

So, retirement is here and with a ton of different components still on hand, several breadboards, a multimeter / passive component tester and a logic probe I decided to get into things again.

While researching some stuff I ran across Oscilloscopes, found EEVBlog and David's excellent videos on them, and ultimately wound up sourcing a Rigol DS1054Z, Rigol's DK-DS6000 Demo Board, a Velleman O-Scope Tutor Board and a couple of books (Heeres' 'Oscilloscopes: A Manual for Students, Engineers, and Scientists' and the ARRL's 'Oscilloscopes for Radio Amateurs').

SO, I'm currently knee deep in reading and learning about the scope (and scopes in general) and how NOT to blow anything up. I have watched David's #279 video several times and am being very cautious! After WWII my Dad got into TV / Radio repair for several years before changing careers and had experience on scopes of that time (50's ~ 60's) but none of that translated to me as he was out of that line of work by the time I hit elementary school. I can't help but think how much he would FREAK over the low cost and capabilities of the basic Rigol scope I just bought! :-)

Enough about me and on to my (hopefully not too ridiculous) question. It started with a reference to 'Generating Lissajous Patterns'. I saw I needed two waveforms of the same Amplitude and Frequency but of different Phases. I have several waveform signal sources but none give me Phase adjustment options so I thought breadboarding something might be an option.

Then I thought about probing two (or more) separately powered boards via separate Channels (with all involved boards producing the same basic signal but one with one Phase, and the other(s) with different Phases). That brought up questions of the Power Supplies for the boards being separate, possibly at different levels and possibly of different types (AC vs DC vs +/- Split DC). Yikes! Am I headed down a wrong (and possibly dangerous - to the boards, scope AND Me) path with that idea?

David's #279 video makes it exceptionally clear that all 4 Channels and USB Cable on the scope share a common, Earth Referenced Ground with each Probe's Alligator Clips and it is here where I begin to wonder about connecting those grounds to boards with differing level power supplies that may be either isolated or Earth Referenced. Add in AC vs DC vs +/- Split DC and I'm definitely back to YIKES again. I think I need to better understand the subject of Potential and how that works.

I've attached a diagram that may more clearly depict the possible combinations I've alluded to above (but lacking the +/- Split DC example), one that uses specific examples of the items I currently have. Any advice on this would be much appreciated as I'm just not far enough along in my education on this to understand the ramifications of the various combinations being simultaneously connected to the same scope via different Channels that share a common, Earth Referenced Ground.

Back to the 'Generating LissaJous Patterns' desire that got me on to this subject in the first place. What I may well be looking for is a single dual-output circuit that produces two signals of common Amplitude and Frequency where one signal output is fixed and a second signal output is Phase adjustable with both signal outputs being powered by the same Isolated DC supply, but the question of using multiple boards with differing level power supplies and types (AC, DC, +/- Split DC) to do the same remains for me.

Barry 

[ledtester]

Here's a simple way to generate Lissajous figures from a PC sound output:

https://www.instructables.com/Generating-Lissajous-Figures-using-a-PCLaptop-Soun/

There are other options - search for "sound card lissajous".

[Brumby]

Let's not dissuade the gentleman from doing things in the real world!

Barry, you are right to be cautious - but, in the case of what you have presented, it's all good.

From left to right .....
 - The Rigol demo board will share an earth reference with the scope - so that's an easy 
 - The EDU06 is powered by a plug pack - which are isolated by design.  Also a 
 - Breadboard circuit #1 is powered by the GS1310 which has 3 fully isolated supply rails, so using one as you have shown will mean there's no problem here either. 
 - Breadboard circuit #2 is powered by a battery, which is the epitome of isolated!  Another 

However, once each of these circuits is connected to the scope, they will become earth referenced and any future connections will need to be done with care.

This includes the case of connecting the EG on the GS1310 to anywhere in these circuits.  Even connecting it to a known point earthed through the scope is not without its own risk - specifically: ground loop.

It started with a reference to 'Generating Lissajous Patterns'. I saw I needed two waveforms of the same Amplitude and Frequency but of different Phases. I have several waveform signal sources but none give me Phase adjustment options so I thought breadboarding something might be an option.

Playing with Lissajous patterns is a great way to have fun - and it's easy enough to do, too.

For starters, you don't need to constrain frequency and phase like that.  Just start out with two sine waves and you will soon find yourself viewing a bedazzling array of displays.  Also, amplitude isn't quite as critical as you might think.

When the X and Y signals are of the exact same frequency, you will get a static display - and it's kinda boring.  Depending on the phase difference, you will get a circle, ellipse or a straight line.  But with a slight difference in frequency, things start to look interesting.  You will get a circle that looks like it is spinning.  Now change one frequency so that it's twice or three times the other .... then try one that is 1.5 times.

You'll have a ball!

[Barry A. Waters]

Thanks for the assistance (and advice) guys! I am making some Sine Wave generators now. This is amazingly fun!

Barry

[Barry A. Waters]

Playing with Lissajous patterns is a great way to have fun - and it's easy enough to do, too.

For starters, you don't need to constrain frequency and phase like that.  Just start out with two sine waves and you will soon find yourself viewing a bedazzling array of displays.  Also, amplitude isn't quite as critical as you might think...

You'll have a ball!

WOW - No truer words! :-)

I made good progress with some breadboarded battery powered 555 and CMOS 4046 circuits but they tended to drift in frequency (and amplitude) over a short period of time so I ended up springing for a Rigol DG822 which gave me quite a bit more flexibility AND stability so I could make small changes in both frequency and amplitude and take my time seeing what impact they had. I have also noted that the DG822 adds another Earth Referenced Ground piece of equipment to the mix so I am mindful of that should I use it in combination with anything I might be probing.

Wow! Just freaking WOW! I'm now up to my eyebrows in COOL Lissajous figure images and have learned a ton about waves, waveforms, O-Scopes in general and the DS1054Z in particular. Thank you Brumby for such encouraging advice!

My latest epiphany was to use a small 8 Ohm speaker as a microphone on a small AM/FM radio to look at Sound Waves. That generated another level of FUN and the Wife (Grand Lady that she is!) got a bit tired of the late night "HONEY! You gotta come see THIS!" calls from my bench. If I had known O-Scopes could do this I would have added one to the Stereo System years ago! And yes, I hooked up Channels 1 and 2 to the same input just to get two different colors of the same plot (with a bit of vertical separation on the O-Scope) just for grins. Both Channels are indeed showing the exact same signal. :-)

Barry
     

[taste_tester]

In the context of using an oscilloscope with different power supplies, is a "rule of thumb" one can follow? That when working with properly isolated power supplies, one doesn't have to  worry about a problem with mains referenced earth?
I watched the EEVblog video "How Not To Blow Up Your Oscilloscope" probably 10 times, but I don't feel that I truly understand the concept, is it really as simple as finding out what circuits are isolated and which circuits are mains grounded and take greater caution poking around on the mains grounded ones?
Just looking for something ......grounded...... to help me apply the theory in a real world situation.