Electronics > Beginners
Need help Understanding Oscope vs Multimeter Readings
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skinny:
I recently put together a 555 timer circuit that changes the duty cycle of a signal but keeps the frequency stable. I used this circuit.

https://www.multisim.com/content/tf7Mwpy2CctfFaaVEr4SeN/555-variable-duty-cycle-constant-frequency-astable-multivibrator/

When I measure the output with my multimeter, I get what I expect. When I adjust the pot and cause the duty cycle to approach 100%, the DC voltage reading approaches the supply voltage. Likewise, as the duty cycle approaches 0%, that DC voltage approaches 0 volts. Additionally, I can see the duty cycle change on the oscope as expected with the frequency staying somewhat stable as the duty cycle changes.

The piece that's occurring that I don't understand is that the entire signal shifts up as the duty cycle approaches 0% and down as the duty cycle approaches 100%. Also, even though the entire signal shifts down where the top of the wave form is at 1V @ 84.8% positive duty cycle, the multimeter reads a voltage close to the supply voltage.

My questions are as follows:
1) What causes this shift? I suspect it's something to do with the triggering but I'm not sure.
2) Why would I be reading one measurement on my multimeter and seemingly another on the oscope?

For reference the signal being examined is at 25kHz, 5Vpp, with a duty cyle that can be adjusted from 17% to 83%.
I am DC coupled with the oscope. I am triggered on the rising edge. The circuit is running off of a battery supply.

Thanks for all your help. I'm really looking forward to understanding this.
JS:
You are saying your circuit's duty cycle can be adjusted from 17% to 83% but you are having trouble over 84.8%, that's an odd one.

What load do you have on your 555?
What voltage peak to peak are you reading when you see 1V as the top of the waveform? can you post a waveform when it's all fine and when it's misbehaving?
What do you see when the duty cycle is low? how low? under 17%?

The first thing that comes to my head is AC coupling, but you said it was DC...

JS
Brumby:
Agreed.  AC coupling.

Op: You say you are DC coupled to the scope, but do you have the scope set on "AC"?  There is usually a switch that has AC, DC and GND.  If this switch is on AC, then that perfectly explains your observation.

What scope do you have, by the way?
Shock:

--- Quote from: skinny on May 29, 2018, 02:45:23 am ---The piece that's occurring that I don't understand is that the entire signal shifts up as the duty cycle approaches 0% and down as the duty cycle approaches 100%.

--- End quote ---

Shifts up, you mean increases in voltage, offset or frequency? Helps to know what multimeter and oscilloscope you're using.

The multimeter is "sampling" (there's a little more to it) at less intervals than the oscilloscope so therefore is more suited to low frequency applications. So it will not be as accurate as an oscilloscope and in many cases totally pointless to use.

That's not to say multimeters aren't extremely useful they are just not ideal for interpreting signals.
skinny:
Thanks for the suggestions and help.


--- Quote from: Brumby on May 29, 2018, 04:01:50 am ---Agreed.  AC coupling.
Op: You say you are DC coupled to the scope, but do you have the scope set on "AC"?  There is usually a switch that has AC, DC and GND.  If this switch is on AC, then that perfectly explains your observation.
What scope do you have, by the way?

--- End quote ---

The vertical shift still occurs regardless whether I put it in AC or DC coupling. I have a Rigol DS1052E.


--- Quote from: Shock on May 29, 2018, 04:04:56 am ---Shifts up, you mean increases in voltage, offset or frequency? Helps to know what multimeter and oscilloscope you're using.
The multimeter is "sampling" (there's a little more to it) at less intervals than the oscilloscope so therefore is more suited to low frequency applications. So it will not be as accurate as an oscilloscope and in many cases totally pointless to use.
That's not to say multimeters aren't extremely useful they are just not ideal for interpreting signals.

--- End quote ---

When I say shifts up, I mean the whole waveform shifts up and down on the vertical/voltage axis of the oscope. I agree that the multimeter is sampling. This is a PWM application so the closer the signal gets to 100% duty cycle, the closer the DC voltage reading will approach the supply voltage. However, the same time this is happening on the meter, my scope is telling me that my peak voltage of the waveform is at 1V, not 6.5V as the multimeter tells me.
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