[SOLVED] O-scope Voltage Percentage Measurement Cursors to measure stage Gain

[danny davis]

What circuit do you have that needs its voltage ratios measured? Please post its schematic.
Do you know what the ratios are supposed to be?
Why did you suspect that the voltage ratios are wrong?
Do you know what causes the voltage ratios to be wrong?

What circuits or stages would you use this on?

I'm using it to to measure summing amps and summing stages , the voltage ratios

What would you use this for?

 

[Audioguru]

I have designed, built and used some audio mixers and I have never found the voltage ratios to be wrong.
An audio mixer is a summing amplifier. Its voltage gain is determined by the ratio of its negative feedback resistor to the input resistor on the input you want.

 

[danny davis]

Yes but I'm talking about other types of summing amps, not just audio summing amps stages

What would you use the Voltage Ratio cursors to measure? for what kind of circuits come to mind for you that you would want to measure the voltage ratio or use it for troubleshooting?

 

[Audioguru]

You never said what your circuits are summing and you never said the voltage ratios they have so we do not know why you are talking about them.

 

[danny davis]

In the O-scope manual it has they use the Voltage Ratio curses to measure phase shifts and phase angles

How does this work? I never measured voltage ratios from phase shifts or phase angles

Does this make sense to you?

What kind of circuits would you measure the Voltage radio of a phase shift or phase angle?

 

[Audioguru]

I use a dual channel 'scope to see and measure phase shifts which are timing differences. Voltage ratios are amplitude differences which is not timing.
Maybe you can use amplitude difference and some math to calculate a phase shift but seeing it on a dual channel 'scope is much simpler.

 

[betwixt]

To download the link in post #19 would take me all day but I do have a modern Tektronix oscilloscope and the service manuals for many older models. I can't find any references to "ratio cursors" so I'm not sure what you are referring to.

No matter what the configuration of amplifier and how many inputs it has, the gain is simply the amount by which the output changes for a given input change. You can measure that on an oscilloscope or if the amplifier is working at low frequency or DC, on a DVM. Ratio doesn't come into it.

Phase shift is just that, a difference in time within one cycle of a waveform. It is measured from a determined place in the input waveform to the same place in the output waveform. Given that one cycle is considered 360 degrees (treated as angular velocity) we measure it as a fraction of a cycle in degrees. For example a signal shifted by one quarter of a cycle would be considered 360/4 = 90 degrees shifted. You CAN use an oscilloscope to measure phase shift by selecting X-Y mode and noting the angle and straightness of the resulting vector. This is not something you have to worry about in your line of work.

Brian.

 

[danny davis]

Brian It will take less than a min. to scroll down to 6-2 and look at the figure of the voltage Ratio cursors

tektronix O-scopes 2445 Manual
GO to 6-2 its Voltage RATIO chapter , shows you how to measure voltage ratios using the ratio cursors on the O-scope
Look at the picture figure 6-2 and it shows you
**broken link removed**

 

[betwixt]

What I meant was it would take me all day to download it. No fast broadband here! The fastest I can download is about 4 Mb per hour so it would take around 3 hours if nothing else was using the line. Unfortunately, it is shared with several other data links.

Brian.

 

[danny davis]

In the picture, they measure the PEAK TO PEAK of the waveform, the Voltage Ratio is displayed at 31 percent ratio , they are comparing the Peak to peak ratio to an output waveform or Reference waveform

What would you measure the peak to peak voltage ratio of a waveform? for what kind of circuits can you think of?

What would you use this peak to peak voltage ratio cursors to do troubleshooting for , for what kind of stages or circuits?

The Voltage Ratio is a Peak to peak amplitude ratio , and they compare it to a reference waveform or an outputwaveform or input waveform , if you want to compare it or not

 

[betwixt]

I looked in the Tek 2465 manual which is similar to the 2445. The 'ratio' measurement you refer to is specific to those instruments and although the technique can be used elsewhere, it isn't common or particularly useful. What they are saying in the manual is that if you adjust the oscilloscope to make a waveform cover exactly 5 'Y' divisions then change to the other signal and press the delta V button, moving the cursor will show the proportion of amplitude rather than it's absolute level. So if the two signals were the same it would show 100%, if one was larger or smaller than the other it would show a higher or lower percentage. It's basically showing how to use the delta (= difference) function. You can do the same by measuring the two signals and using a calculator as I suggested before and the calculator method is more versatile.

The peak-to-peak measurement of a waveform is just that, the voltage difference between the most negative part of the waveform and the most positive part. You can use peak-to-peak to check gain by simply comparing the input measurement and the output measurement. As long as both are measured the same way the gain is output voltage divided by input voltage.

For the most part we don't use peak-to-peak for AC measurements though because it requires an oscilloscope to see where the peaks are and this is obviously inconvenient. Instead we use RMS which is a more user friendly method. You can compare RMS input with RMS output to measure gain in exactly the same way as peak measurements and for the same signals you get exactly the same answer. FOR A GIVEN WAVE SHAPE the RMS and peak-to-peak are related to each other, for example a sine wave RMS value is 0.7071 (1/sqrt(2)) times the peak and for a square wave it is 0.5 times the peak but for other waveforms the figure is different and needs special measurment techniques to derive the value.

Brian.

 

[danny davis]

If the feedback resistor and Rin are the same value, the ratio is a gain of 1
My manager says that it just "passes through" no gain
It's a voltage follower
So when ever you see the same values in both feedback and Rin resistors , it's just a pass through circuit, gain of 1

- - - Updated - - -

So if the two signals were the same it would show 100%, if one was larger or smaller than the other it would show a higher or lower percentage.

Right, wouldn't this be good to use for summing circuits and summing amps? because of the different waveforms it's summing and the percentage and ratio of the summing

I'm trying to measure the percentage and ratio of the summing , not the gain of the summing

It's not the same right? what's the difference?

 

[Audioguru]

If the feedback resistor and Rin are the same value, the ratio is a gain of 1
My manager says that it just "passes through" no gain
It's a voltage follower
So when ever you see the same values in both feedback and Rin resistors , it's just a pass through circuit, gain of 1

WRONG again if you are talking about an opamp inverting summing amplifier.
An opamp with an input resistor is inverting so the signal does not "just pass through", it gets inverted.
Because the signal gets inverted the opamp is not a "follower". A follower DOES NOT invert a signal.
The gain is less than 1 if the source impedance is higher than about 1/10th the input resistor value.

Wouldn't this be good to use for summing circuits and summing amps? because of the different waveforms it's summing and the percentage and ratio of the summing

Why do you ask? You described a summing amplifier.

I'm trying to measure the percentage and ratio of the summing , not the gain of the summing

Why do you need to measure the percentage and ratio of summing? Because the 49 years old carbon composition resistors are too old and frequently change their value?

It's not the same right? what's the difference?

Look up the words "percentage" and "ratio" in a dictionary.

 

[danny davis]

An opamp with an input resistor is inverting so the signal does not "just pass through", it gets inverted.
Because the signal gets inverted the opamp is not a "follower". A follower DOES NOT invert a signal.
The gain is less than 1 if the source impedance is higher than about 1/10th the input resistor value.

But it adds no gain, the gain is less than 1, so it's inverting and passing through

Look up the words "percentage" and "ratio" in a dictionary.

Measure gain and measuring voltage ratios are different right?
what's the difference? from input and output comparing gains VS voltage ratios whats the difference? or our they the same thing?

 

[Audioguru]

But it adds no gain, the gain is less than 1, so it's inverting and passing through

You and your manager should learn English and learn electronics.
"Passing through" is not an electronics phrase. A person is passing through a doorway. A person and a doorway are not electronic components.

Why do you say the gain is less than 1? If the source impedance is low and the feedback and input resistor values are the same then the gain is exactly 1.

Measure gain and measuring voltage ratios are different right?
what's the difference? from input and output comparing gains VS voltage ratios whats the difference? or our they the same thing?

You and I do not know what you are going on and on about.

 

[danny davis]

Why do you say the gain is less than 1? If the source impedance is low and the feedback and input resistor values are the same then the gain is exactly 1.

Because a Gain of 1 , doesn't make it boost or adds gain of 1 , there is NO gain at all, it's passing through

So it's doing what to the impedance? it's change a low input impedance to a high output impedance?

The Rin is 10K and Rf is 10K, the input impedance is 10K , so the output impedance is 10K?

 

[Audioguru]

Because a Gain of 1 , doesn't make it boost or adds gain of 1 , there is NO gain at all, it's passing through

"Passing through" is not an electronics phrase. People are passing through a doorway.
A gain of 1 is a gain of 1.
An inverting opamp inverts the signal.

So it's doing what to the impedance? it's change a low input impedance to a high output impedance?

The gain of an inverting opamp is Rf/Ri but the source impedance is in series with Ri and increases the value of Ri which reduces the gain.

The Rin is 10K and Rf is 10K, the input impedance is 10K , so the output impedance is 10K?

You do not understand ANYTHING in electronics.
The input impedance of an amplifier cannot increase its output impedance. The output impedance of most amplifiers is very low.
The output impedance of a 741 amplifier is shown on its datasheet. With a gain of 1 it is about 0.0004 ohms from DC to 10Hz then increases to about 75 ohms at 1MHz.

 

[danny davis]

But you said that Rin VALUE is the input impedance

The 741 output impedance is very low

"Passing through" is not an electronics phrase. People are passing through a doorway.
A gain of 1 is a gain of 1.
An inverting opamp inverts the signal.

What is the point of having a gain of 1? it doesn't do anything

 

[Audioguru]

But you said that Rin VALUE is the input impedance

EDIT: Now you have me speeking bad Engrish. I should have said, "The source impedance ADDS TO the value of Ri".

The 741 output impedance is very low

The output impedance of MOST amplifiers is very low. The output impedance of most stereo amplifiers is about 0.04 ohms or less at low frequencies so it damps the resonance of a speaker.

What is the point of having a gain of 1? it doesn't do anything

WRONG!
It does not need gain in its circuit. It does a lot of things:
It is a summing amplifier, it has a 10k input impedance and it has a very low output impedance. It is also an inverting circuit.

 

[danny davis]

It is a summing amplifier, it has a 10k input impedance and it has a very low output impedance. It is also an inverting circuit.

True

So it converts a High input impedance to a low output impedance

It's an impedance matcher