oscilloscope and function generator

Thread Starter

PG1995

Joined Apr 15, 2011
832
Hi again, :)

My question was:
I think this scope has internal resistance of 1MΩ. But you will see 25PF. What is this "25PF"?
Jaguarjoe's reply was:
1 megohm/25 pf means that the scope input "looks" like a 1 megohm impedance with a 25 pf capacitor across it.
Could someone please show me by drawing a circuit the actual configuration of the capacitor? Isn't 1 mega ohm resistor alone enough?

Thank you.
 

Thread Starter

PG1995

Joined Apr 15, 2011
832
240 volt mains is 240 volts RMS. The scope measures peak to peak voltage. 1v RMS is 2.82v P-P. 240v RMS is a lot P-P.

The scope has a "Time/Div" selector switch and a "Variable-Cal" control which determines the sweep rate. With the VAR knob full CW to the Cal position, the Time/Div switch will tell you how fast the sweep moves across the screen for 1 major division. If your mains are 50Hz then 1 cycle will occur in 20ms. If you set Time/Div to 20ms/div then you will see 1 cycle in 1 division. If you set it to 2ms/div, you will see 1 cycle in 10 divisions. Generally, in the Cal position, the sweep is only accurate to 3% or so.

1 megohm/25 pf means that the scope input "looks" like a 1 megohm impedance with a 25 pf capacitor across it.
Hi, once again, :)

This is the o-scope under discussion (you can check the attached image instead): http://img8.imageshack.us/img8/5408/oscillo.jpg

The amplitude and frequency of a wave is determined by the device generating the signal such as function generator. A o-scope only graphically shows the wave.

Frequency = 1 / (Time Period)

If I have set TIME/DIV equal to 2 sec and the one wave occupies 4 divisions then we can calculate its frequency. The period is 8 sec, (4 divisions x 2 sec).

Now include the effect of Variable-Cal knob which adjusts the sweep rate. Won't it affect the period and frequency measurements of the wave?

The Jeguarjoe says:
With the VAR knob full CW to the Cal position, the Time/Div switch will tell you how fast the sweep moves across the screen for 1 major division. If your mains are 50Hz then 1 cycle will occur in 20ms. If you set Time/Div to 20ms/div then you will see 1 cycle in 1 division. If you set it to 2ms/div, you will see 1 cycle in 10 divisions


Please help me with the query above. Thank you.

Regards
PG
 

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someonesdad

Joined Jul 7, 2009
1,583
Now include the effect of Variable-Cal knob which adjusts the sweep rate. Won't it affect the period and frequency measurements of the wave?
Yes, it will. You can only make calibrated measurements if the variable calibration knob is in the calibrated position (they often had a click into this position).

A similar comment applies to the adjustable gain for the vertical amplifier.

One might wonder why then would a manufacturer bother to put these variable knobs on the scopes? One reason is that this would let you adjust the waveform either in the time axis or the vertical axis to get the waveform spread over most of the screen. This is handy especially when you want to compare one waveform against another. With modern digital scopes, you'll often just use the measurement function to quantify some aspect of behavior.
 

Thread Starter

PG1995

Joined Apr 15, 2011
832
Thanks a lot for the help.

Yes, it will. You can only make calibrated measurements if the variable calibration knob is in the calibrated position (they often had a click into this position).
I guess by "calibrated measurements" you mean original frequency etc. of input signal (e.g. a signal from a function generator). In the picture in my previous post the VAR SWEEP is in CAL position.

One might wonder why then would a manufacturer bother to put these variable knobs on the scopes? One reason is that this would let you adjust the waveform either in the time axis or the vertical axis to get the waveform spread over most of the screen. This is handy especially when you want to compare one waveform against another. With modern digital scopes, you'll often just use the measurement function to quantify some aspect of behavior.
Exactly. Actually I had access to a o-scope yesterday and the wave under observation was too much 'congested'; I mean peaks were very close to each other. Accidentally I turned the VAR SWEEP knob and one cycle of the wave spread over the larger area along the x-axis (time axis).

But there should be some way to calculate the 'original' frequency etc. of the input signal even when the VAR SWEEP knob is used. Please let me if there is some way. Thank you for your help and time.

Regards
PG
 

someonesdad

Joined Jul 7, 2009
1,583
But there should be some way to calculate the 'original' frequency etc. of the input signal even when the VAR SWEEP knob is used. Please let me if there is some way.
I don't see any a priori reason why there should be. However, you can investigate the question yourself. You need to be able to apply signals of known frequency; a calibrated DDS function generator is probably the tool of choice. Then quantify the relationship between the variable knob's position and the time/div of the scope. If you're going to use this feature on different timebase settings, you'd want to find out whether the knob position vs. sweep relationship you find is independent of the timebase setting (i.e., there could be some subtle interaction between the settings). Frankly, I've never known anyone who would spend the effort doing this.

A modern digital scope typically obviates this need; the main digital scope I use doesn't even have the ability to vary the time base setting like the analog scopes could and I don't miss the feature.
 

tom66

Joined May 9, 2009
2,595
Most digital oscilloscopes use a crystal oscillator for the timebase and are very accurate, for example mine is specified at ±0.005%.

Whereas analog oscilloscopes use an oscillator based on a resistor and a capacitor. An integrator ensures a linear sweep. However the tolerances of the resistor and capacitor are crucial.

A good way to calibrate it would be to use a computer's sound card. The sound card has a master timebase usually derived from a 24.576 MHz crystal. This isn't going to be a super duper crystal, but is probably at least ±0.1%.

Use a program like Audacity to generate a 1kHz wave on the headphone output, and then calibrate your oscilloscope until it shows correctly.
 

Thread Starter

PG1995

Joined Apr 15, 2011
832
Thank you, someonesdad, Tom.

I don't see any a priori reason why there should be. However, you can investigate the question yourself. You need to be able to apply signals of known frequency; a calibrated DDS function generator is probably the tool of choice. Then quantify the relationship between the variable knob's position and the time/div of the scope. If you're going to use this feature on different timebase settings, you'd want to find out whether the knob position vs. sweep relationship you find is independent of the timebase setting (i.e., there could be some subtle interaction between the settings). Frankly, I've never known anyone who would spend the effort doing this.
Yes, I will investigate the question! :) But I don't know why you say that you don't see any reason why there should be a way to know 'original' frequency of the signal etc. Isn't o-scope used to make such calculations? At least this is what I have been told that o-scope graphs the signal and we can make calculations etc. Think about a graphing calculator. Would it be doing its job properly if it's scale doesn't really map the original function? Now I'm curious why you said that. I understand that there would be other functions for o-scope besides calculations.

In the attached picture, I have noticed that there are also additional knobs on the VOLTS/DIV knobs of CH1 and CH2. So, I guess they are used to spread the wave vertically. Correct?

I have unsuccessfully tried to understand what the TRIG LEVEL knob do; though some good people did tell me but I couldn't get their point. Could you please give it a try?

Thank you very much.

Regards
PG
 

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oldtech33709

Joined Sep 24, 2011
26
You can use the built in cal signal near the bottom of the scope. This is a 2V p-p square wave for adjusting the compensation of the scope probe to the scope. Use this signal to check the accuracy of your time base . The trigger level sets the voltage polarity and level that the scope recognizes a trigger to start a sweep. When the trigger knob is pushe in you have a positive going trigger.
 
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