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Signal Generator's actual and expected output differs - is it broken?

Keltis

Mar 18, 2021
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Greetings,

today I received a FY6900 DDS Generator and I think it is faulty. I kindly ask you to affirm this by visiting the facts I present.

My assumption: It says it outputs 1V but the oscilloscope shows 10 Vpp. I am pretty sure that the oscilloscope is working correctly, to the signal generator must have a bug or something.

The first picture shows the function generator. Except the frequency the settings are unchanged. Since AMPL is 01.0000V, I would expect 1 Volt.
function_generator.jpg

The second picture shows the oscilloscope's screen. It has a resolution of 5V per square, so clearly the Vpp is 10 Volt. How can this be?
oscilloscope.jpg

You can find the specification sheet on feeltech site.

Thank you in advance for you insights!
 

kpatz

Feb 24, 2014
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Are you sure the scope is set to 5V/div? It's not clear looking at the image... I don't own that scope so I don't know what the indicators on the display mean and if they represent V/div. Some images I've seen of the same scope on the internet shows it can display Vpp for a waveform, maybe turn that on to verify.

My wild guess is it's the scope settings and not the signal generator.
 

Harald Kapp

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Nov 17, 2011
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Hvae you actually activated the settings on the generator? If not you may simply see the output from the default settings.
The manual states:
At start-up, the dual channels are configured to output a sine
waveform with 10kHz frequency and 5Vpp amplitude by default. Two channels
use default setting saved at Position 1 when power on. Users can configure the
instrument to output various waveforms.
You see 10 VPP, that may simply be due to the output not being properly terminated. The output impedance of the generator is 50 Ω and all data in the specs are given with respect to the output being terminated with 50 Ω load impedance. The 10 VPP you will drop to the 5 VPP stated in the manual once you properly terminate the output with 50 Ω
 

Keltis

Mar 18, 2021
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I carefully checked all your hints, thank you!

By going through the scope settings I stumbled upon the problem, the probe impedance was set to x10 ... after setting it to x1 the scope shows 5 Vpp.

Now I only have to understand why. Guess I therefore have a heavy schedule at the weekend. :)
 

Harald Kapp

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the probe impedance was set to x10 ... after setting it to x1 the scope shows 5 Vpp.
This sounds illogical. Changing the probe settings from x10 to x1 should reduce the displayed amplitude by a factor of 10, not 2.
 

Keltis

Mar 18, 2021
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True, it really sounds illogical. Let me explain:

I set the amplitude of the sinusoidal output to 5Vpp and the frequency to a very small value. I recorded a short Gif to demonstrate what the voltmeter is showing. So the voltage is oscillating between +2,5V and -2,5V, so the signal generator seems to work correctly and indeed the scope has a wrong setting.

This is a cursor measurement of the probe set to x10 impedance in the settings. It shows 50Vpp.IMG_20210319_094034.jpg

And this is the same measurement with the probe set to x1. It shows 5Vpp
IMG_20210319_093447.jpg


Since I am an electronics amateur this is quite magical to me...

Do you still see any evidence that the probe setting is not the reason for the unexpected output?
 

Harald Kapp

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What you see is the expected 1:10 variation. When you change the scope setting for a 1x probe or a 10x probe this doesn't affect the actual measurement, it only tells the scope to mathematically multiply the measured value by 10 (for a 10x probe setting) to compensate the attenuation of the probe when displaying values.
The shown change in values doesn't make sense either. What is the setting of the multimeter? DC or AC? Is the multimeter capable f measuring a 10 kHz signal at all?

To me the observed signal still looks like the default output from the generator. Try changing the frequency to e.g. 2 kHz to see whether this affects the output of the generator.
 

kpatz

Feb 24, 2014
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When you switched from x10 to x1 did you do it in the scope settings or on the probe itself?

The probe should have a x1/x10 switch on it. When set to x1 it passes the signal as-is. When set to x10 it attenuates the signal to 10% of its actual voltage, to allow measuring higher voltages than the scope otherwise would be able to.

If there's a x1/x10 setting on the SCOPE, it should be set to match the setting on the probe. The scope, when set to x10, multiplies the voltage it sees by 10.

If you had the scope on x10 and the probe on x1, and pass a 5v p-p signal, you'll "see" 50v p-p shown on the scope, due to this multiplication (and lack of corresponding attenuation on the probe).
 

Keltis

Mar 18, 2021
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Oh jeez...

it only tells the scope to mathematically multiply the measured value by 10 (for a 10x probe setting) to compensate the attenuation of the probe when displaying values.

I used a BNC to BNC connector between the scope and the generator without switching the probe impedance from x10 to x1... and the actual (physical) scope probe has x10...
 
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