What am I doing wrong?

[pkr]

I'm a bit new to this so I suspect I'm doing something wrong and I'd appreciate any pointers.

I took delivery today of a Siglent SDG1020 to replace the cheap eBay special I already have.  The problem is, it isn't behaving as I would expect.

I've set a 30Hz square wave at 50% with 4vpp ( just playing about, the values aren't significant ) and my scope is telling me the signal is +7.4vpp with an offset -3.84v.



Thinking the problem may be with the cheap scope*, I plug in the old generator with the same settings and I get on the scope what I expect!  30Hz, 4vpp, 0v min, 4v max, 50% duty.



I've tried the 'Self Adjust' menu item with no difference and channel 2 produces a similar effect - although I can't set 4vpp on channel 2 ( SDG1020 limitation ).

I naturally think that it's me doing something wrong but when I see a 4vpp setting and the scope is saying 7.4vpp there is an element of doubt that the SDG1020 is working properly.

Help please! And apologies in advance if it's me being stupid.


* and yes I've seen Dave's video about it and I know his opinion! A new one will be purchased in the new year

[deephaven]

It says "load 50 ohms" so maybe you just need to terminate the signal with 50 ohms, otherwise it will be double what you're expecting. It might not be able to do the full 8V p-p which could account for the lower than it should be value.

[grumpydoc]

deephaven is correct about the 50 ohms load although I would say it is much more likely that the toy oscilloscope is getting the voltage wrong.

The sig gen will default to outputting a singal which is symmetrical around 0V so it will be swinging between -4V and 4V (or -2V and 2V if you terminate it into 50 ohms).

[pkr]

I though I'd tried HighZ, but I was probably confusing myself by then.  I'll try what that again.

Thanks.

[pickle9000]

Do some basics

- Swap your probe leads and see if the dc offset changes.
- Connect a 1.5 v batt to the scope and see what the voltage is, reverse the polarity and check again.
- Place a cap in series with the signal in to strip off the dc.

Any of these will help isolate if the scope is having an issue with the dc.

If the scope has a cal feature do that before and after to see if there is a change.

[fubar.gr]

Allan (W2AEW) has an excellent video on the topic:

[pkr]

Thanks guys.  It was indeed my problem but I've got my head around it now!

I had seen another video about 50R & HighZ - a guy with a German/Austrian accent which explained it very well using an analogue voltmeter.

Thanks again.