ISO-TECH GFG-2110 Sig Gen modulation issue

[rjmusto]

Hi all,
Have recently acquired a GFG-2110 signal generator. In general, it seems in good order and is working.

I have one issue though in that noise spikes appear when AM is activated.  I have tried to find a Service Manual or Schematic but not succeeded.
I also found that touching the top of C118 electrolytic changed its character to more of a burst, so assume the problem lies somewhere nearby.

See attached photos.

Has anybody seen this problem before and/or know where I could get hold of a schematic?

Thanks a lot.

[akimpowerscr]

Welcome to the forum.

Hanmatek DOS1102.....I do'nt trust this scope. 

Have you already checked the signal with another oscilloscope and another BNC cabel to be sure ?

[ataradov]

This is a rebrand of an Owon SDS1102, it is not that bad of a scope.

OP, try to set time scale such that you see the the carrier signal, and set the trigger level slightly higher than the signal level. Do you still see the spikes?

[rjmusto]

Hi Guys,
Thanks for the replies.

The DOS1102 is newly arrived (discount deal from Amazon, seemed too good to pass by), so is on probation you could say. And, yes, my first thought was that this was some artefact of the scope.

I have an old analogue scope here also, but these 'spikes' just don't show up, although it will trigger randomly when set above the main signal level, so my feeling is they are there.  I also have a boat anchor valve sig gen that I can generate much the same kind of modulated signal and this displays fine on the digital scope.

And yes, I can set the DOS1102 to trigger just of the spikes - see pick with distorted waveform. 

So my feeling is they are real.

On a whole different topic:  I have noticed that the DOS1102 does have a kind of aliasing issue on signals below around 2MHz (see second photo), one click lower in time/div and it displays fine.  But maybe digital scopes can have this effect? Not used one before....  And I think I could probably live with it....

[ataradov]

The distortion does not looks like a spike from the initial picture. Can you capture something more severe/discontinuous?

It looks like may be a looping artifact from the signal generator. What are the values for the carrier and modulation frequencies? Try to adjust them by a bit and see if that changes anything.

Most (all?) digital scopes have this aliasing effect. It is possible to avoid it by better post-processing of the signal for display. But it is still will be limited to what was captured in a first place.

[rjmusto]

Managed a quick look at the problem today....

The internal AM is a 400Hz signal. Also tried switching to external modulation input, but left it unconnected, then I can see these distorted cycles appear as soon as AM is switched on. Doesn’t happen with FM.

So whether it’s a fault or just an artefact of this generator, they seem to be real enough.