AFG-2005 Stability - Am I doing something wrong?

[rmel]

Hi Everyone,

I just got a new AFG-2005 function generator today, and I'm not sure if there's a problem with or if I'm doing something wrong.
I put a 48 ohm resistor across the output and measured the signal across the resistor with the scope.

I'm seeing two things:  1) little noise spikes all across the signal, like little hairs, and 2) instability in the frequency, especially at voltages < 1Vpp.

My scope is a Rigol DS2102.

I'm trying to post pictures, but I haven't figured out how to include them yet.

I hope someone can take a look at this and tell me if I need to return it or if it's me.

Edit:

The first picture is a square wave at 200mVpp and 1KHz.  Notice all the ghosting or hairs at the tops and bottoms of the signal.  Also, The Freq measurement at the bottom left bounces continuously between about 995kHz and 1.020kHz.

The second is a sawtooth at 500mVpp and I think also at 1kHz.  The "auto" button of the Rigol could not trigger on a clean signal.

For both of these waveforms, if I bump up the voltage to say 1.5Vpp, it's more stable, but I still see the Freq measurement bounce around a bit.  For the sawtooth, I still occasionally see a waveform 180 degrees out of phase.  This flashes across the screen very quickly and I have to hit run/stop several times to capture this.

[w2aew]

Try connecting the ADG directly to the scope with a coax cable.  It'll be ok at these low frequencies, except for possibly some ringing on the fast edges of the pulse waveform. If the spiky features go away, then you know they were some kind of stray pickup from your probe setup.

The reason that you're not getting a stable waveform is that the scope is sometimes triggering on the spiky features.  If your scope has a HF Reject function on the trigger coupling, try it.

As for the frequency stability, could be due to the spiky features too, or just limited resolution of the frequency measurement on the scope. Try a bit of averaging.

[rmel]

I tried putting the scope probe tip directly into the AFG's output jack, but I'm not sure it helped that much.

I have a 20mhz bandwidth limit on the scope and it seems to help a bit.

I don't have a coax cable I can use to connect the AFG directly to the scope, so I'll have to pick one up.

I think I might have a bit of a noisy environment.  I have a couple of computers running on the same power circuit.  Could these contribute to the noise?

Thanks!

[w2aew]

I tried putting the scope probe tip directly into the AFG's output jack, but I'm not sure it helped that much.

I have a 20mhz bandwidth limit on the scope and it seems to help a bit.

I don't have a coax cable I can use to connect the AFG directly to the scope, so I'll have to pick one up.

I think I might have a bit of a noisy environment.  I have a couple of computers running on the same power circuit.  Could these contribute to the noise?

Thanks!

Possibly. Connect your probe to nothing but the 47 ohm resistor and see if the scope picks up any noise? 

Of course, the direct coax connection between the gen and the scope should tell you for sure if the noise is in the generator.

[rmel]

Hi, sorry for the long delay.  I've been under the weather during the holiday, and I'm just now catching up.

Finally made it to the store and found that BNC cables are quite expensive, and since I don't see a use for a BNC-to-BNC cable other than this test, I thought twice about purchasing one.

I have had a chance to play around some more with this unit, and overall, I think I am happy with it.  I also learned some more about my DSO and the fact that I can pick up noise due to its high bandwidth.  I did find that turning on the bandwidth limit on the scope helped.  Also, trying to keep the resistance across the AFG's outputs close to 50 ohms seems to help too.

Looks like I have a lot to learn!

Thanks again for your help.