Siglent SDG2042X Low Frequency Limitations and Linearity

[Peter_O]

(Hi there, great forum! I've been reading some time and learning a lot, today this is my first post.)

I've go a Siglent SDG 2042X Generator.

Use the ramp waveform to generate a sawtooth signal (highLevel -2V, lowLevel 2V, symmetry 100%)

With a Siglent SDS 1202X-E Oscilloscpe:

Above 50Hz everything seems to be fine. (addet screenshot 50Hz)


Below 50Hz
- the raising slope is increasingly nonlinear.  (addet screenshot 5Hz)
- the signal gets an increasing negative offset. (see same screenshot)


Connection is a RG 58 BNC cable straight from out to input.

1) Is this normal and to be expected?
2) Why?
3) Spec limits of Function-Generator?
4) Beginner's error?

[radiolistener]

I have the same behavior for AC coupling. At 20 Hz it looks like curve.

Could you please show 100 Hz square wave with DC coupling?
Here is how it looks on my scope:

[radiolistener]

here is 5 Hz ramp with AC and DC coupling on my scope:

[Peter_O]

Others can explain it better, but so slow signals and AC coupling does not work. With AC coupling there is a capacitor in the signal path. Change the channel to DC coupling.

Hmm. Theoretically this should have been clear to me. Practically it was not.
Works fine now.
So it was 4) Beginner's error.  :-)
Hoped for that.
Thx a lot Peter for the quick reply. 

[radiolistener]

Works fine now.

Could you please show 100 Hz square wave with DC coupling on your oscilloscope?

[Peter_O]

Could you please show 100 Hz square wave with DC coupling on your oscilloscope?

Sure.

Your shot above might show a misaligned probe compensation.

[radiolistener]

Your shot above might show a misaligned probe compensation.

No, I got screenshot for well matched 50 Ohm connection with direct RG58 cable

[Peter_O]

Just for my curiosity: What are you after when asking for a 100Hz square wave shot?

[radiolistener]

Just for my curiosity: What are you after when asking for a 100Hz square wave shot?

just interesting how your oscilloscope response (at low frequency) is flat in comparison with my oscilloscope.

Your oscilloscope has very flat response at low frequency 

[bdunham7]

just interesting how your oscilloscope response (at low frequency) is flat in comparison with my oscilloscope.

Your oscilloscope has very flat response at low frequency 

Something is funky with your setup....try not using a 50R termination to start with. 

[radiolistener]

Something is funky with your setup....try not using a 50R termination to start with.

All is okay with setup. The setup is just 1 meter RG58 coax cable with BNC connectors connected from 50 Ohm gen output to oscilloscope input. With 1 MOhm input mode the same issue.

This is due to non flat frequency response of oscilloscope at low frequency.

You can also see this issue on the ramp waveform above (SDS00096.png), just look at the bottom corner of the waveform

[bdunham7]

Something is funky with your setup....try not using a 50R termination to start with.

All is okay with setup. The setup is just 1 meter RG58 coax cable with BNC connectors connected from 50 Ohm gen output to oscilloscope input. With 1 MOhm input mode the same issue.

This is due to non flat frequency response of oscilloscope at low frequency.

You can also see this issue on the ramp waveform above (SDS00096.png), just look at the bottom corner of the waveform

So what does a 1 Hz square wave look like?   

[radiolistener]

So what does a 1 Hz square wave look like?   

Here is square wave for 1 Hz and 10 Hz.

There is some change in amplitude between 20 Hz and 100 Hz. Just look for sine wave 20 Hz and 100 Hz.
20 Hz = 1.22 Vpk-pk
100 Hz = 1.16 Vpk-pk

For higher frequency oscilloscope also shows 1.16 Vpk-pk. So, there is a curve at about 20-100 Hz interval.

[radiolistener]

Full set for comparison

1 Hz, 10 Hz, 100 Hz, 1 kHz, 10 kHz, 100 kHz, 1 MHz, 10 MHz square wave.

All wave is 1.20 Vpk-pk.

oscilloscope: Siglent SDS1102X

cable: 1 meter RG58
cable loss:
0.101 dB at 1 MHz
0.070 dB at 10 MHz
0.070 dB at 20 MHz
0.081 dB at 30 MHz
0.080 dB at 40 MHz
0.103 dB at 50 MHz
0.123 dB at 60 MHz
0.142 dB at 70 MHz
0.174 dB at 80 MHz

[Peter_O]

Have you tried another cable?
Have you tried another FG?

[radiolistener]

Have you tried another cable?

yes, I tried RG402, RG316, RG174, RG58, tried  different length - 1 cm, 5 cm, 10 cm, 20 cm, 32 cm, 50 cm, 100 cm, 200 cm. Exactly the same behavior.

Have you tried another FG?

yes, exactly the same behavior with different generators.

Even more, other people with the same oscilloscope have the same issue.


Here is result 100 Hz square wave on another generator (hand made XR2206):

[radiolistener]

And another generator si5351.
Connected directly into oscilloscope connector through SMA-BNC adapter.

10 kHz, 100 kHz, 1 MHz, 10 MHz and 100 MHz square wave with rise time 1 ns.

Since signal rise time is 1 ns, you can also see actual oscilloscope rise time: 2.6 ns. So, the actual bandwidth is 134 MHz.

[radiolistener]

And another generator ADF4351.
Connected directly into oscilloscope through SMA-BNC adapter.

50 MHz, 100 MHz, 150 MHz, 200 MHz, 250 MHz, 300 MHz, 350 MHz, 400 MHz, 450 MHz and 500 MHz square wave.
Amplitude is 0 dBm (635 mVpk-pk).

[Peter_O]

What about Siglent's or dealer's support?

[radiolistener]

they know about that and sent me replacement main board. But I still didn't tried it. It is still sealed. Guarantee period is expired, so need to test a new board. But I'm scary that board replacement may break some calibration and I lose my oscilloscope

[Peter_O]

But as they agreed in this to be a failure of the board, it might be a good idea to take the risk. And nobody here showed up so far, who saw a mistake on your side.