Comparing bode plots on REW+M4 vs Scope

[rpschultz13]

I have a nice scope arrangement: Siglent SD804xHD + SDG1022x Plus, will do automated bode plots (below I used 48 pts/dec). I use a Motu M4 with REW to do the same thing, a lot faster.

But when I compare them, they seem to diverge at low frequencies. Measuring a RC HPF on the same device (DUT):
Knob fully CCW: REW+M4 = 85 Hz
Knob fully CCW: Scope+AWG = 67 Hz
Spice sim: 58 Hz

Knob fully CW: REW+M4 = 335 Hz
Knob fully CW: Scope+AWG = 317 Hz
Spice sim: 285 Hz

Doesn't surprise me that the spice sim is different, same components but there may be unaccounted for impedances. But measuring the same DUT on both a scope and REW, the low end numbers are concerning. I tend to believe the scope. I don't think the M4 has built in HPF, but maybe there are impedances that are raising it's measurements at lower values?

Thoughts?

 

[John Mulcahy]

Did you do a loopback calibration of the M4?

 

[rpschultz13]

I have but maybe I should check that.

 

[rpschultz13]

I have but maybe I should check that.

To me this feels like an impedance issue creating another HPF.

 

[John Mulcahy]

You could try using the M4 headphone output, that should be very low impedance. The TRS outputs are probably 100R on each half of the balanced output. Could also do the scope measurement with the M4 input connected so its input impedance stays in circuit.