PW,
its floating like shown in the red circle in this modules connection diagram.
Itsu
Itsu,
I would like to impose upon you to make a few scope measurements. You will have to be able to make differential measurements with your scope. Please let me know if your are willing to do so.
For now, these measurements only need be a single diff channel with an external trigger. Initially, you can try using the two channels of your lower cost Owon scope to make these measurements.
Are you familiar with using a two channel scope to make a single channel differential measurement (ie, invert and add one channel to the other)?
Here is a brief tutorial:
Use a matched pair of 10X probes. Remove the ground leads from the probes. Connect the probe BNC's to the scope's two channel inputs.
Connect a wire (14-16 gauge) between the scope's ground lug and the ground (lug?) on a function generator. Connect a second ground wire between the scope's ground lug and the push-pull PCB ground.
Connect a third X1 probe (no match necessary) to the external trigger input and the tip of the probe to the scope's calibrator post. Set scope to trigger on the external input.
Connect both of the 10X probe tips to the scope's calibrator terminal. Set both scope inputs to AC coupling and display the cal signal on both channels of the scope. Set vertical gain for around 50-60% of the display height for both channels' displayed square wave. Overlap the square waves and center them about the zero volt base line (center the two square waves vertically on the display). Set the horizontal so as to display one or two cycles of the cal signal. Keep both vertical gain settings identical. You should see two overlapping square waves.
Set the compensation on the probes for best HF response (minimum undershoot/overshoot on leading edge of cal square wave).
Set the scope so as to invert one channel and add it to the other channel (scope used must have this capability, most do). You should now see very little signal. Increase the vertical gain on both channels two or three ranges. You should still be seeing very little signal (keep the vertical gain identical on both channels).
The residual signal that you will probably see at this point could be: very close to nothing, a small amplitude but nicely flat square wave, or a leading edge/HF peak with or without a visible bit of square wave.
If you see a hint of HF peaking, adjust ONE probe's compensation to minimize that peak and flatten any residual square wave.
Depending on the scope used (some may or may not be able to do this):
To eliminate any residual square wave, take one channel's vertical gain out of cal and adjust gain to minimize the display of residual signal. Depending on the scope, if a minimum cannot be reached using one channel's vertical gain, put that channel back in its cal position and try minimizing the residual with the other channel's vertical gain.
Some scopes cant do this with the required resolution, depends on scope design. If not, you will need to live with the residual.
Now:
Disconnect one probe from the calibrator post and touch it to the ground lug on the scope. You should now see the the calibrator square wave and be making a differential measurement. Increase or reduce vertical gain as needed, by reducing the vertical gain on both scope channels. The vertical gains on both channels must be set identically at all times.
You are now measuring the scope calibrator differentially.
Set your function generator to output a 10VPP, 100K sine wave. Connect the FG's sync output to the scope's external trigger input. Trigger the scope on this external input.
Connect one probe tip to FG ground and the other probe to the FG output. The scope should display the FG's sine output. Adjust the vertical gain on both channels to display the FG output at a reasonable height on the scope display (again, keep both scope channels' vertical gains identical at all times). You should be seeing the FG output on the scope display. There should be no clipping of the sine waveform.
Set FG for a 10VPP 100K square wave.
Adjust the time base and both vertical gains to display a cycle or two of the square wave (keep both channels' vertical gains identical at all times).
Now connect both probe tips to the FG output. You should see very little signal. If you see any residual HF peaking on the display, you can adjust ONE probe's compensation to minimize that peaking.
Post a shot of the scope making a diff measurement of the FG output (one probe to FG out, other probe to FG gnd) and another with both probes on the FG output. This demonstrates the diff measurement and the common mode rejection.
Let me know if you have any problems doing this, but if all has gone well, you can now make AC coupled differential measurements with decent enough common mode rejection for the required purpose.
Once you are able to do this, we can discuss the measurements I would like you to make if you are willing to do so.
Thanks,
PW
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Topic: Dally, Shark & Ruslan workbench (Read 310007 times)
