A bipolar pulser for NMR, originally based on a linear amplifier which wasn't fast enough.
2021-09-10
Moved to DMNOIXDD614 which uses a pair of IXDD614 MOSFET drivers to alternately drive the two sides of the load. Shown here with 5 Ohm load and ~9 inches of coax, just driving a single side:
Load is 5 Ohms. Measuring with standard scope probes, with violet trace being A-B. Yellow trace is the driven side and green trace is the undriven side, which jumps up because the undriven side has a 5 Ohm source terminator (as does the driven side). The issue is the relatively long relaxation time after the pulse – almost 40 nsec to return to baseline. The team is hoping for something closer to 10 nsec, and certainly shorter than 20 nsec.
To see how much the cable length affects the measurement, I shortened the cable to a pair of wire stubs, < 2". This is the result:
Less of a reflection caused by the almost shorted cable end, and some improvement in the relaxation time, but it's still there.
2021-09-11
Trying a ToprDOUT2LBP board, lightly modified to create two pulses adjustable from 0V to 10V into no load, with 5 Ohms of source termination.
The ToprDOUT2 has reduced drive capability, but maybe faster? Mounted onto a PBJ and driven with a pair of 40 nsec pulses, this time spaced by about 100nsec so we could see the effects of each pulse more clearly.
I also modified the setup, getting rid of the probe cables and mounting the resistor within 3 inches of the scope inputs:
What you see here are roughly 12" RG58 cables coming from the ToprDOUT2 board, hitting the 5 Ohm resistor mounted onto a heat sink, then heading into the scope inputs. Here is the result:
Note the impressive ringing from the 5 Ohm termination. Clearly the EL7457 used in the ToprDOUT2 board has much faster rise/fall time than the IXDD614, but has the falling edge artifact gotten better? There's so much ringing that it's hard to tell. Notice that the time scale has changed from 20 nsec in the previous pictures to 50 nsec.
I backed up and fed the DMNOIXDD614 into the new setup:
The tail is back, and worse than the scope probe version, but it gives us a point of comparison with the ToprDOUT2.
It looks to me like the ToprDOUT2's driver is coming closer to the desired waveshape than the IXDD614. However, it is also clear that the cable length would need to be kept extremely short to control the ringing.