|Aug 22, 2012, 09:35 PM|
Rage over the switching speed
So the final transducer driver board came together. Got extremely short transition times & very loud pings. 30V transitions in 2us are 33% faster than the LF353, but getting short transitions is a very tempermental affair. The transition speed really does matter as much as the voltage, with these transducers.
There's an optimum ratio of current to drive the pullup & pulldown transistors in the h-bridge. Giving both transistors 0.3mA gives rise & fall times of 3us. Raising the pullup transistors to 3mA gives nanosecond rises, but lengthens the falls to 5us. Giving both transistors 3mA gets the rise to nanoseconds & the fall to 2us. So more current buys more speed, but only if the transistors are balanced.
The pulldown transistors have a hard job. They have to remove the charge built up on a capacitive speaker, but the speaker acts like a charge pump. When 1 pullup transistor starts, it tries pushing the 30V generated by the other one to 60V. In reality, it only gets to 40V before the transistor breakdown voltage stops it. That's causing a 40V spike & delay before the pulldown transistor starts removing charge.
There are ways to optimize it, controlling it with 4 GPIOs & staggering the transistors, using the charge pump effect to double the voltage. A career can be devoted to optimizing pings.
The Rigol resets to writing waveforms, every time it's rebooted. You have to remember to constantly set it to writing bitmaps, or nothing useful gets saved.
Also, 1 of the probes has already stopped working in 10x mode, so nothing over 30V can be measured. Reality of the $350 scope has definitely begun after only 1 week. Fortunately, anything with a BNC connector should work as a probe.
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