| ??? 06/18/09 13:26 Read: times |
#166223 - be careful about your investment ... Responding to: ???'s previous message |
Per Westermark said:
Few people can build good DIY scopes. Just think about delay times and phase shifts for a 100MHz scope. And think about the cost of a gigasample/s-kapable ADC. Of what it means to implement a staggered read where you have a fast sample-and-hold circuit but a slow ADC and capture repetitive signals by capturing the same curve multiple times with a time-shift between each pass. Think about the problems just 10ns of extra delay in your trig circuit will introduce. That's the sort of thing that associates a nice display with an instrument that's really rubbish! The critical item in an oscilloscope is triggering. Until the '80's, Tektronix had the best triggering in the industry. HP had a cheaper triggering circuit that didn't work as well. The cheaper circuit drove the "good" one off the market, and the result is that modern 'scopes don't trigger as well or as cleanly as they did in the '70's. Trigger circuits tend to use tunnel diodes. The cheap things offered on many PC-based instruments use fast comparators instead, and, while these work for most things most of the time, over the long term, they are found to be much-less-than-ideal. It is very easy to do low-end scopes - basically reasonably fast data loggers with a low number of bits of resolution. But if you step up the analog resolution or the sampling frequencies, life isn't so fun anymore. Before one decides on a test instrument, one should first become very familiar with a number of instruments, real instruments, and not PC-based toys, that can be applied in the conventional manner. That's the only way to become aware of the things that are really important in such an instrument. Then, if one finds a PC-based instrument that meets one's expectations and can do all the things conventionally done with a "real" stand-alone 'scope or logic analyzer, he can make a rational decision as to what he should acquire. There are many things that are quite easy and quick to do on a stand-alone instrument that are slow and painful, if at all possible, when using a PC-based instrument. There are exceptions, of course, namely some of the instruments that simply build the PC into the instrument. They have none of the OS-related problems, hence can focus on making the instrument serve the user rather than the OS. RE |
