I've been trying to focus a bit more on Analog circuits lately and found that my design and actual results often don't match up exactly. An example is a Pierce oscillator I built a while back, the circuit worked mostly as expected but the gain required was much less than I calculated for (meaning I got some distortion). Is it common practice to just change the values to what seems to work right and go on or is it better to make sure the theory matches up as well?
To clarify, I mean that after observing the difference and playing around with some values I change the design to match, the circuit then gives the desired output but my initial design was inaccurate. Should I go back and try to identify why or is it just not worth it? ie, no calibration on production units.
Thanks for all the replies, I'm going back to the drawing board because there's clearly something going on that I don't understand. Ignoring it would possibly create other problems.
Best Answer
If your prototype performance doesn't match your anticipated performance this can indicate an error in the design process or a build error. You really should sort this out if you are going to be making several units or more because it will only come back and bite you. ISO9000 talks about "Design Verification" - this is basically proving the design/prototype does what it was intended to do. This is important and shouldn't be short-changed - if your pierce oscillator doesn't produce the correct amplitude I'd want to know why.
"Design validation" checks that production units are still meeting expectations.
A really good test that I always do is spray the circuit with a freezer spray and see what happens and I also touch each chip and transistor with the tip of a soldering iron. This is done on the bench and I'm looking for unexpected brief changes in amplitudes (or some other parameter) as an indicator that something is wrong. If all is OK then nothing beats a good full load test at elevated temperatures. This can find a lot of anomalies on a complex circuit.