>I think I am convincing myself that this would not work well anywhere close to 60 or 120 hz X2 Nyquist sample rate - you are better off with 4 or 5 or 8 samples during each cycle, and sampling over a few cycles. Nyquist was an optimist, just like Murphy! No, but he's often misunderstood. The Nyquist rate is : The reciprocal of the Nyquist interval, i.e., the minimum theoretical sampling rate that fully describes a given signal, i.e., enables its faithful reconstruction from the samples. Note: The actual sampling rate required to reconstruct the original signal will be somewhat higher than the Nyquist rate, because of quantization errors introduced by the sampling process If you want to accurately describe a non-square wave, you need more samples. >So far I have figured on having a low pass RC filter on the input tuned to attentuate 60 hz severely. If you're measuring DC, that works. If not.. read on for a horror story. >I am curious about a "software low pass filter" technique mentioned in another post.... > You could also take multiple samples at some rate that is an integer > multiple of 60 Hz, and mathematically average them. It's important that you > take an integer multiple of samples though, because otherwise you'll be > left with a "stub" of a cycle that won't average out. I got involved with this, as a cleanup in a project where the departed head engineer stuck a stepper motor, with 24W drive, an inch away from a magnetic pickup head, whose output on a good day is a few uA of the desired signal. Famous last words: "If it causes a problem, then I'll fix it." The only good part was that the motor stepping and the ADC sampling off the head amp, were locked in sync, and an exact multiple of 60 Hz, in order to reject power line noise. In our case, the motor noise waveform was an entirely variable quantity, from near zero to several volts, due to unrepeatable internal magnetic balancing in the mag heads. (move the pickup coil 3 mils either way.) Shielding was very effective. Without it, there would have been much more signal. The amplifier output, on the desired signal was about 1V. (+/- 50% in amplitude due to the media) Due to some un-named person's excellent layout skills (me) the amp got no pickup directly from the chopped 1A drive circuits less than an inch away, or anything else to speak of. The only noise source was that which was magnetically coupled into the head. To be fair to the head manufacturer (Vikron) the head design was excellent. The motor field was HUGE (As in Biblical, plague of locusts huge), and their careful design nearly eliminated all the external fields. This rather noisy output was fed into a fun little "ADC", consisting of three Xor gates, arrainged to form a sigma-delta converter, with about 6 bits output, due to timing constraints in the Z8 controller. (the Z8 divides the xtal by 12, and many instructions are multi-cycle) How we got rid of the noise: We sampled at 7200s/s for 5 seconds. During that time, data would happen, and a period where there was no data. We averaged the data into a buffer 120 samples long, inverted it, and added the buffer to the original samples. This gave us an inverted copy of the reinforced noise signal (exactly like in a boxcar averager) at the same amplitude. This gave us motor noise, 60 Hz junk, and 120 Hz junk, all at one fell swoop. NOTE: Analog filters would not have been possible. The data being recovered is also in these bands, and all analog filters have phase shifts around their cutoffs. It would be possible to get rid of the noise, but the data would also have been gone. Unfortunately, this couldn't completely eliminate the noise, since the head moves during the read process, and this amplitude modulates the motor signal, and to a lesser degree, external 60 and 120 hz fields (inverse square law). The period of the modulation was too long for the same technique to be used to remove it's residual. We sold a european version, with the sample rate set to match up to 50 Hz power. FWIW, the mag head mounts directly on the two-layer PCB, which is routed in a serpentine pattern to create a spring, loading the head against the document and drive wheel, and eliminating the need for any cables or mounting brackets. (patented :) -- Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9 -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu