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Thread: The tintinnabulation of the noise
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face BY : David VanHorn email (remove spam text)



>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 :)



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