"Roy J. Tellason" bravely wrote to "MIKE ROSS" (12 Jan
04  12:06:08)
 --- on the heady topic of "bag of chips"

 RJT> Speaking of chunky waveforms...

 RJT> I wanted a function generator for a long time,
[,,,]
 RJT> Here's the thing,  though -- you get real low in frequency,  and all
 RJT> of a sudden the output waveform gets *real* distorted.  Sine or
 RJT> triangle waves have a chunk out of the middle of the top part.  I
 RJT> can't recall offhand what squarewaes do.  I showed this to one guy,
 RJT> and he said to me "get that power supply out of the box",  but it
 RJT> being a transformer and the rectifier and filters being on-board,
 RJT> that's a little difficult to do.  I did add more filter capacitance at
 RJT> one point,  and that didn't help much.  I think that what I'm likely
 RJT> to try next is removing the zener diodes that are in there currently
 RJT> and sticking a couple of 3-terminal regulators in there instead,
 RJT> since I have them on hand.  Along with associated caps and such.
 RJT> Other than that,  I don't know...
 RJT> You guys have any ideas on this?

There seems to be some kind of feedback into the reference voltage
through the power supply. Probably adding like 10 times more filtering
might help it but your regulator idea seems much better. However, you
may need to increase the input AC transformer voltage because the
78xx regulators usually need a couple of volts compliance (i.e. 12 volt
reg needs more than 14 volts input). Don't forget that includes the dip
in the ripple sawtooth, i.e. a 12 volt reg with 16 volts input but with
a 2 volt ripple p-p. A side benefit of using a reg is that it can
tolerate a lot more ripple, thus one can use even less supply filtering.

This doesn't work well for preamps however because 78xx reg ic's have a
relatively low supply noise rejection only about -60dB I think and thus
one needs additional filtering to further reduce it but also because the
78xx is actually quite noisy itself. IIRC I measured about 200microvolts
of wideband noise from them.

In the case of a precision generator like yours the regulator shouldn't
use a noisy zener reference but rather a quiet current source such as a
low noise jfet and the reference amplifier should be a low noise preamp.

 Mike
 ****

... Resistance Is Futile!   (If < 1 ohm)
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