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From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
Subject: Re: Amplifying stage with *negative* Voltage gain??
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Date: Sun, 08 Dec 2002 13:06:15 -0800
NNTP-Posting-Date: Sun, 08 Dec 2002 13:06:17 EST
Organization: Bell Sympatico
Fred Bloggs wrote:
> Mike Monett wrote:
> > Mike Monett wrote:
> >>In order to resonate with 100pf at 3.57 MHz, the ground clip inductance
> >>would be 19.87uH. If the wire were suspended in free space, it would be
> >>about 10 feet long, assuming 0.5uH/ft.
> > Can't divide without my coffee. Make that 40 feet. This is closer to
> > another rule of thumb of 1uH/meter, which would be about 60 feet.
> > Pretty long ground clip, I'd say:)
> If you want to be productive, drop your smartass attitude. That
> resonating inductance would not be far off if there is no gnd clip on
> the probe, other than that you are looking for a non-linearity between
> the emitter follower and the function generator, or some kind of field
> feedback where the inductance is causing peaking- instability of signal
> amplitude with movement around the circuit would be a giveaway there.
Yes, I guess 40 or 60 feet of wire is a bit much. It would probably be a
special order from Tek or HP, and would cost a fortune.
Actually, you could achieve the same results by winding about 12 turns on
a 1 inch diameter rod. This would require much less wire and give far
higher Q. Since the Q would be about 200, it would be desirable to space
the turns a bit to hit resonance before gluing them down. Glyptol is
recommended to maintain high Q.
But be sure to wind only the ground lead. If you use the probe cable
itself, and use a short ground lead, the inductance would be in the wrong
location to resonate with the cable capacitance.
A variation on this technique is sometimes used to isolate the scope from
common-mode noise by breaking the ground path. Ferrite blocks can also be
used to improve the isolation.
Another method that sometimes helps is to make a small preamplifier
located right at the signal source that feeds a 50 ohm coax terminated at
the scope. This improves the SNR by boosting the signal prior to
transmission, and can be used for very low-level or high frequency
As far as the rest of your comment, it sounds like the same technobabble
you use in all your posts. A meaningless collection of words you do not
understand, and no one else does either.
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