[R-390] Ramp Generators

[David Wise]

The canonical Tektronix ramp generator is the "Miller Integrator" used in the sweep section of most of their old analog scopes.  Here, the constant current comes through a resistor.  It's constant because the voltage across the resistor is constant.  The voltage is (nearly) constant because it's the input voltage to a high-gain DC amplifier with feedback.  This is your generic "operational amplifier" circuit: the output will do whatever is necessary (and possible) to maintain the "virtual ground" at its input.  The resistor forms the low leg of the feedback network.  The cap, wired from amp output to input, forms the high leg.  With a constant current trickling into the low end of the cap, the amplifier has to elevate the high end at a constant rate to keep the low end from drooping.

Dave Wise
Tektronix, 1980-1995
________________________________________
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> on behalf of Barry <n4buq at knology.net>
Sent: Friday, April 26, 2019 9:09 AM
To: Roy Morgan
Cc: r-390 at mailman.qth.net
Subject: Re: [R-390] OT: Ramp Generators

Hi Roy,

Ahh, okay.  That's a small detail I missed.  Indeed, the sample I was looking at states it's a steady current (through a transistor).  That makes sense to me now.

Thanks for the explanation.  My electronics world is back in order now.

Barry - N4BUQ

----- Original Message -----
> From: "Roy Morgan" <k1lky68 at gmail.com>
> To: "Barry" <n4buq at knology.net>, r-390 at mailman.qth.net
> Sent: Friday, April 26, 2019 10:55:16 AM
> Subject: Re: [R-390] OT: Ramp Generators
>
> Barry,
>
> Some bits in reply - hopefully useful:
>
> The usual capacitor charging curve shown in texts and explanations assumes
> (usually states) that the charging current comes through a fixed resistor
> from a steady voltage source. We see the familiar exponential curve
> approaching the source voltage as time goes on.
>
> BUT in ramp generators there is usually a constant CURRENT source. This
> produces a linear voltage change with time on the cap.
>
> Tektronix produced a number of explanatory documents related to their
> oscilloscopes, and I am sure one of them tells about time bases. I can send
> more info on that later but in the meantime search for “tekwiki”:
> W140.com/tekwiki/wiki ... main page, scroll way down to find “Concept
> Series”.
>
> The Tektronix folks were/are masters at ramp generators.
>
> Roy sends.
>
> > On Apr 26, 2019, at 11:01 AM, Barry <n4buq at knology.net> wrote:
> >
> > Hopefully this isn't too dumb of a question, but I was wondering how ramp
> > (sawtooth) generators work ...
> >
> > This is confusing to me.  Capacitors don't charge linearly, do they, and,
> > if that's true, then why is the ramp linear wrt time?