[sdiy] Very fast CCO core, pulse CCO!

[Ian Fritz]

Karl --

Looks interesting!  Great to see someone experimenting with new ideas.

One thing I noticed is that the comparitor has a fairly large input bias 
current.  One advantage of the more traditional designs is that optimal 
chips for the integrator and switch can be separately chosen.

Perhaps a super-fast, low bias opamp (OPA602, etc.) would also work here.

Please keep us posted as this develops!

   Ian



At 06:01 PM 6/25/2004, karl dalen wrote:

>Do this around the comparator:
>(look at page 6, "squarewave oscillator" in LM393
>data sheet for assistance.)
>
>1:Take a comparator e.g. LM393, take two 100k resistors
>   connect them serially between +V and AG, connect the
>   middle point to the +input.
>
>2:Take a 100k resistor from the output to the +input.
>
>3:Take a 100k resistor from the output to +V.
>
>4:Remowe feedback resistor and insert either a diode or
>   a transistor connected as a diode (base+collector)and
>   connect the cathode to the -input.
>
>5:Take a 100pF-1nF capacitor and connect between -input
>   and AG or -input and +V.
>
>6:Connect your regular NPN expo converter to the -input.
>
>You now have a saw current/pulse voltage CCO core!
>
>You can use a OP amp instead of a comparator just
>remove the resistor who goes from the output to +V.
>
>An even better approach is to do a PNP/Fet/NPN switch solution,
>of the diode switch. Connect a PNP transistor, emitter to +V,
>collector to +input, and base to the comparator output trough
>a low value current limiting resistor (1k) to the comparator's
>output. ( remember to swap inputs if using PNP tranny).
>
>The capacitor -input junction has sawtoth current flowing,
>if this is buffered with a FET transistor or with a CMOS
>or FET OP amp there will be a sawtoth voltage at the output.
>
>With OP amp buffer this allows:
>1:Polar sawtoth independent of frequency and thresholds.
>2:Bipolar sawtoth independent from frequency and thresholds.
>
>A normal Tierry Michaels VCO don't allow these things.
>If you change the thresholds in a TM VCO you change
>frequency as well as amplitude.
>
>Idea is:
>To get very high oscillations use a fast FET/CMOS comp/OP,
>The "imagined" idea is that its easier to get high frequencies
>out of this CCO that's buffer less its also neat and simple
>for driving logic e.g wave tables!
>
>Quick tests:
>I did a test setup and a LM393 with a 1nF cap and NPN
>instead of PNP switch, 2,2M in the divider and a 22p
>compensation from the divider junction to AG,and 2,2k
>feedback from out to +input, and 2,2k from output to +V,
>it did easily 0.5hz to 840khz at +5V supply!!....Neat!
>
>Now, one might ask the question, what and where is
>the drawbacks of this CCO? Well there must be some
>but at least fewer then the typical ASM/TM buffer in
>chain designs! One obvious, the NPN switch adds
>certain aspects.
>
>Comments wellcome!
>
>Reg
>KD
>
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