[sdiy] Very fast CCO core, pulse CCO!

[karl dalen]

--- Ian Fritz <ijfritz at earthlink.net> skrev: > Karl --
> 
> Looks interesting!  Great to see someone experimenting with new ideas.

Well, actually i cant take all the credits im just expanding on a old
idea BJ showed me some 4-5 years ago. He used this diode coupled CCO
for clocking SRAMs for a 2 osc 8bit mono sampler (to get variable pitch)
he did in his early youth days in school!! These where the days of
"buckets of logic" and "Siclair Spectrums" (used as a sample editor!!)
around 83-84).(imagine the idea to have to tape load the editor and samples!!!)
I just added the NPN/PNP switch, buffer, and sync thing but the basics
core convering was allready done by him. Ooh, thats, well least 20 years old!

As i also forgott, it's syncable to, simply pulse the divider junction
or the, open collector output or the switch tranny, many options here!

BTW, the resistors sets tresholds wich affect frequancy and amplitude.
 
> One thing I noticed is that the comparitor has a fairly large input bias 
> current.

Exactly, i was about to say that but forgot, thanks for pointing it out!
So there is some very low bias devices around the cheapest and quite fast
is national LMV761, 0.2pA bias, 120nS delay, rise and fall 1,7nS. Otherwise
TI have some very, very ,very fast (500Mhz) and very very low bias, but 
very expencive.

>One advantage of the more traditional designs is that optimal 
>chips for the integrator and switch can be separately chosen.

You have the same freedom here aswell, its just the other
way around i think. And i see a clear benefit by having the
saw buffer out of the loop. Its one problem less.  
 
> Perhaps a super-fast, low bias opamp (OPA602, etc.) would also work here.

Most likely.
 
> Please keep us posted as this develops!

Will do.

Reg
KD 

>    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
> >
> >Höstrusk och grå moln - köp en resa till solen på Yahoo! Resor på adressen 
> >http://se.docs.yahoo.com/travel/index.html
> 
> 
>  

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