[sdiy] adding sync to a vco/how does sync work

[Scott Gravenhorst]

Dan Snazelle <subjectivity at hotmail.com> wrote:
>
>I have been finding some great uses for sync lately. i am 
>wondering how easy or hard it is to add sync to a schematic that 
>does not call for it. is it as easy as injecting a pulse wave 
>into the timing cap? 
>
>for instance, the FARM VCO I doesnt have sync (i dont see it at 
>least) how hard would it be to stick it in somewhere? 
>
>http://www.aleph.co.jp/~takeda/radio/img/farmVCO-scheme.gif
>
>is it possible to add it to a 555 oscillator? 

Absolutely yes.  I do it with a FatMan synth.  The 555 (or 7555 which I
prefer), the TTL compatible output on pin 3, can be connected directly
to pin 4 (through a switch).  

http://home1.gte.net/res0658s/fatman/hardsync.html

The pulse on pin 3 then resets the timer causing synch.  This would be
"hard synch".  Your mention of injecting a pulse into the timing cap is
one way to implement soft synch.

>or a transistor 
>oscillator? i guess what i am asking is is it a pretty easy thing 
>to add to an oscillator circuit? thanks 

I think it really depends on the oscillator topology, some are trickier
than others.  7555 or 555 based oscillators are a piece of cake.

>also what is the difference between soft and hard sync

My understanding is that hard synch is an absolute reset at the time the
reset is input is activated regardless of the current state of the
synched oscillator.  Soft synch resets (or not) the synched oscillator
depending on where it is in it's waveform.

-- ScottG

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-- Scott Gravenhorst
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