[sdiy] understanding this one transistor F to V

[Eric Brombaugh]

Yes, it's driving the supply lines of the oscillator. Basically any 
waveform applied to the input gets amplified up to a fairly large AC 
signal that appears on the base of the middle transistor (the one with 
the diode across the E/B). This modulates the supply of the astable 
flip-flop on/off at the input frequency. If you modulate faster than the 
natural frequency of the oscillator (about 100 Hz here) then every time 
the flip-flop is turned on it comes up in the opposite state as on the 
previous cycle, so there will be a slight 1/2 frequency variation on the 
output.

I actually built the circuit up and observed this on the 'scope. The 1/2 
frequency modulation is fairly minor compared to the amplitude of the 
supply modulation, and the primary frequency dominates. Judging by the 
waveforms it probably sounds pretty harsh too, but sometimes that's just 
what you want I suppose...

So, no, this isn't an F/V converter. It's more like a suboctave 
generator via a modulated astable flip-flop.

Eric

On 11/03/2009 10:07 AM, Dan Snazelle wrote:
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> ok but it is driving the supply lines of the oscillator? so isnt it then cr=
> eating a voltage? when i made a few adjustments to the circuit=2C i was get=
> ting some interesting results. a plucked note would make my scope rise from=
>   0 to around 5 volts (roughly) and then it would slowly swoop back down. i =
> hooked it up to a transistor/40106 setup and was able to get sweeps out of =
> the 40106 oscillator but it was hardly in tune with the guitar!! anyway...y=
> our explanation makes sense as i have certainly never seen such a simple F =
> to V before. i was sort of hoping that I could tweak it enough to get this =
> to drive a VCO thanks>>  On Nov 3=2C 2009=2C at 4:24 AM=2C Ren=E9 Schmitz wr=
> ote:>>>  Eric Brombaugh schrieb:>>>  Sure looks like a simple flip-flop divid=
> er to me.>>>>  But thats not a FF but an astable oscillator.>>  Well=2C yes. =
> I've heard the cross-coupled circuit on the right called an>  'astable flip-=
> flop'=2C so let's split the difference. My point though is>  to clarify for =
> the OP that the way this circuit is set up there is no>  explicit conversion=
>   from frequency to voltage. Instead=2C the input>  signal is amplified and d=
> rives the supply lines of the astable flip->  flop in such a way that the re=
> sult is to divide the input signal's>  frequency by two.>>  At least that was=
>   my thought based on a quick glance at the circuit.>  If you've got a differ=
> ent interpretation I'd be interested to hear it.>>  Eric>  __________________=
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