[sdiy] VCO that goes SHARP in highest octaves??

Ian Fritz ijfritz at comcast.net
Tue May 10 10:43:01 CEST 2022


The most common cause of this is spurious hi freq oscillations. Probably the place to start, although not the only possibility.

Ian

http://ijfritz.byethost4.com 
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> On May 9, 2022, at 11:53 PM, Paul Perry via Synth-diy <synth-diy at synth-diy.org> wrote:
> 
> Possibly a capacitor (being smt) is made of material that is behaving in a non-linear way?
> 
> paul perry melbourne australia
> 
>> On Tue, May 10, 2022 at 2:13 PM Neil Harper via Synth-diy <synth-diy at synth-diy.org> wrote:
>> so this one is pretty strange to me...
>> 
>> i breadboarded a few of Thomas Henry's VCO-1 (i'm sure that design is
>> familiar to most, if not it is here:
>> https://www.birthofasynth.com/Thomas_Henry/Pages/VCO-1.html) with 3080
>> replaced with a TI 13700 that worked worked exactly as intended, i was
>> able to adjust the scale tracking and both needed a little bit of
>> high-frequency compensation as expected - with C8 initially being about
>> 10-20cents flat. no problem. built a few thru-hole pcbs, worked exactly
>> the same.
>> 
>> my most recent project was to design a SMT version using the exact same
>> schematic - but with SMT equivalents. I received 5 boards back and all
>> of them go SHARP at C8 (10-15 cents) without any hf-compensation.
>> 
>> what's going on?
>> 
>> my first suspicion was the hf-compensation trimpot never 'went to zero',
>> so I removed the trimpot completely and the diode in the Rossom
>> Compensation circuit - no change.
>> 
>> as another test, I removed C1 ("C1 is a speed-up capacitor; its duty is
>> to drain away any accumulated charge from the base of Q2. This permits
>> the transistor to change states more rapidly"). the oscillator required
>> re calibration, but C8 was now only 2-4 cents SHARP. i feel like this
>> isn't the solution though, and that i was just crippling the
>> high-frequency linearity of the oscillator to compensation for another
>> non-linearity somewhere. but i can't think of anything that could be
>> different going from breadboard to TH PCB and then to SMT PCB.
>> 
>> 
>> 
>> --
>> /// Neil Harper
>> /// Every Wave is New Until it Breaks
>> 
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