[sdiy] 3340 FET sync

[Neil Johnson]

Hi Finlay,

Ok, so to help everyone else who cannot see your full schematic it
would be much more helpful to show us what is happening at the pins of
the AS3340 so that we can then relate what you are seeing with the
datasheets and other folks' experiences.

If, as you say, the triangle bounces between +2V and +6V then I would
start looking into why, as both CEM3340 and AS3340 datasheets specify
the triangle low point at 0V +/- 15mV.  Your readings are way out so
something is not quite right.

For example, what components do you have connected to pin 13?

And have you checked that your GND pin 12 is soldered down and connected to 0V?

Cheers
Neil


On Tue, 21 Jan 2020 at 13:53, Finlay Shakespeare
<futureimage at hotmail.co.uk> wrote:
>
> Thanks Neil! Will correct the HFT trimmer now...
>
> The waveform outputs indeed go through op-amp gain and offset stages, so what's on the scope isn't the raw output of the 3340. However, the problem doesn't lie in clipping these stages, etc. etc...
>
> Probing the cap at pin 11 - the triangle wave never hits 0V on the trough of the waveform. Instead, the triangle wave oscillates between +2V and +6V. When the sync pulse goes high, however, the triangle is forced to 0V, causing this overshoot on the sawtooth (and triangle waveform post-gain + offset)
>
> The problem exists at whatever frequency I run the 3340 - it isn't one of those sync-catching effects as Tony suggested, sadly!
>
> Cheers!
> Finlay Shakespeare
>
> ________________________________
> From: Neil Johnson <neil.johnson71 at gmail.com>
> Sent: 21 January 2020 13:17
> To: Finlay Shakespeare <futureimage at hotmail.co.uk>
> Cc: synth-diy at synth-diy.org <synth-diy at synth-diy.org>
> Subject: Re: [sdiy] 3340 FET sync
>
> Hi Finlay,
>
> Your HFT connection is wrong: the VHFT output should go to the top of
> the trimmer, and the 1M0 from VFCI should go to the trimmer's wiper.
>
> Also, your sawtooth waveform seems to be the wrong amplitude and wrong
> direction. Both CEM3340 and AS3340 datasheets quote the sawtooth peak
> at around 10V and it should be a rising sawtooth, not falling as you
> have (although as you're running from +12V rails it might be a little
> bit lower - the 10V peak is only specified for +15V supply).  Maybe
> you have a divider on the output?  Looking at the top of the bad bit,
> it looks like the output is hitting the V+ rail -- OR it could be the
> -Ve rail if you're using an inverting op-amp on the output?  If you're
> using an inverter then it looks like the output could be going
> negative before climbing up.
>
> More of the circuit would help.
>
> Neil
>
> On Mon, 20 Jan 2020 at 16:51, Finlay Shakespeare
> <futureimage at hotmail.co.uk> wrote:
> >
> > Hi all!
> >
> > Long time lurker here - I really haven't mailed in a long time - but wondering if anyone could shine a light on this...
> >
> > I'm working on a 3340 circuit (using new-generation Curtis chips here) employing MOSFET sync rather than the standard hard/soft sync methods given in the datasheet. This allows for resetting of the waveform to a known(ish) point of the waveform, rather than just giving a sync-soundalike. Known-ish, however, because at certain frequency settings / ratios (between syncing and sync'd oscillator) the waveform will "overshoot" before stabilising on the next cycle.
> >
> > I've attached both a schematic and a shot of the scope here - if anyone has any guidance for minimising (or even better, completely removing) this overshoot, that would be fantastic. On the scope, the blue trace is the pulse input to the sync circuit, and the pink trace is the waveform from the sync'd 3340's ramp output.
> >
> > Please note that the resistor and capacitor values given in the sync network going to Q4 are not necessarily as they currently stand - I've managed to improve the situation by tweaking this network, but seemingly I can't remove the overshoot completely.
> >
> > Cheers!
> > Finlay Shakespeare
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