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Thanks Neil! Will correct the HFT trimmer now...</div>
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The waveform outputs <i>indeed</i> 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...</div>
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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
<i>forced</i> to 0V, causing this overshoot on the sawtooth (and triangle waveform post-gain + offset)</div>
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The problem exists at whatever frequency I run the 3340 - it isn't one of those sync-catching effects as Tony suggested, sadly!</div>
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<div id="Signature">Cheers!<br>
Finlay Shakespeare<br>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><b>From:</b> Neil Johnson <neil.johnson71@gmail.com><br>
<b>Sent:</b> 21 January 2020 13:17<br>
<b>To:</b> Finlay Shakespeare <futureimage@hotmail.co.uk><br>
<b>Cc:</b> synth-diy@synth-diy.org <synth-diy@synth-diy.org><br>
<b>Subject:</b> Re: [sdiy] 3340 FET sync</font>
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<div class="PlainText">Hi Finlay,<br>
<br>
Your HFT connection is wrong: the VHFT output should go to the top of<br>
the trimmer, and the 1M0 from VFCI should go to the trimmer's wiper.<br>
<br>
Also, your sawtooth waveform seems to be the wrong amplitude and wrong<br>
direction. Both CEM3340 and AS3340 datasheets quote the sawtooth peak<br>
at around 10V and it should be a rising sawtooth, not falling as you<br>
have (although as you're running from +12V rails it might be a little<br>
bit lower - the 10V peak is only specified for +15V supply). Maybe<br>
you have a divider on the output? Looking at the top of the bad bit,<br>
it looks like the output is hitting the V+ rail -- OR it could be the<br>
-Ve rail if you're using an inverting op-amp on the output? If you're<br>
using an inverter then it looks like the output could be going<br>
negative before climbing up.<br>
<br>
More of the circuit would help.<br>
<br>
Neil<br>
<br>
On Mon, 20 Jan 2020 at 16:51, Finlay Shakespeare<br>
<futureimage@hotmail.co.uk> wrote:<br>
><br>
> Hi all!<br>
><br>
> Long time lurker here - I really haven't mailed in a long time - but wondering if anyone could shine a light on this...<br>
><br>
> 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.<br>
><br>
> 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.<br>
><br>
> 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.<br>
><br>
> Cheers!<br>
> Finlay Shakespeare<br>
> _______________________________________________<br>
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> Synth-diy@synth-diy.org<br>
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