[sdiy] Signals leaking into the PSU?
Roman Sowa
modular at go2.pl
Fri Feb 24 09:42:39 CET 2023
Don't bother with 470uF on the busboard, I was trying to be sarcastic
about them.
I said it before and repeat it here - you are trying to decouple 40106
with 100nF cap while the chip is literally pumping another 100nF cap
with several volts span. At least it's 100nF now, it was more in
previous schematics. Add 10uF or more to it and an inductor or small
resistor from +5V rail to power pin of 40106.
I can't see any decoupling caps for 7805 except one 100nF. For better
stability 7805 likes to have a few uF at the output. I've seen
oscillators made of 7805 without proper decoupling.
Since you have 5 inverters unused, why don't you use them as current
booster for faster reset of the ramp? It will help with tuning on higher
frequencies. Use one ineverter like, well, inverter, and then parallel
remaining 4 so they can drive the timing cap 4 times faster.
You might also consider replacing 1N4148 with low leakage diode to get
better tuning at low frequencies, or even better - a transistor. Then
maybe even be able to replace 100nF timing cap with 10nF or less, to
further reduce ripples and improve higher end tuning.
OK, I'll shut up now
Roman
W dniu 2023-02-23 o 20:32, A.M. Barrio via Synth-diy pisze:
> So, I did the following:
>
> * Added one 100uF cap between each rail and GND on the busboard (on
> the closest connector to the PSU wires). That's the biggest value
> I could fit on the bus board, I had some 470uF but they were axial
> and had no space for them. Do you recommend a value larger than
> 100uF? If that's the case I'll buy some radials.
> * Breadboarded one VCO following the schematic attached. I added the
> 10uF caps between rails on the LFO board as well.
> * Tested it again. This time the wiggle from the LFO was hardly
> noticeable at the outputs of the regulators. However, although it
> had been reduced, the change on the VCO output frequency was still
> distinguishable on the scope and could definitely be heard.
> * And finally, removed the LEDs from the LFO. This is what did it,
> it is finally working as intended, no wiggling on the VCO output
> nor LFO remnants on the rails. I will look into high efficiency
> leds, driving them through current sources and ways to stabilise
> the amount of current they draw.
>
> Many thanks again guys, you made it very easy for a beginner like me
> to understand the issue :-) If you feel like adding anything else to
> the topic, please do so! I'll be glad to read, I've learnt so much
> from this thread.
>
> Kind regards,
>
> A.M. Barrio.
>
> vco1.png
> <https://drive.google.com/file/d/1O6NkuBz6gKZGsou1AdJfsE4TkF1un0Fb/view?usp=drive_web>
>
>
>
> On Mon, Feb 20, 2023 at 6:10 PM A.M. Barrio
> <albertomunozbarrio at gmail.com> wrote:
>
> Hello again,
>
> First off I'd like to show how appreciative I am about all this
> technical support, I wasn't expecting such extensive and
> informative answers! (and quick too...). I'll try to answer to
> everything that's been talked about here:
>
> * To David Huss, René Schmitz, Lukas Hazen-Bushbaker: thanks a
> lot for your thorough explanation on why this is happening. At
> least I can say I now understand what's going on. David's
> bucket analogy in particular made it super easy for me to
> grasp the idea. In the process, though, you also gave me a
> feeling that these matters might be a little too complicated
> for me to manage haha! I'm no engineer nor do I have as much
> knowledge on electronics as I'd like to... I actually do have
> some, I work as a technician repairing stuff, but the reasons
> behind my problem and the solutions to it caught me off guard.
> I was never taught too much on noise issues back when I was
> studying, I suppose it is explored more deeply in actual EE
> degrees. As a result I've never been aware of it: there are no
> 100uF caps anywhere on the entire synth case, my bus board is
> the crappiest, most DIY thing you can think of and my
> way-to-go has always been /slap in those 100nF caps between
> rails and go test it!/ So much for me to learn on this topic.
>
> * To René Schmitz, Roman Sowa: I must note that the schematic I
> attached on the first email is not the LFO, but the clock,
> that's my bad again. That's why there's only one LED. However
> I just noticed that I didn't rectify the signal before
> plugging it to U1C, so the negative cycles are indeed going
> nowhere. I will change that in the clock circuit. On the LFO
> circuit I do have two LEDs per wave, one for positive cycles
> and one for negative cycles. Took notes about high efficiency
> LEDs as well, right now I'm using a plain common red LED on
> the clock and six green (!) LEDs on the LFO, since it has
> three wave types. I'm still concerned about the LEDs
> however... a total of 7 LEDs might take around 140mA, would
> that be correct? Even if I were to correctly bypass all my
> circuits and properly isolate everything, would the LEDs still
> suppose an issue in terms of sag/wiggle for a PSU that can
> supply much more? (all of the PSUs I tried with can supply
> around 1A max).
>
> * To Mike Beauchamp, Chris McDowell, Roman Sowa: The VCO design
> comes from Moritz Klein's one. I'm attaching a schematic of
> the exact circuit I'm implementing. After reading all the
> responses it is quite obvious to me that it is VERY prone to
> changes in output frequency: the CV pots are tied directly to
> the rails, as well as the CD40106 IC in charge of generating
> the oscillation. Roman, I am taking absolutely no offense from
> your comment: it is indeed a bad design. I am not willing to
> improve /this/ particular VCO design as it was just a test and
> a bad example of me being too excited to have my first panel
> mounted on my case. Please ignore the wasteful use of the
> 40106 and the crappy expo converter, much more care will be
> put into the next VCO I put together. About this: /If you need
> help deciding which ones those are, post the VCO schematics./
> Would you mind pointing them out? I mentioned a couple spots
> above, but I'd like to make sure I cover them all. That being
> said, I'd love to hear about the reference voltages. Chris,
> thanks for the IC examples, I'll check them out. Besides, I
> have some spare 7805 regulators at home. Although they are
> quite big, would they do the job for testing before I go ahead
> and grab some of the examples you mentioned?
>
> * To Ian Fritz: Thanks for welcoming, and that's a good sense of
> humour you've got :-). I can only show my respect to you guys,
> you really seem to be very knowledgeable people.
>
>
> So, to summarize: I must look for a way to improve my bus board
> (uh consider spending some money on a proper PSU/bus board). I
> must plug 100uF to 470uF caps between each rail and GND on every
> module as well as the bus board. 100nF caps between each IC and
> GND are recommended. Sensitive parts of circuits should never be
> tied directly to the power rails, but to a voltage regulator
> instead. Would these measures remove (or at least notably reduce)
> the problem I'm facing?
>
> Thanks a lot beforehand,
>
> A.M. Barrio.
>
> VCO.png
> <https://drive.google.com/file/d/1EBAwnrlCLTUUEoa8b8sTuSTGzpBY5-kL/view?usp=drive_web>
>
> On Sun, Feb 19, 2023 at 5:21 PM A.M. Barrio
> <albertomunozbarrio at gmail.com> wrote:
>
> This is the complete email I wanted to send. There's another
> thread where the attached files and additional info is
> missing. My bad, pressed CTRL + Enter by mistake whoops.
> Please ignore that one! pata at ieee.org and
> mbryant at futurehorizons.com, thank you for your replies, I have
> taken them into account :-)
>
> ------------------------------------
>
> Hello,
> I'm having an issue where the frequency of my VCO is being
> affected by the operation of other modules. I'll give a
> broader explanation:
>
> I'm a beginner on synth DIY, right now I have my home made
> case, a PSU, two VCOs, an LFO, a clock module and an
> amplifier/speaker (built in the case, not externally).
> Everything has been arranged by me, I haven't purchased any
> module yet. The clock and the LFO are the latest modules I
> have built (pretty much both at the same time), and when I
> tested them on their own, they worked just fine. However I
> noticed that when they are powered, the frequency of the VCO
> would stutter following the operation of the clock and the LFO.
>
> That would be: *without connecting the clock or the LFO
> anywhere*, just having them powered up, whenever the clock is
> up or down, or the waves generated by the LFO reset, the
> frequency of the VCO varies a tiny bit. It gets more
> noticeable the higher the pitch of the VCO.
>
> After some head scratching I've realised that the signals
> generated by any of the modules I have (both VCOs, the clock
> and the LFO) are somehow leaking into the +12 and -12 rails of
> my PSU. Measuring any of the rails on my scope (on AC mode, so
> only the noise is shown), I can see that the waves generated
> by the modules I mentioned above are there as well in the form
> of noise (around 20mVpp each of them). That explains the
> little variations in frequency of the VCO. However I have no
> idea why this is happening or how to fix it. I have tried with
> different PSUs I have around:
>
> * 11V 0.750A SMPS into DCDC to get +12 and -12
> * Two 12V 1A SMPS together to get +12 and -12
> * Linear PSU with 7812 and 7912
>
> I have the same problemwith all of them. I'm positive there's
> an issue somewhere with a lack of filtering, but I don't know
> where, or why.
>
> I'm attaching a sample of the VCO output where the stutter can
> be heard and the schematic of the LFO. The way the LFO is set
> up in terms of caps and isolation can be extrapolated to every
> other circuit I have made, I always place those 100nF caps
> between each rail and ground.
>
>
> LFO (square).jpg
> <https://drive.google.com/file/d/1ts1WGcyl2y1Ypp5eP79Ptb5fHXawetZh/view?usp=drive_web>
>
> VCO stutter.mp3
> <https://drive.google.com/file/d/1RrUNX_Of_RGLAwn2CyV7I5fxRrlHvfHL/view?usp=drive_web>
>
>
> In case you have any idea what could be wrong, your reply will
> be greatly appreciated. Thanks beforehand!
>
> Kind regards,
>
> A.M. Barrio.
>
> ------------------------------------
>
> In response to pata and Mike Bryant:
>
> * Yes, the current PSU I'm using can provide 8W in total.
> I'm only powering one VCO and one LFO with some LEDs,
> power shouldn't be the issue in this case.
> * I always place 100nF caps between each rail and GND on
> every circuit/board. However I hadn't heard of placing
> caps on each IC. I suppose it should be between their
> power pins and GND? I will have a look at capacitance
> multipliers as well as I haven't heard about them before.
>
>
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