[sdiy] Signals leaking into the PSU?

[dh at atoav.com]

Bypass caps are important, but one thing to consider is the following:

Theoretically your modular synth caase is ONE big circuit, that means if 
a quick/short current spike on that one module attempts to draw that 
current from your busboard (and not for example from it's internal 
electrolytic capacitors or 100nF bypass caps) the other modules might 
see their rails dip precisely at that point in time.

This is why _proper_ decoupling, using the right value, combination¹ and 
placement of capacitors, sometimes a strategic ferite on the rails and 
the right routing of rails and ground paths is important. As Peter Knoot 
mentioned the lowest impedance path to ground every module "sees" should 
always be the PSU. This is why in digital modules analog and digital 
grounds are seperate traces/planes till the last moment, this is why 
something like star grounding would be the gold standard for signal 
integrity (although there are not many star grounded busboards out there).

Just looking at your LFO schematic the first thing I'd try would be to 
add electrolytic caps (100uF to 470uF) to the power input of the module. 
100nF is a very small bucket of electricity for a module to draw from. 
If that bucket is empty the module pulls the rails down and every other 
module sees that dip of the rails. So just adding another pair of bigger 
buckets might do the trick.

If that is not it, it might be wiring.

Cheers,
atoav/David Huss



¹ There is a video by EEVblog on youtube explaining how the value of a 
bypass cap influences the frequencies at which it is effective, I tend 
to go for bigger capacitances (e.g. 470u) between rails and gnd at the 
power connector of a module and 100nF directly next to each IC

On 2023-02-19 17:45, Peter Knoot wrote:
> This is actually quite a common issue.  The advice given so far is 
> sound; bypass capacitors are certainly a must, but not sufficient 
> solution.  A comprehensive approach must be taken for any Eurorack case 
> (which tends to be far noisier than standalone equipment) to become 
> noise-free.
> Proper filtering requires a good, low impedance ground path.  This 
> includes the module circuitry (proper PCB layout practices), power 
> distribution bus boards, power supply and of course the wall socket 
> ground!  Many folks don't even consider the latter.
> Here's a great white paper by Genus Modu to give you some idea:
> http://www.genusmodu.com/products/libb-wp-01.html 
> <http://www.genusmodu.com/products/libb-wp-01.html>
> I'm not proposing you run out and buy their busboards, although I can 
> attest to their performance.
> The PSU may or may not be a factor, but you won't really know until 
> you've created a low impedance ground path from the module back to the PSU.
> Best of luck,
> Peter Knoot
> 14197 N Hidden Arroyo Pass
> Marana, AZ 85658-4995
> USA
> 650-759-2101
> www.synquanon.com <http://www.synquanon.com>
> This communication (including attachments) is covered by the Electronic 
> Communications Privacy Act, 18 U.S.C. 2510-2521. This communication is 
> confidential and may be legally privileged. If you are not the intended 
> recipient, you are hereby notified that any retention, dissemination, 
> distribution or copying of this information is prohibited. If you have 
> received this communication in error, please notify me by telephone 
> immediately and destroy the information. Thank you.
>> On 02/19/2023 9:21 AM MST A.M. Barrio via Synth-diy 
>> <synth-diy at synth-diy.org> 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 <mailto:pata at ieee.org> and mbryant at futurehorizons.com 
>> <mailto: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.
>>
>> _______________________________________________ Synth-diy mailing list 
>> Synth-diy at synth-diy.org 
>> http://synth-diy.org/mailman/listinfo/synth-diy Selling or trading? 
>> Use marketplace at synth-diy.org 
> 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy
> Selling or trading? Use marketplace at synth-diy.org