[sdiy] Signals leaking into the PSU?

Sun Feb 19 21:15:01 CET 2023

Another thing to consider is your reference voltages. In a VCO circuit there are a few places that connect to a stable voltage, like the the main frequency pots and in the expo converter, that directly influence the oscillation frequency. if these voltages wiggle, your frequency will too. rock solid power distribution is a noble endeavor, but can also use voltage reference ICs in the circuit to separate power supply fluctuations from these sensitive nodes. TL431 and LM4040 are two examples of reasonable and affordable choices that can yield, for example, a stable +5V rail instead of the +12V power rail you might otherwise use as a reference. you'll then need to change associated resistors to compensate for a low reference, but this strategy can go a long way toward better behavior in the face of wiggly power. I can share some examples if you need. I've made a habit of including this in all of my modules and I never have serious problems with PSU wigglin'. 

as others have stated, you need -some- decoupling caps too :) I always put 10 microfarads between power and ground at the power connector, and 0.1uF at each chip, though as Ian points out this is overkill. Just a habit, and unless those caps mess up the layout, they're essentially free. (Ian wrote this as mF, which could me misconstrued as millifarads! I see it more often written as uF) 

Chris McDowell 

> On Feb 19, 2023, at 11:29 AM, Ian Fritz via Synth-diy <synth-diy at synth-diy.org> wrote:
> 
> Hello AM --
> 
> Congratulations on your start on diy synths! There are some of us here who have had up to 50 years of fun doing this, although you may not always get the same answers to questions.
> 
> Proper power supply distribution and filtering are very important to avoid the kind of cross talk you are seeing. There are many discussion threads about this on the online forums. General Google searches will get you where you need to go.
> 
> Most designers will place electrolytic caps where power enters a module's circuit board. 10 mF or so is a reasonable starting value. Series ferite beads or small resistors are also fairly common. A few 0.1 mF bypass cap pairs spread around the board are plenty of bypassing, as carefully studied by chip manufacturers many years ago. (No need to bypass each chip.)
> 
> Power supply lines need to be very heavy -- some manufacturers even use solid bars -- and distributed via a star configuration as far as practical. Ground loops are generally a greater source of cross talk than insufficient bypassing.
> 
> Have fun!
> 
> Ian
> 
> 
>> On 2/19/2023 9:21 AM, A.M. Barrio via Synth-diy 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.
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