Old problem revisited

[Govmnt_Lacky]

A while back, I got a brilliant idea to install an MXR Micro Chorus and Micro Flanger into the same enclosure. Both are etched project PCBs. Both run on the RD5106 Reticon BBD. Both are powered by the 9VDC 1Spot. The LED is a Bi-colored Green/Red and color determines which pedal is activated. 1 at a time only on this build. Stomps are Bypass and the other is effect select.

The video below speaks for itself. I am guessing that this is caused by both clocks running under the same roof. In the video, ONLY the Flanger is active. The noise is less but, still present with the Chorus on.

Can anyone venture to guess how I can get these both to play nice TOGETHER? My final solution is to break them out into separate enclosures BUT... I would like to keep them together.

[DrAlx]

Yes its heterodyning between the two clocks.  It's hard to give advice without more info on how you've connected and powered the 2 PCBs.
Take a look at the advice Puretube posted on the NZF Flanger thread.  He sorted out my heterodyning problem (for a flanger with two clocks).
The problem I had was mainly down to two things.  Both needed to be fixed to remove the heterodyning.

1) The two clocks coupling through the supply lines.
    Daisy-chaining the power supply to the two blocks was my mistake.
    The correct thing to do was make the two supplies paths meet at a common point and put caps at that common point to bleed the RF (an electro and 100nf ceramic in parallel).
    I am guessing you already have the two PCB powered in parallel. Try adding the caps at the common point like this ...

Code Select Expand


                                                        |-------PCB1 +ve supply
(+ve Supply Line)----( Electro + 100nF to to ground )---|
                                                        |-------PCB2 +ve supply

   
2)  Clock signal from one circuit block appearing at the BBD input of the other circuit block.
     In my case, the BBDs inputs were originally directly coupled to each other.
     This is not likely in your case, but you still could have noise from one clock appearing at the BBD of the other.

There is a third way of these things interfering and that is RF pickup "through the air".
That may be the main problem you have in which case all i can suggest is shielding things as much as possible.
I am guessing you have wires going from the audio I/O of each PCB to a foot switch.  If those pick up stray RF from one circuit and
manage to leak it into the other you can get heterodyning.

As an experiment, knock-up a simple RC low-pass filter (3dB point of 5kHz) and use it at the audio input of one PCB.
The idea being to remove RF rubbish from the input wire.  See how much that decreases the heterodyne noise (if at all).
That will at least let you know if the pickup is mainly from those wires, or directly from PCB to PCB.
If the audio I/O wires are part of the problem, use shielded cables for those.
If the problem is down to PCB to PCB interference, then all I can suggest is shielding one PCB from the other.

[slacker]

If you're only using one effect at a time you might be able to rig something up to stop the unused clock.

[gjcamann]

Any pictures of the board layouts? I think a good layout could help with 2 and 3, but 1 is most likely your primary issue.
Adding a little capacitance on the clock lines to smooth out the clock signal could help too.

[Fender3D]

Hey Greg,

It looks like advices here are the same as in the older thread...

I guess a correct physical placement of PCBs and maybe a GND screen as in old EH polychorus between PCBs might help better.

You might also change clocks frequencies, so they're not so close each other...

[Govmnt_Lacky]

Thanks for the advice gents. I think I have come to the decision to seperate and rehouse these circuits in seperate enclosures. I might repurpose this enclosure for another project.

Cheers to all! 

[R.G.]

Here's the problem in a nutshell. Logic switching signals are fast edged, so they radiate into the rest of the universe more easily, and they have big, full-power-supply swings so there's lots of power involved. They also typically move a lot of current around on those fast edges, even if the logic is CMOS and the current moved during the saturated part of the signal is small. That is, they have all the characteristics needed to make them a problem in a circuit that amplifies low audio signals.

The real answer is * you have to do everthing right * to keep clock and logic signals out of the audio. This includes:
- Local decoupling of the logic chips with dipped-ceramic caps right on the logic chips to satisfy power current spikes locally, not from the wires to the rest of the circuit. A few 22uF electros very near the logic chip helps with the lower frequency issues.
- Isolating the current spikes on the power and ground wires by running separate power and ground wires and traces to the logic sections. This makes the issue of where and how many times to connect analog ground and digital ground more critical as the amount of digital power goes up.
- Alternatively trying to make the ground return resistance tiny may help in certain circumstances. This is done with ground planes.
- Eliminating radiated signals by careful wiring. Running clock signals off the PCB is a disaster inside most pedals where the audio also runs in the wiring tangle. Any high impedance audio wires easily pick up the logic spikes. Careful use of shielded wire may help.
- Eliminating fast-edged signal spikes by filtering on the audio lines. This can be resistor/capacitor or inductor (ferrite bead)/capacitor filtering, band limiting in input stages, many things.
- Physically isolating high impedance inputs, especially JFET and FET input opamp inputs. The further these and all wires connected to them are from the fast logic edges, the better you do.
- Worrying about heterodyning. I say "worrying" because there's only so much you can do. The logic stuff generally runs at whatever frequency it needs to and you may not be able to do much about it.

All of these are problems. People find magic "fixes" to the one or two that are the most prominent in a given pedal, then declare victory.

[puretube]

ye olde threade...
how DrAlx` clox et al got solved...