Little/Triple LFO crosstalk

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Noiseconformist
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Little/Triple LFO crosstalk

Post by Noiseconformist » Sun Feb 17, 2019 5:02 pm

Hi, rather an observation and looking for an opinion:

Just today (and it took more than a decade to stumble upon this) I noticed some crosstalk between the outputs of the Triple LFO in my system.

The situation was rather extreme and popped up during testing the Lin FM input of a 3rd-party VCO. This input seems to be very sensitive at some point.

One LFO I used as a DC-source for a quick test.
I.e. set to the lowest possible frequency and with the shape pot all way CW to get a constant voltage.
This was fed into the Lin FM input of this VCO to test.
At some point and at extreme settings I could perceive a vibrato-like modulation.
It took me a moment to find out that there was a very small voltage imposed on the DC-like signal.
It's just about 5mV and soaked in noise, but the signal clearly stems from another LFO in this module.
Changing the frequency of that other LFO made this obvious.

Image

Since in this implementation the LFOs are on individual PCBs, the crosstalk must be caused by my wiring.
Any thoughts on that?
Thanks a lot, Michael.
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Last edited by Noiseconformist on Wed Feb 27, 2019 12:29 am, edited 1 time in total.

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Synthbuilder
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Post by Synthbuilder » Mon Feb 18, 2019 6:40 am

It's pretty much what I would have expected for that particular and very simple design. 5mV on a 8V signal would normally be considered as pretty negligible (-65dB). It's probably due to the LED current being dumped into the 0V (module ground) line. If you remove the LEDs any crosstalk will probably reduce down to a very small level - or maybe use a modern high efficiency LED and increase the resistor that drives the LED. The newer Dual-LFO module uses a different LED driver circuit to reduce this sort of problem.

That said, even without the LED(s) there will still be oscillating control currents being injected in the module's local ground. If this is producing unacceptable levels of crosstalk you'll need to beef up your 0V (pin 2) wiring to the module. One way to do this is to give each Little-LFO board in the module its own power supply lead.

Tony

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Noiseconformist
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Post by Noiseconformist » Tue Feb 19, 2019 1:15 am

Thanks Tony, that fits my picture.
I don't think I'll change anything soon, I never had a problem with that and as mentioned,
this was a test situation with extreme settings.
It's still my favourite LFO, BTW - the 'simple' it is! ;)

Michael.

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