Debugging VCO Pitch unstability continued 2

Testing continues. First up:

Disconnecting all waveshaper outputs:

Disconnecting pulse, saw, saw inv, sine, triangle, sub square 1 and 2, sub saw 1 and 2. No difference in noise

Replacing trimmers with resistors (to disconnect trimmer wires, as trimmers are on the mainboard):

 

No difference. At this point, the wires still connected are +12V/GND/-12V, -5V, In center, symmetry, DCO in, PW CV and Pulse VCA CV.

Stopping the sub oscillator

First, trying to stop the sub oscillator by pulling pin 7 of IC8 to GND

 

That didn't work at all! Instead, the noise quadrupled, probably because the output of the op amp pollutes GND.

Next, by pulling pin 8 of IC8 to GND

Not really any big change there.

While still pulling pin 8 to GND, reducing PW to 0

This DOES have some effect, though we still see spikes for each period. Not sure where they're from

Combining stopped sub oscillator and pw at max

At this point both the sub oscillator and pulse wave outputs are stopped. There still is noise, but less. The spikes are gone, not sure why.

The comparator op amps (IC8B, IC1D) are still running, this is probably where the noise is coming from.

I did also try to reduce the amplitude of the DCO by running it trough a buffered 33k/100k resistor divivder. That way, I could make sure the PW would be completely disabled. Not a big change there either.

Tapping ground directly from power input, not via mainboard. Sub osc and pulse waves are running

 

This is interesting. The noise is halved. It should be quite apparent by now that this is most likely caused by noise on the GND line. Let's reconnect the full waveshaper board but without GND

With waveshaper board A fully connected but GND routed back to power input:

This is good, the noise isn't increased by reconnecting all pins

With waveshaper board A and B fully connected but GND routed back to power input:

Maybe a bit surprising, but the noise level doesn't change when reintroducing waveshaper B, as long as the GND goes back to the power input. This is very nice.

Double check how things normally are - reconnect GND back to mainboard

Sooo, the noise is back up past 10 cents, approximately +5 cents on both sides. In my first measurements the noise was around +/-20 cents, but then the VCO was running at 7k. Here it runs at 15k. Also, at some point I started synching DCO 1 and 2. Still, a bit confusing.

Plugging waveshaper straight into the main board with VCAs in place. Moving VCO off board

Again, noise is much lower, but this time the noise is back to being sort of a triangle shape. I THINK i still kept hard sync on, so this is a bit confusing. The period is still 333Hz, same as the DCO

Increasing the sample rate and VCO frequency for a better view:

The shape stays the same. Weirdly enough, the plot of the DCO is missing the bottom half, what is going on?

Part 3 wrapped up

I am still not able to completely figure out if the noise is caused by the op amp creating the clock pulse for the waveshaper, the pulse wave generator or the flip flop for the sub oscillator, though the last one seems a bit more unlikely as the noise follows the pulse, not the sub oscillator

Disconnecting all outputs from the waveshaper didn't make a great deal of change, whereas moving the GND did. I can only assume this means that the noise, at least partially, travels through the ground plane. On a side node, version 2.2 of the waveshaper used the LM311 in the sub oscillator circuit. This generated a lot of noise that affected the VCO (perhaps not from the LM311 itself but from the spike generation circuit after it? who knows. From my (unpublished) 'Testing the voice card' post:

 

If the noise is in the ground plane, it affects more than the VCO - VCA levels, filter FM and cutoff etc. But noise on these are probably less audible.

 

More to test (hey, we're up for a post number 4!)

- x Set pw to 25% and see how the noise reacts. At this point the sub oscillator should still have pw 50% so we may be able to see a difference? 

- x Connect both VCO and Waveshaper grounds to power gnd, effectively making it star ground

- x Try to measure noise on the ground plane

- Look for info on how op amp comparators may pollute the ground plane.

- Breadboard the offending parts of the waveshaper (pulse and clock generators) to see if we get the same result

- x Remove the mixer PCB and tap VCO directly

- x Enable DCO output in mixer but measure VCO at its output, is the noise even worse? 

- x Measure noise on CV 

- x Measure noise on CV without measuring SPI with same probe. 

- x Measure noise on CV relative to GND on VCO! Move GND back to Voice card 

- x Ground CV/Connect to voltage from modular synth 

- x Disconnect Lin FM input from VCO