Debugging VCO Pitch unstability

After making the frequency analyzer script, I've started a thorough checking of the VCO frequency. It's very clear that something is not quite right, something is modulating the CV (e.g. some kind of noise on the pitch CV input, or perhaps ground).

I initially thought that the linear and exponential FM inputs could be the culprits, so I removed IC9. Here is what that looked like:

The frequency very clearly oscillates around 653Hz. However, don't get fooled by the sinusoid or triangle looking shape - the frequency variations are bipolar, they go in both positive and negative direction. If the noise was indeed a single sine wave, it would not bounce up and down. (Now, this may actually be wrong, see later in the post).

That probably means that we're dealing with more than one noise source, that sometimes cancel out and sometimes doesn't. Almost looks like two oscillators slightly detuned, right? 

Increasing the VCO frequency gives us a better view of what is going on:

With a 7.3kHz wave we get a better approximation of the noise. We get several samples at the same frequency (the small peaks are probably due to the sample rate in logic2 being too low), then it drops and stays low for some time, before going up again. It doesn't take too much imagination to see that the noise may be a square wave of some sort.

If one looks closer at the timeline, the pulses appear to be around 0.003s apart, meaning the noise has a frequency of around 333Hz. 

Now, where on the board can we find two square wave sources, slightly detuned? I got a feeling, let's have a look at the output of the DCOs!

 Oh yeah! The DCO is currently running at 330Hz! 

Lets double check. What happens if we halve the DCO frequency?

 

Ok, the issue isn't as pronounced now, it's dropped from +/-20cents to +/-10 cents - but the period has changed to 0.006s, the noise frequency is now 167Hz. I think it's pretty clear that this it DCO related.
 

Chasing the source

Clearly, the source is related to the DCOs, but what exactly? Is it 
- the DCOs themselves
- the sub oscillator circuit (that made audible noise in the previous waveshaper version!)
- the square wave generator
- the output from the waveshaper 
- the output from the waveform mixer

Here are some things I've tried:

Changing the waveform of the DCOs from square to saw:

There is still considerable noise, but the square wave pattern is gone. 

Disconnecting the waveshaper PCB: 

Now the noise is nearly gone! most fluctuations is reduced from 20 to 2 cents, which is probably an acceptable level. Also, putting back the IC9 FM source switch didn't change anything, which is hardly surprising since the FM sources are gone...

Reintroducing the waveshaper, but removing the DCO inputs:

Still looking good. Again, not surprising but at least it tells us that the waveshaper board by itself doesn't cause the noise.

Reintroducing the DCO inputs, one at the time:

 

DCO A

DCO B

Now, these confuse me. With a single DCO we still get a sine-like behaviour. Looking at DCO A, the VCO frequency (662Hz) is very close to twice the DCO frequency (330Hz), so two and two cycles are probably getting modulated by each top/bottom of the DCO wave, and as the frequencies are not exactly the same, the time at top/bottom moves around, creating the sine/triangle effect?

The DCO B plot is completely bonkers, bot sure what is going on there. The difference in cents is still 10 though. 

 

More to test

- Remove the waveform mixer. If the noise is coming from the waveshaper itself it shouldn't matter (though, as changing the DCO waveform DID matter, I suspect that this doesn't change anything).

- Short the suboscillator flip flop input to ground to stop it. Will tell us if the sub oscillator is the source. NB: Test with a single waveshaper.

- Change the pulse width of the DCO pulse wave, if possible disabling the pulse wave altogether. This is also best tested with a single waveshaper, just disconnect the second DCO input. NB: Test with different waveforms selected, not just pulse. 

 

Now, after these tests we should have a fairly clear view of what the noise source is. Next up is figuring out how the noise enters the VCO. A few things to try:

- Move the VCO off board and connect the pitch input to ground close to the PSU

- Move the VCO off board and connect the ground input to ground close to the PSU  

- Try to measure GND and pitch CV at the VCO input relative to GND at the PSU 

 

A note on noise amplitude

The frequency analyzer predicts the noise amplitude using the ratio cents to volts. In a 1V/octave, 1V corresponds to 1200 cents. The change is linear (well, if the VCO tracking is tuned properly), so 20 cents means 16.7mV.  

The VCO circuit is currently 1V/oct if using a 100k resistor at the input. This is true for the exponential FM input, but for the pitch CV we're using a 39k input. In this case, 1V is 2.56 octaves or approximately 3000 cents. 20 cents will then be as little as 6.7mV!

On the other hand, if the noise is coming through the ground plane, who knows what the amplitude will be.