Supplementary Information for SSI2131 VCO (Patreon)

Here's some additional information about the DIY SSI2131 VCO module.

https://note.com/solder_state/n/nf47861e3c1ed

https://youtu.be/E4mQAlW-ms0

1. Reason for Removing the Soft SYNC Function

The SSI2131 includes a soft sync function. I experimented with incorporating this feature into the module but decided to remove it due to instability under certain conditions. When a pulse signal was input to the soft sync in the standard circuit from the datasheet, it operated stably in the low to mid frequency range. However, as the frequency of the SSI2131 increased, the operation became unstable, and the waveform began to distort. At even higher frequencies, the IC stopped functioning. The operation resumed after reapplying power.

https://youtu.be/xP0kS6ZaRXQ

I couldn't determine whether the issue was with my experiment or with the IC itself.

2. Tuning the Hard SYNC Circuit

For the hard sync circuit, I changed the values of the resistors and capacitors from the standard circuit shown in the datasheet. The picture compares the operation of the standard circuit and the circuit I created for this module.

10kohm and 10nF

100kohm and 100pF

When a long pulse is input to the hard sync pin of the SSI2131, the reset period of the waveform becomes longer. This is not an issue at low frequencies, but at high frequencies, this reset period becomes noticeable.

3. Abnormal Oscillation of the PULSE Output and Countermeasures

There were instances of abnormal oscillation at the rise/fall of the PULSE output.

 This issue was due to instability in the PWM circuit voltage. Adding a capacitor to C11 to stabilize the PWM voltage eliminated the abnormal oscillations.

4. Distortion of the Sawtooth Wave and Its Countermeasures

Distortion occurred around the midpoint of the sawtooth wave output. There was a correlation between the resistance value of the limiting resistor R13 in the regulator circuit and the magnitude of the distortion. Although the cause is unknown, using a 3.3k ohm resistor eliminated the distortion.

R13: 330ohm

R13: 1kohm

R13: 3.3kohm

5. Output Frequency Range of the SSI2131

Here are the results of confirming the IC's performance limits. When C7 was set to 10pF, an output of 230kHz was confirmed. When C7 was set to 100uF, an LFO with a period of over 800 seconds was output. However, the waveform showed distortion, so stable operation might not be expected.