# Compact thermostat for VCO temperature stability

So I have a voltage controlled oscillator with an exponential input made using the THAT2181 VCA IC. The schematic is shown below - the THAT2181 IC is used to change the C4 capacitor charging current and, therefore, the output frequency depending on the LOWCV input.

The problem is the thermal stability of such a solution - THAT2181 has a 3300ppm temperature dependance and for precise audio it is unacceptable - with the same input the schematic will output 807 and 837 Hz signal at 25 and 35 degrees Celsius respectively.

A typical solution for this problem is adding a 3300ppm NTC thermistor divider at the input, but those are hard to find, not exactly precise anyways and manufacturers today move towards "IC heating solutions" in transistor array ICs using one of the transistors in it as a heater.

So, my question is: what will be the most compact, yet cheap solution for THAT2181 IC heating in my schematic? I've looked at the PTC heaters, but the minimal transition temperature of them is about 60-70 degrees Celsius, which is way too hot (I am looking for about 40 degrees) and adding a microcontroller feels like an overkill.

• What are your input output specs and tolerance? And temp. Range. – Tony Stewart EE75 Jun 24 '18 at 21:37
• @TonyStewartolderthandirt Temperature range - around room temperature, but, because I need to keep the temperature stable using a heater, the target temperature is a bit hotter - around 40 degrees. The THAT2181 IC temperature range is 0-70 degrees. – sx107 Jun 24 '18 at 21:59
• Yes for that circuit but we need to know how stable and how sensitive your VCO is. Can you define a proper spec? – Tony Stewart EE75 Jun 24 '18 at 22:05
• @TonyStewartolderthandirt 0.036 voltage change on the input doubles the output frequency - this is the sensitivity and it works from around 10Hz up to 40kHz. The expected output stability needs to be at least as good as the solution with the 3300ppm NTC thermistor divider at the input, which provides around 0.2% stability, which is good enough, but not perfect. The VCO is used for musical purposes and even 6% change already changes the output note, say, from A to A#. – sx107 Jun 24 '18 at 22:10
• is it a SIP or SOIC? The reason determines the size of the insulated oven heater and thermal sensor to regulate above self-heating – Tony Stewart EE75 Jun 24 '18 at 22:18