Transducers

Temperature Controller for Crystals

A simple temperature controller for quartz crystal resonators operates on less than 5 W prime power and can heat a crystal from -10°C to +75°C in less than 45 s. Temperature control is accurate to within 0.7°C.

The short time constant of this temperature-controller is due to its reduced mass and smaller separation between the heater and the sensor. Its temperature error is reduced by an exceptionally high-gain circuit.

The unit consists of a heater/sensor crystal assembly and a high-gain control circuit. The nickel-iron wire used as a sensing and heating element has a highly stable resistance-versus-temperature characteristic. The assembly is arranged as an open ring that fits over crystal containers.

A film of room-temperature-vulcanized compound applied between the heater and the container assures good thermal conduction between the two. The heater is tied to the container surface with a lacing cord, and a heat-shrinkable band is placed over the heater as a constrictive cover.

The control circuit incorporates a sensor, an operational amplifier, and a power transistor. The sensor R₅ connected to a bridge circuit (R₂, R₃, R₄, and R₅) converts the temperature error into voltage error using a 20-mV/°C transfer function developed for this circuit. Resistors R₂, R₃, and R₄ are selected to equal the value of R₅ at the desired regulation temperature, and R₁ = R₆. The latter (R₁ and R₆) limit current through the bridge to keep the bridge elements from overheating.

The bridge error voltage is applied to an operational amplifier that has a static gain of about 3×10⁵V/V. Resistors R₇ and R₈ with capacitors C₂ and C₃ filter the ripple on the prime voltage supply (+28 Vdc), while capacitor C₁ is selected to compensate the amplifier according to the manufacturer's specification. In addition, resistor R₉ is used to limit base current into transistor Q₁ so that full voltage swings of the amplifier output do not drive Q₁ into saturation.