Temperature compensation (choice 2) may not work for all devices, leaving the third choice as the only option: hold the device of interest at a specific temperature. The remainder of this paper will discuss the use of Miniature Crystal Ovens ("Mini-Ovens") for such a purpose.

This tight level of stability assumes two things:

1. Ambient temperatures do not fall below design values.

   The gain of the control circuitry is sufficiently high to cause large power drains if the oven temperature is not within a close tolerance to the desired setpoint. This power is limited with additional circuitry to a value that the customer (or Isotemp) specifies. Moreover, power limiting keeps the heater circuit from destroying itself. In the case where ambient temperatures are approaching the desired setpoint of the oven, very little power is required to heat the oven some incremental amount. As the ambient temperature drops, more and more power is required to keep the oven warm. Insulating the oven helps, but as there is no such thing as 100% effective insulation: The power draw will always increase with decreasing ambient temperatures. A point will be reached where the power required to keep the oven at the specified temperature will be higher than the power limit allows. The limit circuitry will activate, and the oven will not have enough power at its disposal to maintain the proper temperature. An oven with an overly small power limit will warm up slowly and have trouble maintaining its temperature over even a modest temperature range.

2. Ambient temperatures do not approach the set temperature of the oven.

   In the case of a completely empty oven, when ambient temperatures equal that of the oven's setpoint, the oven will need zero power to maintain its temperature (all of the required energy is being supplied by the surrounding environment). If ambient temperatures exceed the oven setpoint temperature, the oven temperature will rise as well. The controller will be asking the heater to remove power from the oven! Without specialized thermo-electric devices, this cannot be done. Note that the previous example is for a completely empty oven. If the oven is holding a power dissipating device, it will dissipate a portion of its power as heat, and this heat is independent of the oven controller. Even if the oven controller shuts down the heater completely, heat is still being added by the active device. This addition of heat from the device is referred to as heat rise.

   Figure 2: Oven Power vs Ambient Temperature

   The heat generated by the device may be small compared to the potential of the heater, but can still make the heater shut down even before ambient temperatures reach the oven setpoint. For this reason, the oven setpoint should be a few degrees higher than the highest expected ambient temperature. Figure 2 graphically demonstrates the principles of the above two sections with a power graph of a hypothetical oven. Below about -40°C, the power is clipped to some maximum value due to power limiting, and the oven temperature will fall out of regulation. Above about 90°C, the power drops to zero as ambient temperatures exceed the setpoint of the oven.

Currently Isotemp produces Mini-Ovens that fit over

Figure 3: Mini-Oven Cross Section

HC-43/U and T05 packages. The device to be heated is simply inserted into the oven (more typically, the oven is seated over the device on a PCB). To ensure that the device being heated makes good thermal contact with the heated surfaces of the oven, Isotemp suggests using a thermally conductive compound between the device and the oven. Refer to Figure 3 for a cut away view of a typical oven installation using thermal compound. Tests have shown that temperature offsets of 1-2°C can occur without the use of a thermally conductive compound. One suggested thermal compound is manufactured by Wakefield Engineering Inc., part number 120. This is the compound used internally at Isotemp.

Figure 4: AT-cut crystal curve

One good use for a mini-oven is for temperature control of the crystal used in a crystal oscillator. A crystal oscillator is an inherently stable device, but by controlling the temperature of the crystal, orders of magnitude of improved stability can be realized. To understand why this is so, we must first understand the nature of a crystal. The most popular and least expensive crystal used in oscillators is the AT-cut type. The term AT-cut refers to the way a piece of quartz is cut to produce the individual crystal blanks. In an AT-cut crystal, the quartz is cut at an oblique angle with respect to one crystal axis (singly rotated). Refer to Figure 4 for the frequency offset over temperature for an AT-cut crystal. It is important to note that the curve in Figure 4 represents only one of a family of curves.

Figure 5: Family of AT-cut crystal curves

Figure 5 shows the family of curves from which Figure 4 was derived. Each different curve is achieved by cutting the quartz along slightly different angles with respect to some crystal axis. The amount of angle change is small, and is typically specified to the minute ('), or 1/60th of a degree (°). A crystal can be specified such that its performance is optimized over a certain temperature range, but the frequency stability will be limited. Even over a modest commercial grade temperature range, stabilities of only a few parts-per-million (PPM) are possible. To achieve frequency stabilities better than this, some type of compensation is required. Refer again to Figure 5: If ambient temperatures swing from -30°C to 70°C, an optimally chosen crystal will change about ±5 PPM. Now refer to Figure 6: The slope around 70°C is near zero. If the crystal temperature could be maintained around this point, the frequency change would be greatly reduced.

Figure 6: AT-cut crystal @ Upper Turn

By using a Mini-Oven to maintain the crystal temperature at the upper turn point of the crystal (the point at which the slope of the curve equals zero) tighter ambient frequency stabilities can be achieved. Low cost Mini-Ovens can maintain better than ±5°C over ambient ranges of -30°C to 60°C or greater. Ambient frequency stabilities can go from many parts-per-million to less than 1/2 part-per-million. Stabilities much tighter than this are not easy to achieve by heating the crystal alone, since the other circuit elements also control the frequency, to a lesser degree. In these cases, a custom TCXO or OCXO may be needed. Isotemp manufactures a full line of these products in addition to the Mini-Oven.