Saturday, January 13, 2018

A simple crystal oven/heater that uses no power resistors

There are times where one needs to elevate a component to a consistent temperature to better-maintain its characteristics, the most common being the need to heat a quartz crystal to attain better temperature stability.

I had such a need when I was constructing my 24 GHz transverter - link - and the 99 MHz crystal oscillator (multiplied by 240 to yield 23.76 GHz for a 432 MHz IF at 24.192 GHz) was locked to a high-stability 10 MHz reference.  This 99 MHz oscillator uses an overtone crystal oscillator, but these are notoriously difficult to electronically tune over much of a range so I needed to maintain the crystal and oscillator at a constant temperature to keep it "close enough" to frequency to be within its narrow tuning capability to allow it to be locked precisely on frequency.

To do this I needed to construct a "crystal oven" - a circuit/device that holds the critical components at a (fairly) constant temperature.

How it works:

Typically one uses several power resistors to heat a crystal, but there are ways that this can be done using no power resistors at all:  Consider the circuit in Figure 1, below.

Figure 1:
Crystal oven using a power MOSFET as the heating element.
R320/Q306 form a constant-current limiter circuit so that the FET can never be fully "on", keeping its resistance higher so that it can function as a heater.  The FET itself, along with thermistor R316 are "thermally coupled" to the crystal being heated.  The (nominal) 0.6 volt turn-on threshold of Q306 and the value of R320 limit the maximum current to about 600mA, but this could be adjusted by selecting appropriate values for R320.
Click on the image for a larger version.
In this circuit Q305, a generic N-channel MOSFET power transistor, is used as the sole heating element:  Its tab could be soldered or bolted directly to the device that needed to be heated such as the crystal or soldered/bolted to a substrate to which the crystal and other components are mounted, etc.  Because the tab of  typical power FET is the drain lead, make sure that it has DC isolation from ground.

The heart of this circuit is the Q305/Q306/R320 combination.  Assuming that voltage has been applied to the gate of the FET via R319 to turn it on (more than 6 volts for the FET specified) when the current through R320 exceeds that which is required to effect an (approximately) 0.6 volt drop, Q306 turns on, pinching off the gate drive and regulating the current through R305 to that level.  With the components shown the maximum amount of current that will flow is that which causes 0.6 volts to appear across R320, which, in this case is (0.6 volts * 1 ohm = 0.6 amps) 600 milliamps.  By limiting the current to a reasonable level the FET's "on" resistance is regulated, allowing it to act as a heater.  Without this current limiting U303 would simply turn FET Q305 "on" and its resistance would be a fraction of an ohm, effectively shorting out the power supply - but more likely it would dissipate a lot of power and blow up if there was sufficient power supply current.

Assuming that there are 9 volts available across Q305 (e.g. the 10 volt supply shown, the drop across R320 and a few other, miscellaneous losses) Q305 will produce about (9 volts * 0.6 amps) 5.4 watts of heat, conducted from its metal tab into whatever it is that is being heated.

To regulate the temperature a simple controller is built around an op amp - in this case, a generic 741 op amp (U303) and a thermistor (R316):  Because of the typical 2-4 volt threshold voltage of typical power FETs, the fact that the output voltage of a common op amp like the 741 doesn't get very close to its negative rail isn't a problem - but this should be kept in mind if you happen to use a "logic threshold" FET that starts to turn on at about a volt.  R316 is "thermally coupled" to the device(s) being heated - but not coupled too closely to the heat-generating component, Q305 that there is constant over/undershoot when the heater is active.

The actual temperature at which the oven will stabilize is determined by matching the value of fixed resistor R315 to that of thermistor R316 at the desired temperature - a resistance that can be determined from the thermistor's 'spec sheet or by experimentation.  The actual value of the thermistor at the operating temperature is not particularly important but it is recommended that it be in the range of 5k-100k for practical reasons.  The thermistor that I happened to use had a nominal resistance of 30k at 25C, decreasing to about 11k at 50C, the target temperature, so I used a value of 11k for R315.

When the oven is cold the resistance of thermistor R316 is going to be higher than that of fixed resistance R315 which causes the non-inverting (+) terminal of the op-amp to be higher than that of the inverting (-) terminal which is biased at mid-supply by two equal resistors.  When this happens the output of the op amp goes high, providing gate voltage to Q305 via resistor R319, allowing it to heat up.

Figure 2:
The oven and its controller.  The tab of Q305 is soldered directly to
a large, electrically-isolated island of circuit board material.  As can be
seen from the picture, the board to which the heater is mounted is actually
smaller than the surrounding enclosure, mechanically "floating" in the center
via four pieces of small-gauge wire that provide both a DC return and RF
ground connections as well as allowing a gap that is filled with an air
space and insulating foam.  The circuit is wired "dead bug" with the op
amp being "leads-up", just to the right of Q305.  At the bottom of the
picture is a 3-terminal 10 volt regulator (a 7810 - not shown in figure 1) that
provides a stable 10 volt source for both the oven and the crystal oscillator.
Click on the image for a larger version.
When the oven comes up to the design temperature (e.g. that which the resistance of thermistor R316 is the same as fixed resistor R315) the voltages at the inverting and non-inverting terminal of the op amp are equal and the voltage being output by the op amp drops, removing the bias voltage from the gate of Q305 and preventing the oven from heating further.  In reality, there isn't an "on/off" action by the oven, but a more gradual "power up/power down" caused by the inclusion of R317 between the output of the op amp and the noninverting (-) input of the op amp.

It's worth noting that using the turn-on voltage of a transistor as a current reference means that the actual current will vary depending on that transistor's temperature (e.g. that of Q306), but because Q306 is located within the oven chamber it, too, will be heated and the maximum "oven" current (e.g. that through Q305) will be quite stable.  What can affect the dynamics of this oven system is a variation in the voltage applied to Q305:  The higher the voltage, the more power (in watts) will be produced in heat.  While the oven controller will help to maintain temperature, if the power supply voltage is quite variable - as could happen when this oven is run from a "12 volt" battery (anywhere from 11.5-13.5 volts, depending on state-of-charge) the thermal input power can change and cause a slight instability in the closed-loop temperature control due to this change in available thermal power.  In this case - with the crystal being externally locked to frequency, anyway, this wasn't too important.

There are a few other component sprinkled about in the diagram as well:
  • C318 and R321 are used to prevent the Q305/Q306 circuit from oscillating.
  • R322 is an optional test point to measure the oven current.  FT304 is an (optional) feedthrough capacitor used to prevent RF ingress/egress along this monitor point.
  • C316 and C317 are power supply bypass capacitors - always a good practice to include.
  • R318 and LED D303 provide an optional "oven on" indication.  The cycling of this LED between full brightness and dim/off after being powered up indicates that the oven is heating/stabilizing.  If it cycles on and off continually this can indicate that there is too much thermal resistance between the heater (Q305) and the thermistor, causing the circuit to overshoot.
  • The values of R313 and R314 are not critical - but they should be equal.
  • The value of the thermistor is not critical, but it should probably be between 1k and 100k at the desired operating temperature.  Select R315 to have the same resistance as the thermistor at the desired oven operating temperature.
  • To provide a temperature adjustment, R315 may be made variable with a good-quality multi-turn potentiometer.  Alternatively, resistors R313/R314 can be replaced with a single 20k-50k multi-turn potentiometer.
What to use this for?
Figure 3:
The "oscillator side" of the circuit shown in Figure 2.  The oscillator is a 5th-
overtone "Butler" type build "dead bug" on a piece of double-sided
copper-clad epoxy board.  The crystal is located directly opposite the
location of Q305, the heater (e.g. the board is rotated 1/4 turn counter-
clockwise from Figure 2).
Click on the image for a larger version. 

As mentioned above, I devised this circuit when constructing a homebrew 24 GHz amateur radio transverter (transmit/receive frequency converter) and needed a "fairly stable" source of a 99 MHz signal to be multiplied upwards and to be locked to an outboard, stable 10 MHz reference (e.g. high-stability crystal or rubidium source.)

Because this crystal oscillator was to be externally locked, it didn't need to be ultra-stable - just stable enough to keep its temperature close enough to the rather limited frequency-pulling range afforded by high-frequency overtone crystal oscillators.  With this relaxed requirement, the crystal could actually vary a few degrees about the set point with no ill effects whatsoever.


I didn't need to optimize this circuit for ultimate frequency stability as the 99 MHz oscillator is locked to an external 10 MHz reference:  All that is necessary is that the frequency be "close enough" - which is to say, within the rather narrow frequency tuning range afforded via the VCXO (Voltage Controlled Crystal Oscillator) circuit.

If this circuit is to be used for a "stand-alone" oscillator where the frequency is directly affected by the temperature, additional care will be required to appropriately thermally couple the thermistor and heater (Q305) - and possibly tweak the value of resistor R317 - to prevent the temperature from oscillating about its set point.

Finally, even though the temperature controller is entirely ratiometric - that is, power supply voltage variations will not affect the temperature set point to a significant degree -  remember that at higher voltages the power going into the heating element (e.g. the power FET Q305) will also increase.  This difference in thermal input can cause the dynamics of the oven to change somewhat and slightly alter the rate-of-change and potentially alter the stability of the feedback loop.  Because this oven was intended to keep an oscillator "close enough to" rather than "dead on" frequency this factor wasn't important.

Additional resources:
  • The OCXO/Si5351A synthesizer - link - QRP Labs sells a version of their Si5351A synthesizer board with a built-in crystal oven controller providing a stability of 1ppm or better.  The assembly manual for this kit (linked on the referred page) uses low-power FETs as heaters and a bipolar transistor as the temperature sensor.  This manual discusses the operation of the circuit and is an interesting read.


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