DIGITALLY CONTROLLED CRYSTAL OVEN S. Jayasimha and T
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DIGITALLY CONTROLLED CRYSTAL OVEN S. Jayasimha and T. Praveen Kumar Signion Systems Ltd. 20 Rockdale Compound Somajiguda, Hyderabad-500082 [email protected] ABSTRACT 1.0 INTRODUCTION Recent developments in integrated miniature Various applications are served by frequency semiconductor temperature sensors, low-cost references of varying accuracy and stability as micro-controllers, predictive finite-element shown in Table 1. Ovenized crystal oscillators thermal analysis and materials (conductive (OCXOs) have the potential to achieve the epoxies and insulating foams with well- stability of low-end frequency standards at less characterized, repeatable properties) now enable than a tenth of their cost and with other attendant the fabrication of low-cost, digitally programmed benefits: reduced steady-state power consumption (temperature set-point and varactor diode bias) and reduced warm-up time. Simultaneously, there crystal ovens, greatly reducing capital equipment are many widely proliferated applications, such as and labor. This paves the way for widespread optical fiber communications, that demand low- deployment of high-stability oscillators. cost, high-stability oscillators, either as back-up or replacements for frequency standards. Description Stability/ Price Power Warm-up time to Applications accuracy rated operation XO, VCXO Crystal oscillator 10-4-10-5 <$1 50mW <10s Watches, phones, TV, PC, toys TCXO Temperature 10-5-10-6 <$10 50mW <10s Wireless, GPS compensated crystal oscillator OCXO (AT-cut) Ovenized Crystal 10-7-10-9 <$200 600mW <100s Instruments, oscillator telecom, radar, satcom Rubidium Rb Frequency 10-10-10-12 <$5000 20W <300s SONET/ SDH, Standard calibration, test, GPS base stations Cesium Cs Frequency 10-11-10-12 <$50,000 30W <2000s SONET/ SDH, standard calibration, test Table 1. Attributes of widely-used frequency references Oven construction and controller architecture greatly influence manufacturing costs of high- stability OCXO’s. A traditional method holds a crystal at a constant temperature (usually 10°C greater than the maximum anticipated ambient temperature) using a thermistor (a passive component whose resistance is inversely proportional to temperature), in an otherwise resistive bridge, and a heater driven by the Figure 1. Oven controller using a thermistor operational amplifier output (Figure 1). The The oven’s sensitivity to ambient temperature heater and thermistor are both attached to the changes is reduced by the oven’s thermal gain. same heat sink (hence thermally connected). High oven gain, together with operation near the conductivity of 0.17 W/m×K) or foam insulation crystal turn-point (where its frequency’s of appropriate thickness (to fit a standard crystal derivative with respect to temperature changes oscillator enclosure2) as shown in Figure 2. sign), may yield a 10 ppb oscillator stability TO-126 resistor (with heat (where the crystal would ordinarily exhibit 10 sink shown filled) TO-126 resistor lead ppm variation over a desired temperature range). Disadvantages of conventional ovens are: • Potentially mismatched oven set point (with Fill this volume respect to crystal turn point) 0.8 mm FR4 with thermal epoxy Copper PCB Solder Ovenized parts (temp. sensor, lead • Analog controllers require an internal voltage crystal, and PWM chip) regulator, creating an uncontrolled hotspot 70mm thick copper metalization in PCB heat spreader • Tuning varactor diode bias potentiometer (to Figure 2. Crystal oven section set frequency) is time consuming. Mistuning due aging and shock may occur. The required steady state power is estimated, • Component aging effects PID controller using materials’ geometry and conductivities as performance inputs, by a 3-D finite element analysis tool, (assuming perfect temperature control at the Our crystal oven, sans these disadvantages, uses: temperature sensor). For this example, the steady state power consumption is 2.5W, for oven and · 8-pin SOIC low-cost micro-controller with ambient temperatures of 363°K and 243°K internal EEPROM that retains: respectively, assuming a 18.5´11´6 mm3 § Temperature set-point enclosure, 18´10´0.8 mm3 FR4 PCB (thermal § Frequency tuning (digital potentiometer1 conductivity of 0.273W/m×K) with an oven changes varactor diode bias) § Temperature controller parameters 2 · Digital temperature sensor Assuming purely conductive heat transfer, the relative · Digital PID controller with feed-forward to dimensions of the oven (constructed using thermal conductors with high thermal capacity) and its blanket suppress oscillations around set-point (constructed using thermal insulators with low thermal · Unregulated power for digital electronics/ capacity) may be understood by considering the oven to be heater, minimizing uncontrolled heat sources a homogenous sphere of radius r1 and its blanket to be a concentric homogenous shell between radii r1 and r2. The heater is assumed to be negligibly thick shell sandwiched 2.0 PACKAGING, POWER CONSUMPTION AND at radius r1. The thermal capacity of the oven is TRANSIENT RESPONSE 3 proportional to r1 while its thermal resistance to ambient 2 is proportional to (r2-r1)/r1 ; thus, the oven’s thermal lag The crystal is maintained at its turn temperature with respect to the ambient is proportional to r1×(r2-r1), (75-90°C) inside an oven that consists of a cavity which, for a fixed r2, has a maximum at r1=0.5×r2.of 2 between the heat sink of a TO-220 or TO-252 0.25×r2 . Since the thermal capacitance seen by the heater is 3 proportional to r1 and the thermal resistance between the power resistor and an electrically isolated copper 2 pour of a printed circuit board. This cavity is heater and the oven is proportional to 1/ r1 , the thermal lag of oven with respect to its heater is proportional to r1. Oven filled with thermal epoxy (e.g. Dow-Corning controllability, the ratio of its thermal lags with respect to STYCAST4954 with conductivity of 1.3 controlled parameters (heater duty cycle and supply W/m×K). The temperature sensor (in the cavity) is voltage, the latter being “controlled” as its variations are, intimately bonded to the heat spreader. The power as shown in section 4, quickly compensated) and uncontrolled parameters (such as ambient temperature, resistor is connected to the supply voltage through barometric pressure, air flow and humidity) is proportional a MOSFET switch. This entire assembly is then to (r2-r1), the insulating blanket’s thickness, and must be surrounded with an insulating potting compound greater than 1. The power consumed by the oven is 2 (e.g., Dow-Corning SYLGARD184 with proportional to (qh-qa) ×r1 /(r2-r1) which is minimized for a given Dq, the temperature difference between the heater, at temperature qh and the ambient, at temperature qa, at a 1 Digital potentiometer wiper capacitance (typically 25pF) practically impossible r1=0. With ideal control, must be considered while designing the varactor network. qh=qo=constant, where qo is the oven temperature. 3 volume of 14´9´2.5 mm , and four (Vcc, ground, control is provided, i.e., the feedback constant K crystal output and serial communication port) will be replaced by a first order FIR system) to 8mm long, 1mm diameter copper (thermal illustrate the effect of heat spreader/ temperature conductivity of 385 W/m×K) pins. The oven’s step sensor delay (while the ensuing discussion is for response depends on the components’ placement integer D, the analysis may easily be extended to relative to the heater and by the materials’ the non-integer case) is shown in Figure 3. volumetric thermal conductance and capacitance. qh(n) qs(n) + z-D 3.0 SEMICONDUCTOR TEMPERATURE SENSOR WITH DIGITAL OUTPUT + -1 + e(n) az + ³T |×| All semiconductor temperature sensors make use of the relationship between a bipolar junction - transistor’s (BJT) base-emitter voltage to its K collector current, VBE=(kq/q)×ln(Ic/Is), with k being qref + Boltzmann’s constant, q the absolute temperature, q the electronic charge and Is the current related to Figure 3. Temperature controller without (a=1) and with the geometry and temperature of the BJT, under temperature feed-forward (a<1) the assumption that VBE>200mV, where Early effects may be ignored. If N identical transistors In Figure 3, at sample n, the heat spreader’s share the current Ic equally, then the base-emitter temperature is qh(n), while the sensor’s voltage is VN=(kq/q)×ln(Ic/N×Is). The temperature temperature is qs and e(n) is the observation noise dependence of Is is eliminated by differencing the (assumed white). It is simpler to consider qs as a two voltages, i.e., DVBE=(kq/q)×ln(N), providing a deviation from the reference temperature; i.e., - D voltage output that is linear with respect to q s ( z) - Kz qref(n)=0; thus, = , resulting in a temperature. This voltage is amplified, digitized e(z) 1-az -1+Kz -D using a first-order sigma delta modulator and high-order sensor spectrum (with at least one transmitted using popular two-wire protocols. spectral peak). For most temperature sensors with digital output, the case-to-junction delay is of the The repeatability of the temperature measurement order of the sampling period, T=1/fs. Thus, D=2 is determined primarily by the amplifier’s dc drift or 3 and the spectrum is unimodal. The natural with respect to age (specified at a nominal frequency for D=2 (when the roots are complex) is f = 1 cos -1 ( a ) f , 0£K<1, |a|<2ÖK. The temperature). The drift at the oven’s operating 2p 2 K s temperature is calculated using the Arrhenius natural frequency for D=3 (when two roots are -1 -1 acceleration factor, AF=exp[Ea/k(q1 -q2 )], where complex and the third is real) is k is Boltzmann’s constant and Ea, the activation 5K - a f » 1 cos-1 ( 3 ) f (with exactitude energy, is assumed to be 0.7eV in the ensuing 2p 54 K 2 + 2a -1 -10 K s calculation.