GB2163008A - Miniature, temperature controlled phase detector - Google Patents

Abstract

A miniature temperature stabilized phase detector is packaged in a hybrid circuit module. The phase detector includes a diode bridge 20 which is bonded to a thick film heater resistor 16 for localized heating and temperature control. A thermistor temperature sensor 22 bonded to the heater resistor and a control circuit 24 maintain the heater resistor temperature at a relatively constant value during ambient temperature variations by controlling the heater current. <IMAGE>

Description

SPECIFICATION Miniature, temperature controlled phase detector This invention relates to a miniature phase detector and, more particularly, to a miniature phase detector employing localized heating of detector diodes for temperature stabilization.

Phase detectors are used in a wide variety of applications to detectthe phase difference between two RF signals. Such detectors commonly employ a semiconductor diode bridge as the phase detection element. It is well known that semiconductor diodes are highly sensitive to temperature variations. As a result, phase detectors without provision fortempern ture stabilization are susceptible to large phase measurement errors. This is almost always a problem since the environmental temperature can rarely be relied upon to remain constant. For example, military equipment is required to operate over a temperature range of -55 Cto +800C.

In the past,temperature sensitivity has been overcome by enclosing the entire phase detector in a temperature controlled oven which maintains the circuit at a prescribed temperature. The oven approach has provided generally satisfactory performance in terms of stabilizing the phase detector output. However, the size, weight and power consumption are substantially increased by the oven.

These factors are important considerations in certain applications such as portable or airborne equipment.

The power consumption of an oven controlled unit can be reduced by improving the insulation. However, the size and weight are increased proportionately.

Anothertechnique for overcoming the temperature sensitivity of phase detectors has been to employ curve-fitting compensation in the video amplifier which follows the phase detector. However, since the phase detector characteristics are nonlinear, the curve-fitting circuitry has been complex and expensive.

According to the invention there is provided a temperature controlled phase detector as set out in claim 1, or a temperature stabilized circuit apparatus as set out in claim 7 ofthe claims ofthis specification.

An example ofthe invention will now be described with reference to the accompanying drawings in which: Fig. 1 illustrates a temperature control system for a phase detector in accordance with the present invention; Fig. 2 is a schematic diagram ofthe temperature control circuit of Fig. 1; and Fig. 3 is a schematic diagram of a typical phase detector.

Atemperaturecontrolsystemfora phase detector, in accordance with the present invention, is illustrated in Fig. 1. A partial, cross-sectional view of a hybrid circuit module is shown. A substrate 10, typically alumina, is mounted in a metal chassis 12. The chassis 12,with its cover 14, encloses the elements ofthe phase detector and the temperature control circuitry for the phase detector. Athickfilm heater resistor 16 is bonded to the upper surface of an insulation block 18 which, in turn, is mounted to the substrate 10. A schematic of the phase detector circuitry is shown in FIG. 3 and includes a quad diode bridge 20 as the detector element. The quad diode bridge is contained in a hermetically sealed package and is adhered to the top surface of the heater resistor 16.Athermistor 22 is bonded to the top surface of the heater resistor 16 in close proximity to the diode bridge 20. The leads of the thermistor 22 are connected to a temperature control circuit 24, which is shown in block diagram form in FIG. 1. The resistance of the thermistor 22 varies with the temperature of the heater resistor 16 and the diode bridge 20, since these elements are in close thermal contact. The outputs ofthetemperature control circuit 24 are connected to opposite ends of the heater resistor 16 and supply a variable currentto the heater resistor 16. The temperature control circuit 24, in conjunction with the thermistor 22 and the heater resistor 16, is operative to maintain a relatively constant heater resistor 16temperature during variations in ambienttemperature.The elements of the temperature control circuit 24 are mounted within the chassis 12, thereby forming a temperature controlled phase detector module.

The phase detector circuit shown in FIG. 3 includes capacitors 25,26, transformers 27,28 and diode bridge 20 coupled in a known double-balanced, class I, low-level mixer configuration. The diode bridge 20 is the principal temperature-sensitive element of the circuit. The leads of the diode bridge 20 shown in FIG.

1 are connected to the other elements of the phase detector in the hybrid circuit module.

Itwill be understood thatthe diode bridge 20 shown in FIG. 1 is gratly enlarged for illustration purposes.

The actual size of the diode bridge 20 is typically 0.25 x 0.25 x 0.12 cm. The heater resistor 16 is somewhat largerthan the diode bridge 20 with a dimension, in the present example, of approximately 0.6 cm long x 0.3 cm wide. The size of the heater resistor 16 is selected to uniformly heat the diode bridge or other element being heated. However, it will be apparent thatthe heating is localized in the region of the diode bridge 20 and that very little heating of other circuit elements occurs. The resistance value depends on the voltages used and the required amount of heat. An adhesive, such as a thermally conductive epoxy, is utilized to bond the diode bridge 20 and the heater resistor 1 6for maximum thermal contact.The insulation block 18, which can be G-1 0 glass epoxy, insures thatthe heat generated by the heater resistor 16 is not conducted to the substrate 10 and the chassis 12.

The temperature control unit, including the thermistor 22, the temperature control unit 24 and the heater resistor 16, are illustrated in schematic form in FIG. 2.

The thermistor 22 is connected in a bridge circuit with resistors R1, R2 and R3. The resistor R1 and the thermistor 22 are connected in series at a junction point 30, between the supply voltage Vcc and ground.

The resistors R2 and R3 are connected in series at a junction point32, between supply voltage Vcc and The drawing(s) originally filed was (were) informal and the print here reproduced is taken from a later filed formal copy.

ground. Resistors R1 and R2 are selected to be equal, and resistor R3 is selected to be equal to the resistance of the thermistor 22 at a prescribed temperature. In the present example,thethermistor22 has a value of 100 kohmsat25'C.With this arrangement, the voltage between the junction point 30 and the junction point 32 is close to zero at the prescribed temperature. As the temperature goes up or down from the prescribed temperature, the resistance of the thermistor 22 changes; and a voltage difference is developed between the points 30 and 32. The thermistor 22 has a lowthermal resistance connection to the heater resistor 16, as indicated bythe dotted line 34 in FIG. 2.

The junction point 32 of the thermistor bridge is coupled through a resistor R4to the inverting input of an operational amplifier 36. The junction point 30 of the thermistor bridge is coupled through a resistor R5 to the noninverting input ofthe operational amplifier 36. Afeedback resistor R6 is coupled between the inverting in put and th e output of the ampl ifier 36. A resistor R7 is coupled between the non inverting input ofthe amplifier36 and ground. The output ofthe operational amplifier36 is coupled through a seriesconnected resistor R8 and diode CR1 to the base of a transistor Qi. The emitter of the transistor Q1 is coupled to ground, while the collector ofthe transistor Q1 is coupled through the heater resistor 16to the supply voltage Vcc.A second heater control circuit can be coupled tothe output ofthe operational amplifier 36, as described hereinafter.

In operation,the operational amplifier36, the transistor Q1 and the associated circuitry control the currentthrough the heater resistor 16 so as to maintain a relatively constanttemperatu re during wide variations in ambient temperature. The operating temperature of the heater resistor 16 and the diode bridge %0 is selected to be at least as great as the maximum ambient temperature. In the present example, the operating temperature is +80 C. Point 32 of the thermistor bridge establishes a reference voltage for the operational amplifier 36.When the temperature of the heater ? 6 decreases, the value of the thermistor22 increases, causing an increase in the voltage atthe point 30 and a corresponding increase at the output of the amplifier36. The increase in the voltageatthe outputofthe amplifier36supplies base currentto the transistor Q1 and causes an increase in collector currentthrough the heater resistor 16 and a corresponding increase in temperature. Similarly, when the temperature of the heater resistor 16 increases, the thermistor 22 decreases in resistance value, the collector current drawn by the transistor Q1 through the heater resistor 16 decreases, and the temperature of the heater resistor 16 decreases. Thus, the circuit shown regulates the temperature ofthe heater resistor 16 at a relatively constant value.The operational amplifier 36 is chosen to have a low offset driftto eliminate offsetvoltage and associated error in the feedback loop. A high input impedance amplifier is utilized to avoid loading ofthe thermistor bridge. A suitable type is Analog Devices AD517. The diode CR1 prevents junction breakdown of the transistor Q1 in the eventthatthe desired operating temperatire is exceeded.

In an alternative embodiment, the operational amplifier 36 is replaced with a comparator circuit having a digital, or on/off, output, depending on the relative values of its two inputs. With this arrangement,thetemperature ofthe heater resistor 16 is sensed bythethermistor 22; and the transistor Q1 is turned on or offto regulate temperature. Thetemperature of the heater resistor 16 alternates between upper and lower limits. This embodiment is suitable in cases where some amount of temperature variation is acceptable.

As noted hereinabove, the temperature control circuit shown in FIG. 2 is optionally used to control a second heater resistor 40 utilizing a second transistor Q2, a diode CR2 and a resistor R9 coupled to the output of the operational amplifier 36. Such an arrangement is appropriate in cases where two or more phase detectors are located in close proximity so that both are subject to the same ambient temperature. The resistor R9 is initially selected to provide the prescribed temperature of the heater resistor 40. It is then assumed that the temperature of the heater resistors 16 and 40 will track as the ambient temperature is varied.

In one example of a temperature controlled phase detector, in accordance with the present invention, two phase detector circuits and a temperature control circuit, as shown in FIGS. 2 and 3, were packaged in a hybrid circuit module measuring 2.16 x 2.26 x .48 cm and weighing 10 fm. The powerdissipationofeach heater resistorvaried between 0.92 watt at -550C and 0 watt at +80 C. The diode temperature varied by less than + 0.5 C, overa temperature range from -22"C to +73 C.

The present invention has been described in terms of a phase detector wherein the temperature of the diode bridge is locally controlled. It will be understood that local temperature control oftemperature-sensi- tive elements can be applied in other types of circuits.

The heater resistor size, value and operating temperature can be selected forthe particular operation.

Although the invention can be applied in printed circuit board situations, it is particularly advantageous in hybrid circuit modules where the heater resistor can be bonded directly to the temperature-sensitive semiconductor device, thereby assuring good thermal contact. In addition, it will be understood that other arrangements can be utilized for controlling the temperature ofthe local heating resistor.

While there has been shown and described what is at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the artthatvarious changes and modifications may be made therein without departing from the scope ofthe invention as defined by the appended claims.

Claims (10)

1. Atemperaturecontrolled phasedetectorcom- prising: a phase detection circuitincludingapluralityof circuit elements at least one of which is a temperature- sensitive semiconductor device; and means for locally controlling thetemperature of said semiconductor device without controlling the temperature of the remainder of said circuit elements.

2. The phase detector as defined in claim 1 wherein said meansfor local temperature control includes heater means positioned in close proximity to said semiconductor device for local heating thereof, means for sensing the temperature of said semicon- ductordevice, and heater control means responsive to said temperature sensing means for energizing said heater means so as to maintain said semiconductor device at a relatively constanttemperature.

3. The phase detector as defined in claim 2 wherein said semiconductor device comprises a quad diode bridge.

4. The phase detector as defined in claim 3 wherein said heater means comprises a thickfilm resistoradheredto said diode bridge.

5. The phase detector as defined in claim 4 wherein said temperature sensing means comprises a thermistor adhered to said resistor in close proximity to said diode bridge.

6. The phase detector as defined in claim 5 wherein said heater control means includes means for varying the currentthrough said resistor in response to the signal from said thermistor.

7. Temperature stabilized circuit apparatus comprising: an electrical circuit including a plurality of circuit elements, at least one of which is temperature sensitive; and means for locallycontrolling the temperature of said at least onetemperature-sensitive element, whereby the operation of said electrical circuit is substantially stabilized against temperature variations.

8. The temperature stabilized apparatus as defined in claim 7 wherein said temperature-sensitive element comprises a semiconductor device.

9. Thetemperature stabilized apparatus as de- fined in claim 8 wherein said meansforlocal temperature control includes heater means positioned in close proximity to said semiconductor device for local heating thereof, means for sensing the temperature of said semiconductor device, and heater control means responsiveto said temperature sensing means for energizing said heater means so as to maintain said semiconductor device at a relatively constanttemperature.

10. Temperature controlled apparatus substantial- ly as hereinbefore described with reference to and as illustrated in the accompanying drawings.