High Stability Buffered Phase Comparator

This application is related to an application for an invention entitled, "High Stability Amplifier", (GSC 12,646-1) U.S. Ser. No. 284,290, filed in the names of the present inventors on July 17, 1981. The application is also assigned to the assignee of this invention.

1. Technical Field

The invention relates generally to electrical signal phase comparators and more particularly to a phase comparator for RF signals which exhibits a relatively high degree of RF isolation and stability with respect to ambient temperature changes.

2. Background Art

In applications where it is required to compare the phases of two RF signals that are nearly the same frequency with a higher degree of precision, such as being able to measure the phase difference to within 1.times.10.sup.-12 seconds (picoseconds) without the signal sources interacting with each other, it is necessary to provide a relatively large amount of signal isolation therebetween. This is achieved by means of buffer amplifiers between the signal sources. While known prior art signal buffers operate to provide a certain degree of isolation, the means by which phase comparison has heretofore been made has been found to be inadequate particularly where temperature stability is concerned. It has been found that unless a relatively high stability with respect to ambient temperature changes is maintained, phase noise results which prevents the measurement of phase difference with a desired degree of resolution.

Accordingly, it is an object of the present invention to provide a low noise phase comparator which is operable at RF frequencies.

Another object of the invention is to provide a low noise phase comparator which exhibits a relatively high degree of output stability with respect to changes in ambient temperature.

Still another object of the invention is to provide a low noise phase comparator which provides a high degree of isolation between relatively high precision frequency sources.

Still a further object of the invention is to provide a phase comparator adapted to meet the variety of precise time and frequency measurement and distribution tasks.

These and other objects are provided by means of a high stability buffer phase comparator comprised of a pair of high stability, driver-buffer amplifier circuits coupled to a double balanced mixer located in a common housing along with a common heat sink into which are embedded the active elements of the amplifier and mixer circuits. The output of the balanced mixer is fed to a signal level crossing detector which is located in an adjoining housing and is adapted to generate a square wave whose positive and negative going edges correspond to the phase difference between two RF signals applied to the amplifiers.

The foregoing as well as other objects, features and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

FIG. 1 is an electrical block diagram generally illustrative of the invention;

FIGS. 2A and 2B depict a detailed electrical schematic diagram of the preferred embodiment of the invention;

FIG. 3 is a perspective view generally illustrative of the composite housing structure for the circuitry shown in FIGS. 2A and 2B;

FIG. 4 is a perspective view of the common heat sink for a portion of the circuitry shown in FIG. 2A;

FIG. 5 is a cross sectional view of the housing shown in FIG. 3 taken along the lines 5--5 thereof;

FIG. 6 is an electrical block diagram of a typical application of the subject invention;

FIG. 7 is an electrical block diagram helpful in understanding the configuration shown in FIG. 6; and

FIG. 8 is a graph illustrative of the stability characteristic for the application shown in FIG. 6.

Referring now to the drawings and more particularly to FIG. 1, disclosed thereat is a system for comparing the phase of two relatively high precision RF signal generators 10 and 12 whose frequencies are substantially the same. While the system is adapted to operate with RF signals covering the range from 30 KHz to 45 MHz, in the preferred embodiment the frequency sources 10 and 12 comprise atomic frequency standards such as a cesium frequency standard or a standard implemented by means of a hydrogen maser where 5 MHz signals are provided thereby. In order to compare their relative frequencies, the sources 10 and 12, which for example provide a 5.0 MHz and 4.999999 MHz frequency output, respectively, are coupled to a double balanced mixer 14 by means of a pair of signal amplifiers 16 and 18 which provide amplification and buffering for the two signals from the sources 10 and 12. The mixer operates in a well known manner to heterodyne the two RF signals and provide a difference signal, comprising for example a one Hertz beat frequency signal, which is applied to a zero crossing detector 20 having a low pass filtered input and providing a very fast, e.g..about.20 nanosecond (1.times.10.sup.-9 sec.), rise time output pulse. The zero crossing detector 20 generates a square wave output whose positive and negative going edges correspond to the phase difference between two RF inputs from the sources 10 and 12. An event clock 24, when coupled to the output of the zero crossing detector 20, is adapted to measure the time of the edge crossings of the positive and negative going edges and provide an indication of the phase difference between the 5.0 MHz and 4.999999 MHz inputs.

In order to provide an accurate measure of the phase difference between the extremely stable frequency sources 10 and 12, it is necessary that the circuitry implementing the balanced mixer 14 and the amplifiers 16 and 18, and to a lesser extent the zero crossing detector 20 not degrade the phase stability of the RF signals translated thereby and thus there is a requirement that a relatively high degree of output stability must be exhibited particularly with respect to ambient temperature changes over a relatively long period of time.

In order to achieve such a performance, the circuitry embodying the balanced mixer 14 and the amplifiers 16 and 18 as shown in FIG. 2A is located in a common metallic housing 26 shown in FIG. 3 and which also includes therein a common heat sink 28 for all of the active circuit elements used to implement the mixer 14 and the amplifiers 16 and 18. The heat sink 28 comprises a relatively large mass and operates to slow down the effect of ambient temperature changes and reduces temperature gradients across the active circuit components, thus improving the cancellation of temperature effects inherent in such devices. The heat sink 28 is shown in detail in FIG. 4 and will be considered in further detail subsequently. In addition to the common housing 26, a second metal housing 27 is also shown in FIG. 3 which is utilized to house the circuitry shown in FIG. 2B which circuitry includes the circuit components for implementing the zero crossing detector 20 shown in FIG. 1. The two housings 26 and 27 are contiguously fastened together by means not shown.

Referring now briefly to FIG. 5, which is a transverse cross section of FIG. 3, the interior of the housing 26, in addition to including the heat sink 28 also includes a circuit board 29 placed in close proximity to the heat sink 28. Three members 30, 32 and 34 of seven are shown mounted on one side of the circuit board 34 projecting into the heat sink 28. These members 30, 32 and 34 constitute the top portion or covers of hermetically sealed packaging means for the active circuit elements (semiconductors) included in the amplifier 16 and fit into the bores 31, 33 and 35 of FIG. 4. In the same fashion the bores 37, 39 and 41 (FIG. 4) receive the packaging means containing the active circuit elements in the amplifier 16 while the bore 43 accepts the active circuit elements in an integrated circuit embodying the double balanced mixer 14. The circuit board 29, moreover, is held in position by opposing spacer members 36 with the circuit board being secured thereto by means of screw type hardware 37. Insofar as the adjoining housing 27 is concerned, it also includes a circuit board 38 centrally located therein. It is held in position by means of spacer elements 29 and flat headed screws 40 passing through the top cover plate. FIG. 5 additionally discloses an input connector 42 located in one end (far) wall 43 of the housing 26. This connector is adapted to receive one of the RF inputs to the amplifiers 16 and 18 shown in FIG. 1. FIG. 5 also shows a connector 44 located in the far end wall 46 of the housing 27 which has for its purpose coupling of power supply potentials to the circuitry included in both housings 26 and 27.

Referring now to the schematic diagram of the electronic circuitry included in the preferred embodiment of the invention, reference will now be made to FIGS. 2A and 2B. Referring first to FIG. 2A which includes the buffer amplifiers 16 and 18 and the mixer 14, a pair of RF signal input terminals in the form of co-axial connectors 42 and 48 are adapted to be coupled to the RF signal sources 10 and 12 (FIG. 1). Connector 48 is located in the opposite end wall 50 of the housing 26 as shown in FIG. 5. Both of the buffer amplifiers 16 and 18 are comprised of three major components, namely a voltage regulator, a driver transistor and an operational amplifier. More specifically with respect to buffer amplifier 16, the RF input signal coupled from the source 10 to connector 42 is applied by way of lead 60 to a single driver transistor 62 through a coupling capacitor 64 and a resistor voltage divider network 66. The driver transistor 62 typically comprises a Motorola type 2N2857N-P-N transistor coupled in a feedback circuit arrangement to the non-inverting input (+) of an operational amplifier 68 which typically comprises a National Semiconductor LH0033 integrated circuit module. Both the transistor 62 and the operational amplifier 68 are adapted to receive a regulated +V.sub.1 supply potential provided by a RF signal isolating voltage regulator 70 which receives an unregulated +V.sub.1 power supply potential from the connector 44 (FIG. 2B) via lead 71 after being filtered by a combination of inductor 72 and capacitor 74. The voltage regulator 70 typically comprises a Motorola type MC1569G integrated circuit module. What is significant, however, is that the transistor 62, the operational amplifier 68 and the voltage regulator 70 are encapsulated in packages which are mounted on the circuit board 29 and include respective cover members 30, 32 and 34 as shown in FIG. 5 which are embedded in the recesses or bores 31, 33 and 35 of the common heat sink 28.

In operation, the transistor 62 whose collector is connected to a load resistor 63 and whose emitter is connected to ground through the resistor 65 and capacitors 67, acts as a signal gain device. The operational amplifier 68, on the other hand, operates as a unity gain signal isolation device. The gain for the composite feedback amplifier implemented by the combination of the transistor 62 and the operational amplifier 68 is determined by the component values of a feedback resistor 76 coupled from the output of the operational amplifier 68 back to the base of transistor 62 as well as a capacitor 78 coupled from the collector of transistor 62 back to its base.

In a like manner, the RF input signal from the source 12 applied to the input connector 48 is coupled to the base of transistor 82 via the circuit lead 81, the coupling capacitor 84 and the resistor voltage divider network 86. Also coupled to the transistor 82 is a collector load resistor 83 and a grounded emitter resistor 85 which is shunted by capacitor 87. The transistor 82 is coupled to the non-inverting input(+) of the operational amplifier 88 in a feedback amplifier circuit arrangement with both circuit components being powered from a regulated +V.sub.1 potential applied from a second RF signal isolating voltage regulator 90 which is coupled to the unregulated supply potential +V.sub.1 via the inductor 92 and the capacitor 94 coupled to lead 71. Both voltage regulators 70 and 90 effectively increase RF isolation on each respective channel by reducing cross-talk through the power supply, not shown, providing, inter alia, the power supply potential +V.sub.1. The feedback circuit arrangement including transistor 82 and the operational amplifier 88 also includes the feedback resistor 96 and feedback capacitor 98 which again operate to control the gain of the signal output from transistor 82 while the operational amplifier 88 comprises a unity gain device for providing proper signal isolation. The single transistor 82, the operational amplifier 88 and the voltage regulator 90 also are encapsulated in packaging means having cover members, not shown, which are embedded in the recesses 37, 39 and 41 of the common heat sink block 28 shown in FIG. 4; however, these cover members are identical to the members 30, 32 and 34.

In addition to having the active semiconductor components of the amplifiers 16 and 18 embedded in the heat sink 28, the heat sink 28 also includes a centrally located recess or bore 43 as shown in FIG. 4 which is adapted to accommodate the cover portion, not shown, of the integrated circuit module 110 implementing the mixer 14 and which has the outputs of the operational amplifiers 68 and 88 coupled thereto via the resistive-capacitive networks 80 and 100. The recess 43 is generally rectangular in shape due to the fact that the physical configuration of the housing of an MCL type SRA1-1 double balanced mixer which is utilized in the preferred embodiment has such a shape. Accordingly, the shape of the recesses formed in the common heat sink 28 depends upon the physical characteristics of the housings for the active components embedded therein. What is significant about the component mounting on the circuit board 29 in the housing 26 is that the active components as shown in FIG. 5 are mounted on one side of the circuit board 29 while the passive circuit components which are shown by a general designation 112, (FIG. 5) are located on the other side of the circuit board. Such an arrangement provides a means whereby the effect of ambient temperature change on the individual devices is not only slowed down, but temperature gradients therebetween are reduced, which has the added advantage that the temperature effects inherent in matched semiconductor devices are cancelled. This is particularly true for the semiconductor devices utilized in the double balanced mixer 14 which uses four matched diodes, not shown.

The output of the double balanced mixer 14 is filtered by means of the combination of a series inductor 114 and two capacitors 116 and 118. The filter output signal from the mixer 14 is fed by a signal feedthrough conductor 120 to the zero crossing detector 20 located on the circuit board 38 contained in the housing 27 and shown by the phantom line 113 of FIG. 5. This circuitry is shown in FIG. 2B.

Referring now to FIG. 2B, a first operational amplifier 122 is shown having its non-inverting (+) input terminal coupled to the inductor 114 of FIG. 2A via feedthrough 119 (FIG. 5) by means of resistor 124 while its inverting (-) input terminal is coupled to ground (zero potential) by means of resistor 126. While a common ground exists between the two circuit boards 29 and 38 due to the metal contact between the housings 26 and 27, it is shown schematically by a circuit lead 121. The operational amplifier 122 is powered by means of +V.sub.2 power supply potential and -V power supply potential appearing on the circuit busses 128 and 130 having respective filter capacitors 129 and 131 coupled therefrom to ground. The operational amplifier 122 operates as a voltage comparator referenced to ground and is thus able to operate as a zero crossing detector; however, the circuit also includes a capacitive-resistive feedback network 123 which implements a low pass filter circuit to provide a low pass filtered input. In order to provide abrupt leading and trailing edges (.about.20 nanoseconds) of the output from the operational amplifier 122 which occurs at circuit junction 130, a second operational amplifier 132 is coupled to circuit junction 130 by means of resistor 134. Resistor 134 is coupled to the non-inverting (+) input terminal of operational amplifier 132 while its inverting (-) input terminal is connected to ground by means of resistor 136. The operational amplifier 132 thus also acts as a zero crossing detector; however, there is also coupled thereto a feedback network comprised of oppositely poled parallel diodes 138 and 140 which provides a square wave at junction 142. Extremely fast operation is provided even though the input is filtered by the network 123. In order to provide further amplification, a third operational amplifier 144 is utilized having its non-inverting input terminal coupled to the junction 142 by means of resistor 146, the operational amplifier 144 additionally operates as a comparator by means of having its negative input connected to ground by means of the resistor 148. Thus all three operational amplifiers 122, 132 and 144 are configured as comparator amplifiers referenced to ground with the output amplifier 144 being coupled to a terminal 150. The resistive-capacitive networks 125, 127, 135, 137, 145 and 147 are shown coupled to the operational amplifiers to establish the required bias levels for proper operation.

By measuring the time difference between the edge crossings of the zero crossing detector 20 as evidenced by the positive and negative going edges of a square wave output which appears at the output connector 150, the phase difference between the RF inputs from sources 10 and 12 can be determined when applied to the event clock 24 shown in FIG. 1. With the circuitry shown in FIG. 2A having its active circuit components embedded in the heat sink 28 which comprises a relatively large mass in comparison to the mass of the active elements and the circuitry as shown in FIG. 2B which implements a zero crossing detector, high stability is achieved despite changing ambient temperature. A high electrical isolation exists between the input RF signals due to the fact that individual voltage regulators 70 and 90 provided on each channel act to reduce cross-talk through the power supply. Enhancement of operation also results due to the double balancing of the mixer 14 and the use of four matched diodes in the mixer 14.

One typical application for the subject invention is illustrated in FIG. 6 and involves a phase comparison system employing a dual mixer technique where it is desired to monitor the phase between two extremely stable frequency sources 152 and 154 and to alter the phase of, for example, source 152 to bring it into synchronism with source 154. The source 152 comprises a controlled oscillator which can be digitally controlled from a signal applied from a digital data buss 156 in accordance with the output of a multi-channel event clock 158 coupled to a pair of mixers 160 and 162 which have a common offset frequency applied thereto from an offset oscillator 164 which has a frequency of, for example, 4.999999 MHz while the frequency of the sources 152 and 154 is 5.0 MHz. Components 166, 168, and 170 comprise amplifier circuits corresponding to the amplifiers 16 and 18 shown in FIG. 2A while amplifiers 172 and 174 are separate amplifier configurations shown, for example, in the cross referenced application Ser. No. 284,290 (GSC 12,646-1)

Referring now to FIG. 7, in the double mixer technique, the offset oscillator 164 acts as a transfer oscillator of nominal angular frequency .omega..sub.O -.epsilon. which is used to heterodyne the output frequency .omega..sub.O from two reference sources A and B which consist of the oscillators 152 and 154. As shown, a time interval counter 158 will measure the time difference between the zero crossings of two beats from the mixers 160 and 162 wherein one beat is operable to provide a start control for the event clock 158 of FIG. 7 while the other is adapted to provide a stop signal for the clock. This time difference corresponds to the phase difference between the 5.0 MHz sources 152 and 154. This measurement in digital form outputted from the event clock 158 of the data buss 156 is adapted to track and bring the controlled oscillator 152 into phase with the source 154. As shown in FIG. 7, the system also includes an RF output control circuit 157 which comprises an RF switch which is adapted to couple a second 5.0 MHz signal from a controlled oscillator 153 to the mixer 160 in the event that another source is to be utilized for comparison or the controlled oscillator 152 for some reason or another becomes inoperative.

A typical longterm stability characteristic achieved by the subject invention is shown in FIG. 8 as it pertains to the phase comparison system shown in FIG. 6. Referring now to FIG. 8 where, for example, ambient temperature as evidenced by graphical curve 175 varies over a 5.degree. C. temperature range for a period of time (15.times.9900 sec.) which corresponds to several days, the phase stability of the comparison between the two sources 152 and 154 which is measured at the event clock 158 is shown by graphical curve 176. Phase stability on the order of one picosecond (1.times.10.sup.-12 sec.) is shown to result notwithstanding a frequency variation of the transfer oscillator 164 as evidenced by the graphical curve 177 which varies in accordance with the ambient temperature.

Thus the system as shown in FIG. 6 is adapted, for example, to intercompare a plurality of hydrogen masers, not shown, with sub-picosecond resolution. Such a system is also adapted to provide a means of tracking the average phase of multiple cesium frequency standards. Also in a remote distribution of measurement system, such an arrangement can be used to monitor an RF signal sent from a main site down a cable and return the same RF signal to the main site via a parallel cable and by monitoring the changes in phase in the parallel cable, the main site can determine a correction to bring a remote RF signal source of essentially the same frequency in phase with the RF frequency at the main site.

Having thus shown and described what is at present considered to be the preferred embodiments of the invention, it should be noted that the detailed specification set forth above has been made by way of explanation only and not limitation and accordingly all modifications, alterations and changes coming within the spirit and scope of the invention are herein meant to be included.