GB2149256A - Phase measurement - Google Patents

Abstract

The system includes a transmitter 11, a receiver 30, and a coupling network 14 which connects the transmitter 11 and receiver 30 to the antenna 20. The network includes a switch 34 which selectively enables the receiver 30 to receive signals either from the transmitter 11 directly or from the antenna 20. A phase measurement device 32 determines the relative phases of the signals received by the receiver 30. Another switch 38 is provided to ground the connection 12 between the transmitter 11 and the network 14 when the receiver 30 receives a signal from the antenna 20, thereby removing the transmitter 11 impedance and making the phase shift of the connections from the transmitter 11 to the antenna 20 substantially equal to the phase shift from the antenna 20 to the receiver 30. A detection loop 52 close to the antenna 20 is connected via the coupling network 14 to the receiver 30 to allow the phase delay through the antenna coupling hardware to be calculated. The difference between the phase of a transmitted signal measured at the transmitter 11 and one measured at the detection loop 52 near the antenna 20 represents the total phase delay of the antenna hardware. This delay can be removed from the calculated phase difference to reflect the true phase delay at the antenna 20. <IMAGE>

Description

SPECIFICATION Improved phase measurement system and method BACKGROUND OF THE INVENTION The present system generally pertains to systems which determine a quantity of interest by measuring the phase difference between a transmitted signal and a signal received in response to the transmitted signal. Particularly, the invention is directed to an improvement in the phase measurement equipment of such systems.

Systems which employ the measurement of phase differences to determine quantities such as distance, angle, and altitude are wellknown in the art. In such systems, a timevarying signal is transmitted from a first point to a transponder located at another point to which the distance, angle, or altitude is to be determined. The transponder responds to the transmitted signal by transmitting another signal which is received at the first point. Typically, an apparatus at the first point is provided to measure the phase difference between the transmitted and the responsive signals, and to calculate the desired value based upon the measured phase difference.

Examples of these systems are disclosed in U.S. Patent No. 4,011,562; U.S. Patent No.

4,229,737; and U.S. Patent No. 3,427,615.

In order to obtain an accurate measure of the desired quantity, it is necessary to measure no more than the phase shift resulting from two-day transmission between the points. It is evident that phase delays which are artifacts of the measuring and transponding apparatus must be eliminated from the phase difference calculation. Means for eliminating internal phase delays of measuring systems are found in U.S. Patent No.

4,011,562; U.S. Patent No. 4,041,490; and U.S. Patent No. 3,427,615.

While U.S. Patent No. 4,011,562 does teach an internal measurement system phase correction, the system's operation is dependent upon the proper functioning of an electro-mechanical switch. The switch alternately connects the input of the receiver to an antenna for detecting the phase of a received signal and, through an attenuator, to the output of a transmitter which supplies the antenna with the signal to be transmitted.

Internal circuitry is provided which permits an operator to calibrate the apparatus by eliminating the internal phase delay of the circuitry so that the phase difference can be accurately calculated. However, as is known in the art, eiectro-mechanical switches generate noise which can interfere with the operation of circuits in which they are used. Moreover, they exhibit significant and variable switching times which are not accounted for the internal phase adjustment of the taught systems and which decrease the system's accuracy.

In the system of U.S. Patent No.

4,041,490, an RF diode switch is used to alternately connect a receiver to a receiving antenna and to the attenuated output of a transmitter which is connected to a second antenna. While connected to the transmitter, the receiver enables the measurement system to calibrate its phase difference measurements by accounting for the internal phase shift of the measurement system. As the disclosed apparatus utilizes a high-speed diode switch, the effect of a switching time constant is substantially reduced in the measurement of the internal phase shift. However, the apparatus requires the provision of two antennas which necessarily increases its cost when compared with a system utilizing a single antenna.Furthermore, as will be understood by those skilled in the art, it is unlikely that the complex impedances exhibited by the pair of antennas are equal, with the result that the phase delay measured through the receiver during the calibration of the apparatus may not match the phase delay through the receiver when it is connected to the receiving antenna. In this case, the factor provided to correct for the internal phase delay of the measuring circuit may not guarantee an accurate calculation of phase difference.

In U.S. Patent No. 3,427,615, a phase measuring system is calibrated to account for internal phase delay in a manner similar to the first two cited patents. That is, the input of a receiver is alternately switched between an antenna during a normal operation mode and the output of a transmitter during a calibration mode. During the calibration mode, the receiver provides the signal transmitted by the measuring system to the phase detection circuitry, while the received signal is provided during the normal operating mode. However, the switching is done by relatively slow-acting servo motors and electro-mechanical switches which, as noted above, can affect the accuracy of the phase measurement.

A further limiting factor of each and all of the above-cited reference patents is that none accounts for the phase delay introduced by the antenna coupling system. As is known in the art, it is customary to provide an antenna loading unit (ALU) at the signal port of an antenna in order to match the impedance of the antenna system with its associated transmitter or receiver. This is an especially important provision in cases where the antenna is connected to an associated active device through a long cable, and where varying local environmental conditions cause the characteristic impedance of the antenna to change with time. It is evident that the cumulative phase delay introduced by the antenna, tuning of the ALU, and coupling cable can contribute a significant error in the measurement of signal phase differences if unac counted for.

The inventors have attempted to calibrate a phase measuring system by providing an antenna circuit phase offset measurement circuit which is described in a product bulletin entitled "ARGO Phase Stabilization System," dated June 1980, and available from Cubic Western Data, San Diego, California. The offset measurement circuit comprises an electromagnetic detection device located near an antenna being fed by a transmitter. A receiver is switched between the transmitter output and the detection device. The difference between the phase of a transmitted signal measured at the transmitter output and the phase of the transmitted signal measured at the antenna by the device provides a measure of the antenna circuit phase shift.However, the transfer impedance of circuitry used to couple the receiver, transmitter, antenna, and detector together is not symmetrical, resulting in a significant difference in phase delay between signals transferred in toward the antenna and signals received from it. Without any means to correct for this lack of symmetry, calculations based upon measurements made by this system are frequently in error.

Hence, there exists a need for an improved phase measuring system which is accurate, effective, economical and which accounts for all of the delays introduced into the phase measurement by the measuring system.

SUMMARY OF THE INVENTION The present invention provides an improved system and method for measuring the phase difference between time-varying signals transmitted and received by the system. The system includes an antenna for transmitting and detecting time-varying signals, a transmitter which provides time-varying signals to be transmitted by the antenna, and a receiver for receiving time-varying signals from either the transmitter or the antenna. Associated with the receiver is a processor system for measuring the phase of signals received by the receiver and for calculating the phase difference between transmitted and received signals. A shielded loop is placed proximate to the antenna so that current flowing in the antenna induces a corresponding voltage in the loop.

The antenna, transmitter, and receiver of the system of the invention are interconnected by a coupling network which connects the transmitter to the antenna and which selectively enables the receiver to receive signals from the antenna or from the transmitter or from the loop. A switch is provided between the transmitter and the coupling network to ground the connection therebetween while the receiver is enabled to receive signals from the antenna.

The antenna network phase delay is calculated by measuring the phase of a timevarying signal provided by the transmitter to the coupling network and then measuring via the shielded loop the phase of a time-varying signal while-it is being transmitted by the antenna. The difference in the two measurements constitutes the phase delay of the antenna network.

Provision of a grounding switch between the transmitter and the coupling network decouples the transmitter impedance from that of the network while the receiver is connected to the antenna. This ensures that the phase shift of the network from the transmitter to the antenna is substantially equal to the phase shift from the antenna to the receiver. Therefore, measuring the phase shift of the network in one direction will permit the phase shift in the reverse direction to be inferred by the application of the reciprocity theorem of linear networks.

The system and method of the invention thus provide for an accurate measurement of phase difference between two time varying signals by accurately measuring the internal phase delay of the total measuring system which can be accounted for in the determination of phase delay between the transmitted and received signals.

It is therefore an object of the invention to provide a new and improved system for measuring the phase difference between a transmitted and a received time-varying sig nal.

It is another object of the invention to provide a new and improved phase measuring system that is accurately calibrated to account for all phase delays introduced by the measur ing system.

Other objects and many advantages of this invention will become more apparent upon a reading of the following detailed description of the invention and examination of the draw ings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic representation of the system of the invention.

Figure 2 is a schematic representation of a directional coupler used in the system of the invention.

DESCRIPTION OF THE PREFERRED EMBODI MENT The phase measurement system of the invention, indicated generally by the numeral 10 in Fig. 1, comprises a transmitter 11 which provides a continuous time-varying car rier signal on a signal path 1 2 to a coupling network 14 through which the signal is con ducted, as explained in greater detail herein below, to a signal path 1 5 and then to an antenna coupling cable 16. The antenna cou pling cable 1 6 conducts the signal to an antenna loading Unit (ALU) 1 8 which couples the signal to an antenna 20 wherefrom it is transmitted. The transmitted signal is then detected at a second antenna 26 which is connected to a transponder 28.The transponder 28 responds to the signal transmitted from the antenna 20 by causing a signal to be transmitted from the antenna 26, which response signal is detected on the antenna 20.

The detected signal is conducted from the antenna 20, through the ALU 1 8 and the antenna cable 1 6 to the coupling network 1 4.

The coupling network 1 4 is configured to selectively connect the detected signal to a receiver 30. The receiver 30 amplifies and processes the detected signal and provides it to a processor system 32. The processor system 32 comprises circuitry and an appropriately programmed processor which give it the capability of measuring signal phases, processing the measurements, and performing calculations relating to phase differences.

As described to this point, the system illustrated in Fig. 1 is useful in radio ranging systems such as are disclosed in U.S. Patent No. 4,011,562 and U.S. Patent No.

4,229,737, both of which are incorporated herein by reference. Another example of a radio ranging system is one produced by Cubic Western Data, San Diego, California, under the trademark ARGO.

Further, the processor system 32 can correspond to the processor disclosed in U.S. Patent No. 4,041,490, which is incorporated herein by reference. As taught in that patent, the disclosed processor is capable of measuring signal phases and performing relevant phase calculations and conversions.

The novelty of the system 10 can be understood with further reference to Fig. 1 wherein the coupling network 1 4 comprises a directional coupler 34 connected between signal paths 1 2 and 1 5. A transmitted RF signal from the transmitter 10 is conducted on the signal lead 1 2 and enters the directional coupler 34 through port 34a and is conducted therethrough directly to port 34b and therefrom over signal path 1 5 and to the antenna 20 for transmission. A received RF signal power is conducted from the antenna 20 to the coupler 34 on signal line 1 5 and enters the coupler 34 through port 34b wherefrom it is conducted to port 34a.In addition, a 20dB coupling path exists between port 34b and 34c so that a portion of the detected signal is provided from port 34c.

A signal path 35 is directly connected to the signal path 1 2 to conduct the RF signal provided by the transmitter 11 through an attenuator network 36 which is connected to terminal 37a of a solid state switch 37. Port 34c of the directional coupler 34 is connected to terminal 37b of the solid state switch 37 over signal path 42.

A solid state grounding switch 38 is connected to the signal path 1 2 which conducts the output of the transmitter 10 to the coupling network 14. The settings of the switches 37 and 38 are determined by control signals provided by the processor system 32 over a control data bus 38 which connects to the control port C of the processor system 32.

The provision and timing of signals which are appropriate to control the switch 37 and 38 will be well understood by those versed in the art of processor and switching system design.

Such signals can be appropriately implemented through the provision of a switching routine in the software of the processor 32. A processor which is operative to control a switch in a predetermined sequence is taught, for example, in U.S. Patent No. 4,041,490.

In operation, the range between the system 10 and the transponder 28 can be calculated by measuring the phase difference between the time-varying signal transmitted by the antenna 20 and a signal transmitted in response thereto by the transponder 28 and detected by the antenna 20. The patents incorporated hereinabove by reference provide ample explanation of phase measurement and processing for this purpose. Generally, the ranging schemes disclosed in those patents are based upon the relationship expressed in the following equation: ; AT = A (1) Where O/ is the phase of a signal received at the antenna 20, AT is the phase of a signal transmitted from the antenna 20 and 6111 is the signal phase difference which is converted to, for example, distance.Past ranging schemes have approximated A by use of the following expression: R T = I\ (2) Where R is the phase of the detected signal as it is received at the receiver 30 and T is the phase of the transmitted signal measured at the output of the transmitter 11.

Measurement of R and T' will provide a reasonably accurate solution of equation (2) only if a negligible phase shift between the antenna 20 and the coupling network 1 4 is assumed. However, if the antenna 20 is connected as illustrated in Fig. 1 and described hereinabove, the coupling cable 1 6 and tuning of the ALU 1 8 can impose a significant and variable phase shift, represented by A' which is not accounted for in equation (2).

Instead, the terms R and T of equation (2) actually represent the values expressed below by equations (3) and (4): OR = AR + A (3) T = AT A (4) and the relationship expressed by equation (2) is more accurately given by the following equation: 0 = 0, = R - 2A (5) Thus, the expression of equation (2) is in error by 2 A- In order to correct for the error inherent in equation (2), a Faraday shield loop 52 is provided immediately adjacent the antenna 20 and is connected by way of signal path 53 to terminal 37c of the switch 37. This permits the receiver 30 to detect, at the antenna 20, a portion of a transmitted signal which is provided to the antenna by the transmitter 11.Measurement of the phase of this signal by the processor system 32 will permit the processor to calculate A according to the following equation: A TL T (6) where TL is the phase of a transmitted signal detected by the Faraday loop 52.

In order to obtain the values necessary for the solution of equation (5), the processor system is appropriately programmed to implement the following measurement method. In the first step of the method, the processor system 32 provides control signals to set the switch 37 to terminal 37a, reset switch 38 to an open position, and key on the transmitter 11. The signal from the transmitter is provided on signal path 1 2 and conducted through the directional coupler 34 to the antenna 20 along the path described hereinabove. At the same time, the receiver 30 receives a sample of the transmitted signal through terminal 37a of the switch 37. The signal is provided to the processor system 32 which detects its phase and holds the value as T for calculation.

In the second step, at the end of the transmission period, the transmitter 11 is keyed off, the switch 37 is set to terminal 37b, and the switch 38 is set to ground signal lead 1 2 as illustrated. After a predetermined period of time, the transponder 28 sends its response signal which is detected by the antenna 20 and conducted to the directional coupler 34. A portion of the detected signal is conducted from coupler terminal 34c to terminal 37b of the switch 37 where it is detected by the receiver 30. The phase of the received signal is measured by the processor system 32 and used as the value .

Finally, switch 37 is set to terminal 37c, switch 38 is opened to remove the ground at port 34a of the coupler 34, and transmitter 11 is keyed on and the value 0,, is measured and stored. Solutions of equations (6) and (5) can now be obtained, and the result of equation (5) can be converted to any desired value of range, altimetry, or angle.

The method described above can be understood with reference to Figs. 1 and 2. When the transmitter 11 is keyed on, its output signal can be modeled as a voltage, VT, which is present in the signal lead 12, and which induces a current in the antenna 20 through a network structure including the coupling network 14, cable 16, and ALU 1 8. The current in the antenna 20 causes it to radiate the transmitted signal. A portion of Vr is received by the receiver 30 through the attenuator 36 and the phase of the received signal is treated as T Measurement of TL through the shielded loop 52 provides the second value necessary to calculate A in equation (6).

Although the phase delay in the reverse direction, that is, from the antenna 20 through the ALU 1 8, cable 16, and coupling network 14, is not directly measured, it is inferred from the measurement of the phase of a current, l5, which is caused to flow in the signal lead 1 2 by voltage induced in the antenna 20 when it is irradiated by a response signal from the transponder. The current Is is detected by the receiver 30 through a current transformer 45, and its phase is measured by the processor 32. The inference of the phase shift in the reverse direction is based upon the assumption that the network structure linking the antenna 20 and the signal lead 12 is reciprocal.If the network is reciprocal, then the phase of 15 can be treated as R and corrected by A measured in the forward direction when equation (5) is solved.

The network structure which is necessary for reciprocity to apply is achieved through the provision of the grounding switch 38 which removes the impedance of the transmitter from the network structure during reception of a response signal and permits Is to flow into ground. Hence, the current Is due to the antenna voltage experiences the same phase shift A as the current in the antenna induced by the voltage VT.

In the preferred embodiment, the directional coupler operating as a current transformer imposes 20dB of attenuation from port 34b to port 34c. However, this does not degrade the performance of the system of the invention because the atmospheric and manmade noise in the frequency band of interest are greater than 45dB above KTB, making even a 40dB + noise figure acceptable.

Obviously, many modifications and variations of the invention are possible in light of the above teachings, and it is therefore understood that the invention may be practiced otherwise than specifically described.

Claims (8)

1. A system for measuring phase differences between signals transmitted and detected by an antenna apparatus, comprising: transmitter means for providing a signal to be transmitted by said antenna apparatus; receiver means for receiving a signal; processor means for measuring the differ ence in phase between different signals received by said receiver; coupling network means connected to said antenna apparatus, said transmitter means, and said receiver means for selectively enabling said receiver means to receive a signal from said antenna apparatus or from said transmitter means; and switch means for selectively placing a ground potential between said transmitter means and said network means.

2. The system of claim 1 wherein said processor means is connected to said network means and to said switch means and includes means for causing said network means to selectively enable said receiver means and for causing said switch means to place said ground potential between said transmitter means and said network means when said receiver means is receiving a signal from said antenna apparatus.

3. The system of claim 1 further including pickup means proximate said antenna apparatus for detecting signals transmitted by said antenna apparatus, wherein said network means is further connected to said pickup means for selectably enabling said receiver means to receive a signal from said pickup means.

4. The system of claim 3 wherein said processor means further includes means for automatically operating said network and said switch means according to a predetermined sequence.

5. A method for measuring the phase difference between signals transmitted and received by a measurement system which includes an antenna apparatus for transmitting and detecting said signals, a transmitter for providing a signal to be transmitted by said antenna, a receiver for receiving a signal, a means for measuring the phase of signals received by said receiver means, and a network for interconnecting said antenna apparatus, transmitter, and receiver, comprising the steps of: a.) providing a first signal from said transmitter to said antenna through said network; b.) measuring the phase of said first signal at a connection between said transmitter means and said network; and c.) providing a ground potential on said connection between said transmitter means and said network while measuring the phase of a second signal provided from said antenna to said receiver through said network.

6. The method of claim 5 further including the steps of: d.) providing a third signal from said transmitter to said antenna through said network; and e.) measuring the phase of said third signal at said antenna apparatus.

7. A phase measurement system constructed and arranged substantially as herein described and shown in the drawings.

8. A method of measuring phase substantially as herein described with reference to the drawings.