This invention relates to circuits for use in the detection and location of a fault or faults in a device under test (DUT).
One known such circuit is shown schematically in FIG. 1 of the accompanying drawings, and is typically used in the detection and location of a fault or faults in a transmission line, or a component constructed using transmission line, required to operate satisfactorily over a range of microwave frequencies.
Referring to FIG. 1, the known circuit comprises first, second and third ports 1, 3, 5, a power dividing network 7, and first and second sub-circuits 9, 11. Sub-circuit 9 comprises a power divider 17, a fault detection section 14, and a reference impedance 13. Divider 17 comprises a delta-shaped impedance network comprising three impedances 19, 21, 23 of equal value equal to the characteristic impedance to which fault detection measurements are referenced, i.e. the impedance of reference impedance 13, see below. Fault detection section 14 comprises a diode 15 and capacitors 16, 18. Sub-circuit 11 comprises a power divider 31, an impedance network 25, and a fault location section 24. Divider 31 comprises a star-shaped impedance network comprising three impedances 33, 35, 37 of equal value equal to one third of the characteristic impedance to which fault detection measurements are referenced, i.e. the impedance of reference impedance 13, see below. Fault location section 24 comprises a diode 29 and capacitor 27. The aforespecified circuit components are connected as shown in FIG. 1.
In use of the circuit in the detection of a fault or faults in a DUT 39, the DUT 39 is connected to port 1, and to port 5 is connected a signal source 41 which generates a plurality of signals of different frequencies which extend over the aforementioned range of microwave frequencies.
The source 41 generates a signal of a first frequency of the plurality of frequencies, which signal is divided by network 7 between sub-circuits 9 and 11. The signal from network 7 which passes to sub-circuit 9 is divided by divider 17 such that a first fraction thereof passes to DUT 39, and a second fraction thereof passes to reference impedance 13.
If DUT 39 is not faulty a predetermined relationship, dependent on the frequency generated by source 41, will exist between the impedance of reference impedance 13 and the impedance presented by DUT 39. Whether or not this predetermined relationship exists will be indicated by the voltage across diode 15 of fault detection section 14, as measured at 43. The voltage at 43 is a measure of the return loss or VSWR (voltage standing wave ratio) of DUT 39 at the frequency of source 41.
The voltage at 43 is measured for each of the remaining frequencies,of the plurality of frequencies of source 41. The return loss/VSWR characteristic comprising the voltage measurements made at 43 plotted against frequency is studied to determine whether the DUT 39 is operating satisfactorily, if not a faulty DUT 39 is indicated.
In use of the circuit in the location of a fault or faults, detected as described above, in DUT 39, DUT 39 is removed from port 1 and connected to port 3. The source 41 generates a signal of a first frequency of the plurality of frequencies, which signal is divided by network 7 between sub-circuits 9 and 11. The signal from network 7 which passes to sub-circuit 11 is divided equally by divider 31 between DUT 39 and the branch of sub-circuit 11 comprising network 25 and fault location section 24.
A fault or faults in DUT 39 will partially reflect the signal incident on DUT 39. This reflection will add to the signal received from divider 31 by the branch of sub-circuit 11 comprising network 25 and fault location section 24. Consequently, the voltage across capacitor 27 of fault location section 24 will be altered from what it would otherwise have been at the frequency of source 41. The voltage across capacitor 27 is measured at 45.
The voltage at 45 is measured for each of the remaining frequencies of the plurality of frequencies of source 41. A plot of the voltages measured at 45 versus frequency comprises a ripple pattern which contains information as to the location of the fault or faults in DUT 39. This ripple pattern is processed by a scalar analyser (not shown) which extracts this information and determines the location(s) of the fault(s).
In sub-circuit 11 network 25 matches fault location section 24 to the impedance presented thereto by divider 31 and attenuates the signals received by the branch containing network 25 and section 24 such that diode 29 operates within the linear region of its operating characteristic.