Broadband high impedance pickoff circuit

A measuring system minimizes the parasitic affects of lumped circuit elements. The system includes two or more in-situ interfaces configured to conductively link a source to an internal load and an external load. The in-situ interfaces are linked to a shunt conductor. Two or more linear and dynamic elements conductively link the in-situ interfaces in series. The dynamic elements are configured to overwhelm the parasitic self-capacitance of an input circuit coupled to at least one of the in-situ interfaces. A shield enclosing at least one of the linear and dynamic elements has a conductive surface to fields and electromagnetic interference. The shield has attenuation ratios that substantially dampen the parasitic capacitance between the linear and dynamic elements that bridge some of the in-situ interfaces.

BACKGROUND OF THE INVENTION

1. Technical Field

This application relates to a closed circuit, and more particularly to a broad band multi-terminal network that mitigates parasitic elements.

2. Related Art

Electromagnetic signals within a wideband may be distorted by reactive elements. The inductance and capacitance within a circuit may distort the frequency and phase response of high frequency inputs or communication signals. Parasitic elements may affect inputs and outputs by reducing bandwidth or by creating unwanted oscillations and ringing.

Unwanted inductance and capacitance may occur through links that connect electronic components or by their proximity to each other. Measurement systems that display voltage across or current passing through these components may add interference. Such circuits may require open access to measure inputs or outputs and distort waveforms or signals through their parasitic capacitance and low direct current resistance.

SUMMARY

A measuring system minimizes the parasitic effects of lumped circuit elements. The system includes two or more in-situ interfaces configured to conductively link a source to an internal load and an external load. The in-situ interfaces are linked to a shunt conductor. Two or more linear and dynamic elements conductively link the in-situ interfaces in series. The dynamic elements are configured to overwhelm the parasitic self-capacitance of an input circuit coupled to at least one of the plurality of in-situ interfaces. A shield enclosing at least one of the linear and dynamic elements has a conductive surface to fields and electromagnetic interference. The shield has attenuation ratios that substantially dampen the parasitic capacitance between the linear and dynamic elements that bridge some of the in-situ interfaces.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In-situ measuring or pickoff devices may interface open or closed networks (or circuits). The systems measure output with little magnitude or phase distortion with respect to frequency. Some devices have large dynamic ranges (e.g., single volts, kilo-volts, mega-volts, etc.) that interface one or more circuits. Some measuring devices do not require separate power sources and are not susceptible to electro-static discharge damage or over-voltage conditions. Due to shielded enclosures, high density layouts, and/or high output ranges, there may be little need for large amounts of insulation or warnings against shock when operating these systems. The pickoff devices may provide low parasitic capacitance (e.g., Cp≅0) and high linear input impedances. The devices may achieve improved high frequency responses and lower loading than alternative passive and/or active measuring devices.

The in-situ measuring or pickoff device may include two or more interfaces. InFIG. 1the device includes an input102, an output104, and a measuring tap106. The helical or spiral ridges that may be a unitary or integrated part of an insulating enclosure100may join or fasten a conductor or an input circuit (or an enclosure containing such elements) to an output conductor or load (not shown) and to a measuring tap106. A ground108lying within a multi-sided plane (e.g., shown as a four sided plane in cross-section) may provide a conducting path to a ground potential or a conducting body serving as a ground potential (e.g., such as a safety device). Conductivity paths110may facilitate power or signal flow between the input102, output104, and measuring tap106. A shield128insulates a portion of the pickoff device. The shield128may reduce electromagnetic interference by redirecting field lines to its conductive structure (or outer surface) that may have high attenuation characteristics. In some systems one or more foil or sheets may absorb and attenuate fields by terminating the fields at a lower potential such as ground. Combinations of shielding (e.g., foils, sheets, screens, etc.) that may (or may not) comprise two, three, or more materials or alloys protect a portion of the pickoff circuit from electromagnetic interference. In these systems, combinations of materials may be used when high magnetic saturations are approached (or are expected).

Parallel combinations of linear and dynamic elements112-116are conductively linked through conductivity paths110. Two, three (shown as112-116inFIGS. 1 and 2), or more linear and reactive components may link the input102to the measuring tap106. By their proximity to one another, a parasitic capacitance may provide a low impedance path between input102and a node connected to a terminating linear element120(shown as a resistor)120. The voltage between the parasitic capacitance202(shown inFIG. 2) and ground may shunt the voltage across a linear and dynamic element118. That voltage may be a fraction of the total input voltage that reactance118is of the total reactance (e.g., linear and dynamic elements112-116and may include the optional reactive elements (capacitors)122-126). Optional reactive elements (e.g., capacitors)122-126may comprise mechanical, electronic, or tunable elements such as varactors (e.g., semiconductor in which the capacitance is sensitive to an applied voltage or current like tunable element) that may be manually or automatically adjusted to the linear and dynamic elements112-116or in some alternate systems to the input frequency or complex impedance received/measured at the input102. In some systems, the optional reactive elements may be tuned through a comparison of outputs across an attenuator load and the pickoff device in response to a pulsed source with a fast or substantially fast rise time.

FIG. 2is a schematic of an exemplary pickoff circuit that may be received by the exemplary insulating enclosures100ofFIGS. 1 and 12. Parallel combinations of resistors and capacitors112-118may render a high impendence through predetermined element values and rated input frequency ranges. The complex quantities may be selected so that the output of the measuring tap106is finger-safe (e.g., reduces shock hazards, arcing, undesired events or accidents). Parasitic capacitance Cpmay be minimized or substantially eliminated through the shielding of terminating linear element120, linear and dynamic element118, and optional reactive elements (e.g., capacitors)122-126.

Voltage division may express the voltage across linear and dynamic element118.

V4=V3⁢R4||C4R4⁢C4+R3⁢C3=V3⁢R41+ⅈω⁢⁢R4⁢C4R41+ⅈω⁢⁢R4⁢C4+R31+ⅈω⁢⁢R3⁢C3|=R4⁡(1+ⅈω⁢⁢R4⁢C4)R4⁡(1+ⅈω⁢⁢R3⁢C3)+R3⁡(1+ⅈω⁢⁢R4⁢C4)
When linear and dynamic element116is substantially equal to linear and dynamic element118, the voltage at V4may be a fraction of the total voltage of V3.

R4⁢C4=R3⁢C3⇔V4=V3⁢R4R4+R3
When the linear and dynamic elements112-118are substantially matched (e.g., R4,C4≅R3,C3≅R2,C2≅R1,C1the voltage at V4may be a fraction of the total input voltage of Va.

V4=Va⁢R4R4+R3+R2+R1
To ensure element matching or a finger-safe output, some exemplary systems may cascade large highly rated precision resistors (e.g., about 50 kΩ). To minimize some parasitic effects, the systems may precisely match the resistors and may couple other reactive elements (e.g., capacitive elements) to overwhelm the parasitic self-capacitance of some input circuits, measuring devices, or sources. In systems that are not well matched, or do not include additional reactive elements, or are not shielded (as described), the parasitic capacitance of each linear component may be estimated to be about 0.5 pF. At this level, the time constant of the parallel combinations of resistors and capacitors may be about 25 nano-seconds or have a cutoff frequency of about 40 MHz. When components are not well matched or do not include the exemplary shielding, small matching errors may affect the output bandwidth greatly.

FIG. 3is a magnitude comparison (in dB) of outputs from input102to measuring tap106with parasitic capacitance (shown as graph302) and without parasitic capacitance (shown as graph304).FIG. 3shows how the magnitude of an output may distort the output from Hnto H0. Phase is also affected.FIG. 4is a phase comparison of outputs with parasitic capacitance (graph402) and without parasitic capacitance (graph404).

FIG. 5is an exemplary board layout of an exemplary pickoff circuit. InFIG. 5, the exemplary circuit board may comprise two (or more) single layer or multilayered about 0.125″ circuit boards. A distal board (or layer) may comprise or include a ground502. A proximal board may provide electrical connections between components and connections to other layers. In some layouts a main power trace may have impedance matched to about 50 Ohms. Some linear elements (or resistors R1, R2, and R3) may be rated at about 50 k Ohm, about 3 kV, about 1%, and about 1 W by Ohmite and may couple the L shaped traces504-508shown inFIG. 5. Dynamic elements (or C1, C2, and C3) may be positioned adjacent to R1-R3, respectively and may be rated at about 12 pF, about 3.15 KV, and about 5% by Murata. A 7/16 connectors or input102may comprise a 7/16 female; 4 hole panel mount; solder cup contact by Pasternack. The measuring tap106may comprise an 82-97 square flange female N-connector by Amphenol. Other linear elements may be rated at (R4) about 2 k Ohm, (R5) may comprise about a 475 Ohm resistor. Other dynamic elements (or C5-C7) may be rated at about 25V.

To minimize or substantially eliminate undesired interference, selected combinations of linear and dynamic elements may be enclosed by the shield128. While this initial attenuation ratio of the shield128may be substantially uniform, in alternate pickoff circuits the attenuation ratio of the shield128varies. In some systems shield variance may be based on position to or characteristics of a source (e.g., input102amplitudes or input frequencies) or an output (amplitudes and/or frequencies occurring at the measuring tap106or load104) or may be based on other characteristics. The thickness, width, and/or shape of the shielding structure may be uniform, vary with the topography of the shielded pickoff elements that comprise the device, or vary with the insulating enclosure100(or may comprise enclosure100). In some systems, linear and dynamic elements R4, C4, R5, C5, C6, C7, and measuring tap106are substantially or fully enclosed within the shield128.

In the exemplary board layout ofFIG. 5the traces between R3/C3and R4and the trace between R5and measuring tap106may be discontinuous. A ground plane may be positioned on top of one or more micro-strips. Layers of a polyimide film that may remain stable across a wide temperature range (e.g., −273° C. to +400° C. such as Kapton manufactured by Dupont®) may be positioned above the ground plane. Traces positioned on the polyimide film may comprise the conducting or communication links between the elements. A second polyimide layer may be positioned above R4, R5, and C5-C7and their respective connections. In some pickoff devices, an ancillary shield of uniform or varying attenuation ratios may be positioned above (or within alternative pickoff devices below/above and below) the polyimide films.

FIGS. 6 and 7show partially assembled and assembled exemplary pickoff devices. The pickoff devices are configured to be placed or integrated in circuit (or load) to collect data or signals at the actual location of the circuit (as opposed to a remote sensing device or ancillary probes). The devices do not require open access (e.g., open circuits or unrestricted component or measuring point access) to monitor and measure signals that some measuring devices require. The pickoff device allows the measurement of current and voltage values too large to be directly measured by other devices. A shunt conductor is placed in series between the source and external load to ensure nearly all of the current or voltage to be measured will flow through the conductor without disturbing or significantly affecting the signal flowing from the input102to the output104. The linear and dynamic elements112-116may draw safe signal levels with minimal signal or power losses. In some alternate pickoff devices, safety devices may protect ancillary circuits coupled to the measuring tap106or output104from excessive current or voltages. The alternate pickoff devices may include elements that melt or open when current passing to the measuring tap106exceeds a predetermined level. Other alternate systems include elements that prevent current or voltage surges and/or reduce or substantially eliminate short-lived unpredictable increases in power from reaching the measuring tap106.

FIG. 8is an exemplary output of magnitude and phase of the exemplary circuit ofFIG. 1through a vector network analyzer. The magnitude shown in the upper graph as802has a low pass characteristic that starts at about 125 kHz (1), passes through about 106 MHz (2) before stopping at about 180 MHz (3). In the upper graph the 3 dB relative difference in power occurs at about 106 MHz. When bandwidth (BW) is defined by the algorithm following this paragraph, the bandwidth of the exemplary circuit is about 160 MHz. The lower graph shows a substantially flat phase804with small or negligible dispersion out to about 180 MHz. In comparison to a circuit without the described shielding, the phase was flat to about 20 MHz.

A second measuring device of an exemplary pickoff device includes a pulse circuit (e.g., a Time Domain Reflectometer or TDR), a test load904, and an oscilloscope906shown inFIG. 9. The output (shown inFIG. 10) traces the output of the scope probes1002, an exemplary pickoff device1004, and the attenuator load1006on a screen rendered by a display. A comparison of the output shows the ripple and noise content that may be associated with charge and discharge events of the input and output capacitance. The ringing or ripple may be highly dependent on the circuit topology, circuit parasitics, and circuit board layout. As shown by the output1002, the oscillations or noise spikes may be highly dependant on the capacitance of the probes and/or circuit layout. In some systems the large ground loop formed by the probes may magnify the interference. In these systems even a short ground wire length may result in significant noise spikes. Like the output measured at the attenuator load1006, the output of the exemplary pickoff device1004shows little noise pickup.

FIG. 11is a method of assembling an exemplary pickoff circuit that may monitor an output with little or no noise pick up. Initially, linear and dynamic elements are selected at1102. In some assemblies resistors and capacitors are selected within a tolerance level (e.g., capacitors may be selected within about a 0.5% tolerance). At1104, parallel combinations of linear and dynamic elements are connected in series. The parallel combinations of linear and dynamic elements are conductively linked through conductivity paths to prevent current, voltage or power levels from reaching a designated output. At1106a low inductance path to ground or shield is positioned or wrapped around the entire or selected circuit elements (e.g.,118,120-136). The shield reduces electromagnetic interference by redirecting field lines to its conductive structure (or outer surface) that may have high attenuation characteristics. In some methods one or more foil or sheets may enclose selected circuit elements to absorb and attenuate fields by terminating the fields at a lower potential such as ground. Combinations of shielding (e.g., foils, sheets, screens, etc.) that may (or may not) comprise two, three, or more materials or alloys protect a portion of the pickoff circuit from electromagnetic interference. The circuit layout and shielding may be coupled to or linked to an insulating or conducting housing like the enclosure100shown inFIG. 12. Using a pulse source, such as a TDR, the output of the exemplary assembly may be automatically or mechanically tuned to an input source/circuit and/or output source/circuit. In some assemblies the devices are tuned by applying secondary source (e.g., a voltage or current apart from an input source to a tunable reactance) or by mechanically inserting a selected reactance.

The methods and systems described above may interface or be integrated within high power circuits and devices such as a Medium Energy Beam Transport (MEBT) like a MEBT power pulser. The MEBT may have a normal pulse at about 2.5 kV (or above, e.g., 100 kV/or below) at a 7% (or greater/lesser) duty cycle into about a matched 50 Ohm load. Some exemplary pickoff circuits have a demonstrated bandwidth of about 160 MHz and a low phase dispersion that extends to about 180 MHz. Some microstrip pickoff circuits may covey micro-wave-frequency signals that are shielded from unintentional radiation. The pickoff circuits are finger safe, mitigate the parasitic behavior of elements, and are not dependant on or substantially affected by the length of cable with which it is desired to monitor the signal levels.