Measurement probe having an internal alignment fixture

Details Measurement probe having an internal alignment fixture Measurement probe having an internal alignment fixture

1999-04-28

2001-04-17

Metjahic, Safet (Department: 2858)

Electricity: measuring and testing

Measuring, testing, or sensing electricity, per se

With probe, prod or terminals

Reexamination Certificate

active

06218826

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention related generally to measurement test probes and more particularly to a measurement probe having an internal alignment fixture.
Measurement instrument probes generally consist of a probing head having an electrically conductive tube with an insulated and coaxially disposed probing tip extending from one end and a transmission cable, such as coaxial cable, extending from the other end. Disposed within the electrically conductive tube is a substrate having electrical components mounted thereon for compensating the probe head and/or terminating the head in the characteristic impedance of the transmission cable. The electrically conductive tube and a portion of the transmission cable extending from the tube is surrounded by insulating material. The other end of the transmission cable is connected to a compensation box having electrical circuitry therein that compensates for aberrations in an acquires signal and a connector for connecting the measurement probe to a test instrument, such as an oscilloscope or the like.
Two type of electrical measurement probes using the above design are passive voltage probes and active FET probes. Passive voltage probes generally have a large value series resistor, shunted by a capacitor, in the probing head coupled via the transmission cable to the series connected resistor and shunting capacitor in the measurement instrument. The voltage divider network developed by the series connected resistors presents a high impedance to a circuit under test at low frequencies, such as 5 MHZ or lower. As the frequency of the test signal increases the impedance level of the passive probe decreases. Active FET probes includes active components, such as field effect transistors or other active devices, that results in higher input impedance without the loss of the signal, i.e. low input capacitance that is typically less than 0.4 pF to 2 pF and high input resistance values that are typically greater than 100 K ohms. Active probes also have greater bandwidth and linear dynamic range than passive probes as well as driving the transmission cable at its characteristic impedance of 50 ohms.
As probe head size decreases and the bandwidth requirement increase into the multi-gigahertz range, the accurate positioning of the probe head components become critical. Traditionally, assembly fixtures are used to hold the various elements of the probing head in position as the head is assembled. This includes holding and accurately positioning the substrate in the electrically conductive tube for soldering, glueing or the like. However, because of the decreased size of the probing head and its associated components, traditional assembly fixtures cannot be used for holding and securing the substrate in the electrically conductive tube.
What is needed is an assembly fixture that holds and accurately positions a substrate in an electrically conductive tube of a probing head of a electrical measurement instrument. The fixture should be electrically neutral so an not to add electrical aberration to the electrical characteristics of the probing head. The fixture should also be small enough to be incorporated as part of the probing head.
SUMMARY OF THE INVENTION
Accordingly, the present invention is to a probe head for a measurement probe having a sacrificial assembly fixture as part of the probe head. The probe head has probing contact at one end thereof and a transmission cable extending from the other end thereof and includes a housing having interior surfaces with the probing contact disposed in one end of the housing and the transmission cable extending from the other end. A substrate is disposed within the housing and is electrically connected to the probing contact and the transmission cable. An alignment fixture has a base with deformable ribs formed on one surface thereof and opposing sidewalls extending from the base opposite the surface with the deformable ribs. Each sidewall has an interior surface with a channel formed therein at the exposed ends of the sidewalls for engaging the substrate. The substrate and the alignment fixture are inserted into the housing with the deformable ribs disposed against a first interior surface of the housing for positioning the substrate adjacent to an opposing second interior surface of the housing and the sidewalls align the substrate between opposing interior surface opposite the first and second interior surfaces. The base and sidewalls of the alignment fixture may have cutouts formed therein for exposing components on the substrate. The probe head may also have a carrier on which the substrate and the transmission cable is secured with the alignment fixture positioning the carrier against an opposing second interior surface of the housing.
The objects, advantages and novel features of the present invention are apparent from the following detailed description when read in conjunction with appended claims and attached drawings.


REFERENCES:
patent: 5898299 (1999-04-01), Fodali

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