Telescoping Pin Probe

The following invention relates to a spring loaded, telescoping electrical contact pin probe having a high input impedance and a wide band frequency response characteristic which can be mounted on a conventional "bed of nails" test fixture.

Telescoping pin probes are commonly in use as test probes for printed circuits or other components. A telescoping pin is a telescoping rod mounted in a spring loaded sleeve and having a contact point for touching various desired locations on a printed circuit board. Often a number of telescoping pins are placed in a test fixture in an array or matrix. Such fixtures are commonly known in the industry as "bed of nails" test fixtures because of the use of a plurality of conical tipped probes held upright in the test fixture and placed relatively close together. A device under test such as a printed circuit board is placed in a chuck and lowered onto the test fixture until the probes make contact with the desired points on the circuit. The probes are connected by cables or transmission lines to a test instrument which then performs a series of diagnostic tests.

Most probes of this type are DC, uncontrolled impedance probes. The pencil-thin shape of such probes, which allows them to be placed in test fixtures of the type described above, prevents the use of active circuitry near the tip of the probe. Active probes have been available only in hand-held units which are considerably bulkier than the telescoping pin probes used in a test fixture. Therefore, the previously available test fixture probes are capable only of DC or very low frequency measurement and also have a low impedance input characteristic which may place undesirable loads on the circuit under test.

The present invention comprises a telescoping pin probe including a telescoping pin which is held within an insulating sleeve, and a circuit element, also held within the sleeve and electrically connected to the telescoping pin. The circuit element is connected to a transmission line which is connected to an auxiliary electrical circuit for providing the telescoping pin with a high input impedance and a wide band frequency response characteristic.

The telescoping pin probe of the invention may utilize a thin, elongate thick film technology circuit mounted on a substrate and having an input at one end of the substrate and an output at the opposite end. Electrical contact between the telescoping pin and the thick film technology circuit, and between the thick film technology circuit and the transmission line is provided by conductive elastomers affixed to the input and output ends of the thick film technology integrated circuit. The entire assembly may be fitted within an outer conductive sleeve which may be placed within a conventional telescoping pin test fixture.

In order to provide a ground connection, the outer sleeve may be connected to a ground plane, and the ground plane may, in turn, be connected to another telescoping pin which makes contact with a region of ground potential in the device under test.

It is a primary object of this invention to provide a telescoping pin probe having a characteristically high impedance and wide bandwidth frequency response and yet capable of being located within a conventional telescoping pin test fixture.

The foregoing and other objectives, features and advantages of the present invention will be more readily understood upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings.

FIG. 1 is a perspective view of a telescoping pin probe constructed according to the present invention mounted in a test fixture and connected to a compensation circuit box suitable for connection to an electronic test instrument.

FIG. 2 is a partially sectional side view of the telescoping pin probe situated in the test fixture shown in FIG. 1.

FIG. 3 shows a schematic circuit diagram illustrating a circuit which is used in conjunction with the probe of FIGS. 1 and 2.

Referring to FIG. 1, a test fixture 10 includes a telescoping pin probe 12 and a ground probe 14. The ground probe 14 is connected to the telescoping pin probe 12 by a ground plane 16 which is situated on the surface of the test fixture 10. A coaxial cable 18 serves as a transmission line to connect the telescoping pin probe 12 to a compensation box 20 which houses active circuitry for providing the telescoping pin probe 12 with the required electrical characteristics. The compensation box 20 is powered by a power input line 22 which may be connected to any suitable power supply. The compensation box 20 includes a BNC connector 24 for connecting the compensation box 20 to the input of an electronic test instrument.

Referring to FIG. 2, the telescoping pin probe 12 includes a telescoping pin 26 comprising a spring loaded sheath 28 and a sharpened contact point 30. The telescoping pin 26 is held inside an insulator 32. A circuit element 34 which may be an RC attenuator is also held within insulating sleeve 32 and is connected to the spring loaded sheath 28 of the telescoping pin 26 by a small wad of conductive elastomer 36. The output of the circuit element 34 is connected by another small wad of conductive elastomer 38 to the signal path 40 of a transmission line 18. The transmission line 18 also includes a coaxial ground shield 42 which is brought into electrical contact with a metal receptacle 44 held within the test fixture 10. The metal receptacle 44 is connected to a ground plane 16 which is in turn connected to ground telescoping pin 14.

The circuit element 34 may be a thick film technology circuit printed on a thin elongate piece of alumina substrate. The circuit 34 may comprise a resistor R.sub.1 and capacitor C.sub.1 connected in parallel, which combine with the impedance provided by coaxial cable 18 and the circuit elements in active compensation box 20 to give the probe its characteristically high impedance and wide bandwidth frequency response. The telescoping pin 12 has a characteristically inductive tip as shown by inductor Lp. The circuit element 34 reduces the signal level encountered at the probe tip, acting as one half of a voltage divider. The other half of the voltage divider comprises a variable capacitor C2 and resistor R2. The two aforementioned portions of the voltage divider are linked by a high impedance cable 18 which has a nominal impedance of 120 ohms. The signal conductor of the cable 18 terminates in a damping resistor R4 and in a compensating peaking resistor R3. The voltage divider output is applied to the gate of high input impedance FET Q1. Q2 provides a current source for Q1 and thermally compensates for the heating of the FETS Q1 and Q2. The current gain of the attenuated probe tip signal is provided by Q4. Thermal compensation for Q4 is provided by Q3. The output of the amplifier comprising Q1, Q2, Q3 and Q4 is connected to a back termination network comprising R12, a 50 ohm resistor, connected in parallel with R13 and C4 in one branch and C3 in another branch. R13 and C4 compensate for skin effect losses which may occur in 50 ohm transmission line 46 at high frequencies. C3 provides a high frequency bypass to sharpen the leading edges of signal pulses to compensate for the initial capacitance presented by cable 46.

Cable 46 is a standard 50 ohm cable which is connected to a test instrument 48. With the circuitry contained in compensation box 20, the test instrument 48 need not be connected directly to the compensation box 20 but may be located remotely from it and connected by a cable 46. Thus, the transmission line 18, whose impedance must be high in order for the attenuator located on circuit element 34 to function properly, is long enough to provide ample clearance for the probe 12 so that it may be located in the test fixture 10. The transmission line 22 does not, however, provide the link by which the compensation box 20 is connected to the test instrument. Rather, it is the back termination network that provides the compensation box 20 with the capability of driving the conventional 50 ohm cable 46, which may be of any arbitrary length.

The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.