1. Field of the Invention
The invention in general relates to electrical test probes, and more particularly to an improved probe tip structure which reduces the input capacitance of the probe.
2. Statement of the Problem
Electrical test probes are often used to connect electrical circuits to test instruments, such as oscilloscopes or voltmeters, while minimizing the loading of the circuit under test. Two desirable electrical characteristics of an electrical test probe are; first, the probe should not influence or "load" the response of the circuit under test; and secondly, that the signal response at the test instrument should be an accurate, though possibly attenuated, representation of the probed signal over the range of frequencies of interest; that is, the signal on the output of the test probe follows the signal at its input. Generally, the first characteristic is accomplished by making the impedance at the probe input as high as possible to prevent the test probe and instrument to which it is connected from drawing significant current or otherwise significantly altering the electrical parameters on the node to be tested. The impedance at the output is generally a value that meets the impedance needs of typical test instruments. Thus test probes may be thought of as essentially impedance buffers. Some probes, generally called divider probes, may also attenuate the voltage of the circuit under test and decrease the capacitance and increase the resistance presented to the circuit by an attenuation factor. For example, a probe with an attenuation factor of 10 will reduce the voltage of the input signal, reduce the input capacitance of the probe, and increase the resistance of the probe by a factor of 10. This invention will be implemented in terms of a passive divider probe; however, it is applicable to active divider probes and other probes in general.
The accomplishment of the second characteristic, the accurate reproduction of the signal, is usually more complicated. Frequencies of electrical signals that a test probe are called on to transmit to a test instrument can vary from zero, in the case of DC signals, to the gigahertz range, in the case of extremely fast digital circuits. Thus it is essential that test probes have a response that is flat to a high degree of accuracy across a wide range of frequencies or band width. However, the capacitance of the probe connecting cable and the probe amplifier circuit interacts with the voltage divider resistors in the probe, limiting the possible bandwidth. Thus most test probes include a frequency compensation circuit near the probe tip which compensates for the capacitance of the probe cable and amplifier and thus broadens the band width. This compensation circuit usually includes a compensation capacitor, which is difficult to miniaturize. See for example U.S. Pat. No. 5,172,051 issued Dec. 15, 1992, on an invention of Thomas J. Zamborelli.
It is not physically possible to locate the compensation circuit at the very end of the probe tip that contacts the circuit to be tested. Thus, there is always a portion of the probe tip and attached probe tip lead that exhibits stray capacitance to grounded portions of the probe tip, especially the shield that is often present. The stray capacitance of this portion of the circuit will be seen entirely by the circuit under test. This capacitance will not be reduced by the attenuation factor. Thus it is critical to minimize the stray portion of a test probe circuit.
Current trends toward miniaturization in integrated circuit packaging, multiplication of leads on integrated circuits, and increased signal speeds tend to aggravate the problem of stray capacitance. As the number of chip leads increases, it becomes necessary to utilize probes with multiple probe tips, so as to provide the possibility of multiplexing test signals and/or simultaneous testing of several leads at once. As lead spacings in integrated circuits shrink, the tips of multiple tip test probes are forced closer together, and it becomes increasingly difficult to locate the compensation circuits, particularly the compensation capacitors, near the probe tips. Though not to the same extent as for multiple tip probes, this phenomenon also affects single tip probes since large diameter probes can cause shorts and mechanical difficulties in placing the probe tip on an integrated circuit lead. Thus the trend is toward long probe tips and tip leads that exhibit high stray capacitance. If left unshielded, these longer probe tips and tip leads result in high self inductance and both inductive and capacitive coupling to adjacent probe leads. These factors, combined with the increasingly high frequencies of electronic circuits, result in increased crosstalk and decreased signal fidelity. To combat crosstalk and allow faithful transmission of signals, shielding these long tips and tip leads is necessary. However, shielding increases stray capacitance between the tip lead and the shield. Thus there is a need for a electrical probe in which the stray capacitance, and therefore the input capacitance is reduced.
3. Solution to the Problem
The invention solves the above problem by incorporating the stray capacitance into the frequency compensation circuit.
The invention provides an intermediate or middle conductor between the probe input conductor and the shield conductor.
The middle conductor is very thin, preferably formed by a thin-film process. It has been found that the thinner the middle conductor the better, so long as the middle conductor is not so thin that its resistivity becomes a factor in the compensation circuit. The thinness of the middle conductor permits the probe tip according to the invention to be the same size as or only slightly larger than the conventional probe tip. Thus it can easily be adapted to miniaturization.
The capacitance between the middle conductor and the probe input conductor takes the place of the compensation capacitor, thus a compensation capacitor separate from the tip structure itself is no longer necessary, further contributing to miniaturization.
When implemented in the standard coaxial probe tip structure, the invention provides a triaxial probe tip. The invention may also be implemented in almost any other probe tip structure, such as in the flat probe tip structure used in multiple tip, flex circuit type probe leads.
The total input (or loading) capacitance of a probe having the probe tip according to the invention may be reduced by as much as 50% in comparison to the conventional probe. The possible reduction in capacitance is found to vary with the attenuation factor of the probe. For a probe with an attenuation factor of 10, the possible reduction is 34% and reaches 50% for an infinite attenuation factor.