GB2158592A

GB2158592A - Electrical probe assembly

Info

Publication number: GB2158592A
Application number: GB08411087A
Authority: GB
Inventors: William Burrows; John David Middleton
Original assignee: Marconi Instruments Ltd
Current assignee: Marconi Instruments Ltd
Priority date: 1984-05-01
Filing date: 1984-05-01
Publication date: 1985-11-13
Also published as: GB2158592B; GB8411087D0

Abstract

An electrical probe assembly is used with a test instrument to accurately attenuate an input signal. The assembly uses relatively movable capacitor plates 6, 11 to vary the signal level applied to an amplifier 14 mounted adjacent to one of the plates. The plates are surrounded by a conductive sleeve 1 so that attenuation is directly proportional to the distance between the plates. <IMAGE>

Description

SPECIFICATION Electrical probe assembly This invention relates to an electrical probe assembly, and is particularly applicable to a test probe assembly which provides an interface between test apparatus, and an electrical circuit or component to be tested by the apparatus. In order that the act of coupling the test apparatus to the circuit or component, as the case may be, does not itself modify the electrical parameters to be monitored or measured, it is generally very desirable that the probe assembly exhibits a very high electrical impedance value. However, to allow for widely varying magnitudes of the aforesaid parameters, and for variations in the sensitivity of the test apparatus which can be dependent on the range setting selected, a single fixed impedance value is not satisfactory.As the value of the parameter received at the test apparatus can be directly dependent upon that of the impedance, great care must be taken to use a high impedance having a value known to an accuracy which is at least as great as that of the measurement to be performed. Thus, it is difficult and expensive to provide an interface of this kind which is useful over a wide range of single levels.

This invention seeks to provide an improved electrical proble assembly.

According to a first aspect of this invention, an electrical probe assembly includes a high input impedance amplifier mounted in close proximity to one of two capacitor plates which are spaced apart from each other by a variable distance along the length of a conductive screen which surrounds both plates in a tubular manner such that an electrical signal applied to the other capacitor plate is coupled to said amplifier.

According to a second aspect of th is invention, an electrical probe assembly includes two capacitor plates mounted face to face within a conductive duct; means forvarying in a controlled manner the distance between the plates along the length of the duct; means for coupling an input signal to one plate; and a high input impedance amplifier, the input port of which is coupled to the other plate, with the amplifier being mounted within the duct in very close fixed proximity to said other plate.

The probe assembly is suitable for use at electrical frequencies extending over a very wide range, but because it utilises capacitive coupling, it does not extend down to zero hertz, i.e. it does not handle d.c.

measurements. Its upper frequency is determined by the transverse demensions of the duct. Adjustment of the spacing of the two capacitor plates gives a variable coupling capacitance, and the impedance thereof is directly proportional to the spacing for frequencies for which the duct acts as a waveguide operating below"cut-off" frequency. As is well known, electromagnetic energy propagates as a wave along a waveguide only if the frequency is above a critical value which is dependent upon the transverse dimensions of the waveguide, but for the present purposes the frequency handled by the probe is chosen to be below the critical value. Under thins condition the variable impedance presented by the capacitive coupling constituted by the two capacitor plates is linearly proportional to the distance by which they are spaced apart from each other.Thus, by accurately adjusting the position of one plate with respect to the other, an input signal can be attenuated by a known amount.

Preferably the duct is formed in two parts each of which holds a respective capacitor plate, with the two ducts being movable one inside the other, whilst maintaining axial alignment. Preferably, again both parts of the duct are of a circular section, and co-operate by means of a precision screw thread. Rotation of one part relative to the other, produces relative movement of the capacitor plates along the axis of the duct.

The invention is further described by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a schematic view of the assembly, and Figure 2 is an explanatory circuit diagram.

Referring to Figure 1, the electrical probe assembly consists of two major parts, a hollow circularly cylindrical outer sleeve 1 which is of slightly larger outer diameter than a cylindrical rod-like portion 2 with which it co-operates by means of a screw thread 3. An elongate probe tip 4 is coaxially mounted in a nose cone portion 5, of the hollow cylindrical sleeve 1 and the inner end of the probe tip 4 supports a cylindrical capacitor plate 6 in the form of a disc which is coaxial with the sleeve 1.

The cylindrical portion 2 contains a central passage 7 which carries electrical connection which is to be made between the probe assembly and the test instrument with which it is to be connected by means of a cable 8. The inner end of the portion 2 has a part of the circumference removed to leave a relatively thin segment 9 which supports one end of a conductive rod 10, the other end of which carries a capacitor plate 11 which is similar to the capacitor plate 6 and which is spaced apart therefrom by a distance D. The inner end of the rod 10 is directly connected to an amplifier circuit 12 having a very high impedance input, and it is the output of the amplifier 12 which is connected via the channel 7 to the cable 8. The rod 10 lies along the axis of the sleeve 1, and the plate 11 is circular and is centred on the rod 10.

In operation, a high frequency signal to be tested or monitored in a circuit board or the like is applied to the tip of the probe 4 and is capacitively coupled to the amplifier 12. The attenuation A of the capacitive coupling is dB per unit length is given by the expression:

where A= wavelength in free space Ac= cut off wavelength of the duct This reduces to A = 20.85 dB/unit length if A > > Ac, where r is the radius of the waveguide duct.

Thus, an attenuation proportional to the distance between the two plates 6 and 11 is achieved so long as the frequency of the signal to be measured is very much less than the critical frequency of the waveguide represented by Ac. Provided this criteria is satisfied, the attenuation is substantially independent of frequency, and the input impedance of the probe 4 is largely determined by the capacitance to ground of the launching plate 6. It is important to notethatthe probe assembly uses a sleeve 1 which acts as a waveguide for signals having frequencies which are below the critical or cut-off frequencies of the waveguide.That is to say, the frequencies with which the invention is used are below those for which the sleeve 1 can act as a conventional waveguide which propagates those frequencies in wave-like manner Under these conditions in which the sleeve 1 does not support propogation of a travelling electromagnet wave, the coupling is purely capatitive, and the linear relationship with attenuation exists.

Referring to Figure 2, there is shown therein a circuit diagram associated with the electrical probe assembly illustrated in Figure 1. The same reference numerals are used to indicate the sleeve 1 and the two capacitor plates 6 and 11, together with the probe tip itself. The amplifier 12 of Figure 1 is represented by a field effect transistor 14 with the gate electrode of the amplifier being connected directly to the capacitor plate 11. The gate electrode is biased in a d.c. manner by means of a high value resistor 15 (typically 10 MQ) which is connected to a negative voltage of about -5v supplied by an associated test apparatus 20. The probe assembly is linked to the test apparatus 20 by means of a long flexible cable represented diagramatically by the bundle 21.The signal path is represented by the centre line 22 of a coaxial cable 23, the earth of the coaxial cable being connected to the casing of the test apparatus 20 via a small bias box arrangement 24 which plugs directly into the front panel of the test apparatus 20. The box 24 includes a small capacitor 25 (typically about 10 nF) and resistor 26 which together act as a filter and act to decouple the d.c. connection of the amplifier 14, the resistor 26 being connected to a low positive voltage of about + 1 5v supplied by the test apparatus. A resistor 27 having a standard value of 50Q or 75Q is connected to the input signal path within the test apparatus.

Thus, in use the probe assembly is positioned so that the probe tip 4 is brought into contact with a circuit or electrical component which carries a high frequency signal having a characteristic to be measured.

Depending on the amplitude of the signal, its level is attenuated to bring it within the operating range of the test apparatus. The value of the attenuation is directly related to the spacing of the capacitor plates, and hence the surface of the portion 2 can be marked with calibration lines, with the position of the end of the sleeve 1 on the calibration lines giving a direct reading of attenuation. A vernier type scale can readily be provided. In practice, the existence of the capacitive coupling will introduce a certain attenuation, which cannot be reduced, and this is likely to be a factor of 2. To compensate for this, the amplifier 12 is arranged to give a constant gain of 2. Typically the amplifier uses a gallium arsenide FET device which has a low gate-to-source capacitance and high input resistance which permits the probe assembly to be used over several decades of bandwidth.

Claims

1. An electrical probe assembly including a high input impedance amplifier mounted in close proximity to one of two capacitor plates which are spaced apart from each other by a variable distance along the length of a conductive screen which surrounds both plates in a tubular manner such that an electrical signal applied to the other capacitor plate is coupled to said amplifier.

2. An electrical probe assembly including two capacitor plates mounted face to face within a conductive duct; means for varying in a controlled manner the distance between the plates along the length of the duct; means for coupling an input signal to one plate; and a high input impedance amplifier, the input or port of which is coupled to the other plate, with the amplifier being mounted within the duct in very close fixed proximity to said other plate.

3. An electrical probe assembly as claimed in claim 2 and wherein the duct is of circular section, and one of the capacitor plates is rigidly fixed with respect to the duct.

4. An assembly as claimed in claim 3, and wherein both capacitor plates are circular, and mounted centrally on the axis of the duct.

5. An assembly as claimed in claim 4, and wherein the amplifier is mounted on a portion of the assembly to which the other capacitor plate is rigidly fixed.

6. An assembly as claimed in claim 5 and wherein the duct cooperates with said portion by means of a precision screw thread, such that rotation of the duct about its longitudinal axis with respect to said portion alters the spacing between the two capacitor plates.

7. An assembly as claimed in claim 6, and wherein said amplifier is positioned on said portion at a location which is between the screw thread and said other plate.

8. An assembly as claimed in any of claims 2 to 7 and wherein said amplifier includes a gallium arsenide FET device.

9. An electrical probe assembly substantially as illustrated in and described with reference to Figure 1 of the accompanying drawings.