GB2090986A - Non-contact signal pickup arrangements - Google Patents

Abstract

An electrostatic non-contact probe device is interconnected via a capacitive coupling C to a logic device GSD, which provides an output signal indicative of the sensed signal, which may be digital or analogue.

Description

SPECIFICATION Non-contact signal pickup arrangements The present invention relates to non-contact signal pickup arrangements.

Connections between electronic devices for the pickup of test signals for example have traditionally been made by means of conductive plugs and sockets which have the disadvantages of requiring large contact pressures and they rely on careful preparation of the contact surfaces.

An aim of the present invention is to provide a coupling which overcomes the above disadvantages in an effective manner.

According to the present invention there is provided a non-contact signal pickup arrangement comprising an electrostatic non-contact probe device interconnected via a capacitive coupling to a logic device which provides an output signal indicative of the signal sensed by the non-contact probe device.

According to a further aspect of the present invention the logic device is a metal oxide silicon (MOS) logic gate.

According to yet a further aspect of the present invention the pickup arrangement senses digital logic signals.

According to yet a further aspect of the invention the pickup arrangement senses analogue signals.

The invention will be more readily understood from the following description which should be read with reference to the accompanying drawings wherein Figure 1 is a principle of the invention and shows a non-contact digital connection made with a (MOS) transistor, Figure 2 shows a logical data acquisition system;; Figure 3 shows a practical arrangement of a pickup of signals for printed tracks of circuit boards, Figure 4 shows the measured results when the arrangement shown in Figure 3 is used in conjunction with a MOS gate pickup circuit, Figure 5 shows measured results when the arrangement of Figure 3 is used in conjunction with a Schmitt trigger pickup circuit, Figure 6 shows a probe arrangement for logic pickup of signals from printed circuit tracks, Figure 7 shows a pickup probe suitable for acquiring logic signals from insulated wires, Figure 8 shows a non-contact analogue coupling arrangement, Figure 9 shows the effect of variation of coupling capacity in respect of the arrangement shown in Figure 8, and Figure 10 shows a frequency response curve of a complementary MOS (CMOS) amplifier with capacitive input coupling.

Referring to Figure 1, MOS transistor operation is based on essentially capacitive coupling between a control gate G and source and drain output connections S and D respectively. It is possible to pass digital signals into MOS devices, such as CMOS devices through external capacitive couplings consisting of a capacitor C and resistors R, R1 The external capacitor C can be constructed to withstand large voltages and is not subject to the practical restrictions of MOS integrated circuit gate isolation processes.

It is possible to pass not only fast signals but also very low speed digital signals through capacitive couplings. These signals act to produce normal CMOS logic gate "saturation" effects so that the ouput of the gate is equivalent to that achieved with DC logic couplings.

Referring to Figure 2, a logical data acquisition system will now be discussed. Consider a length of insulated wire or printed track PT connected to a source of pulses, as shown. To pick up a signal, it has previously been necessary to make ohmic contact either by means of a spring-loaded probe held against the printed circuit conductor or by puncturing the insulation of the wire from a conductive connector. It is possible to place an electrostatic pickup wire PW in close proximity to one of the existing insulated conductors to achieve reliable detection of logical activity by means of electrostatic coupling.

Referring to Figures 3 and 4 a practical arrangement for the pickup of signals from a printed circuit board will now be discussed, where planar electrostatic contacts with the printed tracks are produced.

The arrangement consists of two copper tracks CT1, CT2 having a strip of paper or PVC tape PT interposed between them. A printed transmitting plate TP supports one of the copper tracks CT1, and a printed receiving plate RP supports the other copper track CT2. An aluminium clamp plate ACP is mounted upon receiving plate RP and the arrangement is secured by a nut and bolt fixture NB. Referring to Figure 4, results are shown where the arrangement of Figure 3, is used as a probe the outputs of which are successively switched between foul differ bat sic ial tracks.The input traces I/P are fast 5 volt signals in the order of 12 KHZ, 128 KHZ, 132 KHZ and 16 KH. and are transmitted through the arrangement shown in Figure 3. The output traces O/P show the signals from the pickup circuit which is a CD401 1 B CMOS gate. The vertical scale is 5 volts per division and the horizontal scales are 0.5 microsec per division, 2 microsec per division, 5 microsec per division and 20 microsec per division, respectively. Referring to Figure 5, slow input pulses and measured output pulses are shown. The input I/P traces show slow 5 volt signals transmitted through the arrangement shown in Figure 3. The output traces O/P show the signals from the pickup circuit which is a CD4083B Scum it trigger circuit.The vertical scale is 5 volts per division and the horizontal scales are0.5 millisec per division, 5 millisec per division, 50 millisec per division and 0.5 sec per division respectively.

Referring to Figure 6, a probe arrangement is shown for logic pickup of signals from printed circuit tracks. The probe consists of an insulated wire IW for connection to a CMOS pickup gate. The wire enters an insulated holder IH where it is connected by a soldered joint J to a pickup wire PW. The pickup wire PW consists of two parallel lengths of 2 mm diameter bare tinned copper wire covered in 0.1 mm of paper tape. The working end of wire PW is 2.5 cm long which is a suitable length for achieving good pickup.

Referring to Figure 7 a pickup probe is shown for acquiring logic signals from insulated wire. The probe consists of a spring clip SC which accommodates a portion of the insulated wire IW. The clip SC is connected to a pickup gate by way of insulated wire IC. Table 1 beiow shows the performance of the pickup probe shown in Figure 7 when various insulated wires carrying 5 volt square wave pulses at two repetition rates are monitored by the probe.

TABLE 1 Minimum length of wire Overall Required in spring clip Insulation Size Diameter To maintain pulse output mm mm 1 KHz 10KHz PVC 50/0.25 3.5 14 mm 7 mm PVC 24/0.2 2.15 23 mm 18 mm PVC 1/0.5 1.0 23 mm 18 mm PTFE 7/0.2 0.97 30 mm 30 mm PTFE 7/0.125 0.75 30 mm 30 mm KYNAR 1/951 0.76 20 mm 10 mm The above description has been in respect of digital signal pickups but may similarly extend to the pickup of analogue signals in the form of audio frequency sine wave signals. Furthermore speech signals may be detected through capacities as low as 1 picofarad.

Referring to Figures 8 to 10 a practical arrangement of an analogue pickup will now be discussed.

The pickup uses a DC401 1 B CMOS gate. The input coupling capacity C1 will vary when non-contact pickups are used. Figure 9 shows the theoretical mid band AC gain as capacitor C1 varies. Once capacitor C1 exceeds 5pf, the effect of variation is small. The DC gain achieved with the feedback shown in Figure 8 is close to unity. The low frequency cut-off frequency depends upon the RD feedback time constant which is 10 ms. It is possible to obtain response down to 1 5 HZ through a 1 pf coupling.

Referring to Figure 10 experimental frequency response with input coupling is shown for a CMOS feedback amplifier according to Figure 8. It will be seen that a flat response is obtainable from 50 Hz to 50 KHz.

The non-contact signal pickup arrangement described can be used where access connections are to be implemented without puncturing insulation.

In respect of the analogue pickup, it may be used in the presence of noise spikes since the energy which can pass through couplings of one or two picofarads is small enough to reduce damage to electronic circuits. Also, audio cross point connections could be implemented which require no copper contacts but would use CMOS or other amplifiers having field effect transistor input stages.

Claims (14)

1. A non-contact signal pickup arrangement comprising an electrostatic non-contact probe device interconnected by way of a capacitive coupling to a logic device which provides an output signal indicative of the signal sensed by the non-contact probe device.

2. A non-contact signal pickup arrangement as claimed in claim 1 wherein the logic device is a metal oxide silicon (MOS) logic gate.

3. A non-contact signal pickup arrangement as claimed in claim 1 or 2 wherein the pickup arrangement senses digital logic signals.

4. A non-contact signal pickup arrangement as claimed in claim 1 or 2 wherein the pickup arrangement senses analogue signals.

5. A non-contact signal pickup arrangement as claimed in any preceding claim wherein the probe device is a pickup wire which is placed in close proximity to an insulated conductor carrying the signal to be sensed.

6. A non-contact signal pickup arrangement as claimed in any preceding claim 1 to 4 wherein the probe, for use in sensing signals from printed circuit board tracks, is arranged to make planar electrostatic contact with the tracks, and includes a plate bearing at least one copper track which is clamped to the printed circuit board with the track or tracks thereon aligned with the track or tracks on the plate and which further includes an insulating layer therebetween, the sensed output signals being switched successively to the logic device.

7. A non-contact signal pickup arrangement as claimed in any preceding claim 1 to 4 wherein the probe device comprises an insulated wire for connection at one end to the logic device, and at the other end to two parallel lengths of bare tinned copper wire covered in paper tape and having a working length adapted for the pickup of signals from a printed circuit track.

8. A non-contact signal pickup arrangement as claimed in any preceding claim 1 to 4 wherein the probe device is a spring clip connected to the logic device via an insulated wire and used for the pickup of signals from an insulated signal carrying conductor.

9. A non-contact signal pickup arrangement substantially as herein described with. reference to Figures 1 and 2 of the accompanying drawings.

1 0. A non-contact signal pickup arrangement substantially as herein described with reference to Figures 1 and 3 of the accompanying drawings.

11. A non-contact signal pickup arrangement substantially as herein described with reference to Figures 1 and 6 of the accompanying drawings.

1 2. A non-contact signal pickup arrangement substantially as herein described with reference to Figures 1 and 7 of the accompanying drawings.

1 3. A non-contact signal pickup arrangement substantially as herein described with reference to Figures 8 and 2 of the accompanying drawings.

14. A non-contact signal pickup arrangement substantially as herein described with reference to Figures 8 and 3 of the accompanying drawings.

1 5. A non-contact signal pickup arrangement substantially as herein described with reference to Figures 8 and 6 of the accompanying drawings.

1 6. A non-contact signal pickup arrangement substantially as herein described with reference to Figures 8 and 7 of the accompanying drawings.