GB2264788A - A wideband switchable gain active probe - Google Patents

Abstract

A wideband, switchable gain, active probe e.g. for oscilloscopes, spectrum analyzers or logic analyzers, includes a switch (1) which can be moved to select a unit (X1) or tenfold (X10) attenuation in an input attenuator circuit (8-13) in a high frequency path (3-5, 18, 19) in the probe; and a ganged switch (2) which controls a MOSFET (14) to turn OFF or ON whereby to provide similar attenuation in a low frequency path (6, 7) in the probe. In Figure 3 (not shown) the position of the high frequency switch (1) is sensed (70) whereby to provide low frequency attenuation control. <IMAGE>

Description

A WIDEBAND SWITCHABLE GAIN ACTIVE PROBE This invention relates to a high impedance wideband electronic measurement probe which may be used in conjunction with a variety of test equipment such as oscilloscopes, spectrum analysers or logic analysers.

In prior art designs, active probe designs have made use of removable barrel attenuator units to increase the dynamic range of the instrument. These barrel attenuator units are typically constructed from a turned meta) casing into which is housed a small printed circuit board attenuator unit and a suitable coaxial connector arrangement for making contact with the main probe body. In a typical probe kit, a multiplicity of barrel attenuator units would be provided for example xlO, xlO0 to allow a range of measurements to be made. It may also be possible to cascade these attenuator units for further stages of attenuation. Such a system effectively lengthens the probe head assembly by typically 30mm for each attached barrel unit.

The effect of this length increase e TS to inhahce the rotational moment resulting from the weight of the umbilical cable relative to the fulcrum at the probe tip. This puts extra stress on the tip and any connector or circuitry under test.

Another disadvantage with barrel attenuator units is that they can become lost or mixed up. For instance, when an array of similar probes are in use, the barrel attenuators use variable capacitors which match precisely to the probe input capacitance for a particular unit. If these barrels are mated up with a another unit with slightly different input characteristics, the pulse response may be non optimal. None of these problems exist with the new switched probe design.

Although switchable passive oscilloscope probes have been available for a number of years, it is generally recognised that the high frequency performance is often inferior to a fixed gain passive probe operated on the same gain setting. Also, because of the stray capacitance associated with the switch, the input capacitance of the probe can be similarly adversely affected.

In an active probe, however, the expected input capacitance figures would be around an order of magnitude lower than those of a passive probe, and hence their reason for use. The incorporation of a switched attenuator system would, therefore, appear to make this situation very much worse. In the new system to be described, a typical example of the FET active probe unit exibits a 1 Megohm input inpedance with a shunt load capacitance of 3 picofarads.

This value remains constant for direct and attenuated positions.

The -3db bandwidth of the probe is 500MHz. These performance figures have been found to compare very favourably with other probes using the prior art barrel attenuator design.

According to the present invention, using a prior art field effect transistor active probe, a switchable attenuator network system has been added which extends the dynamic measurement range of the probe. Since this is integrated into the probe assembly, it provides~many advantages over the prior art designs which make use of removable barrel attenuator systems. By reducing stray capacitance and inductance in the design and use of the prior art bootstrapping technique, the probe performance equates or improves on performance found in prior art designs.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:- Fiql: shows general prior art active probe design with barrel attenuator sections.

Fig2: shows the switched active probe design of the present invention.

Fig3: shows a further enhancement of the present invention using a simplified switch system.

Fig4: illustrates use of guard tracing to reduce the probe input capacitance.

In figure 1, is shown detail of a prior art design using a barrel type attenuator system. In a typical product, this would be supplied with a multiplicity of the barrel attenuator units (1! intended for connection to the main probe unit (2) to provide the desired level Oc attenuation, commensurate with the working signal level.

In figure 2, is shown a simplified schematic of the new design employing the switched attenuator system. In this example, only two switched positions are provided or, giving an attenuation = 1 or 10 as selected by the ganged double pole changeover switch (1) and (2) However, the design could, allow more ranges, only being limited by the probe physical dimensions and suitable available switch designs. The system uses a standard two path amplifier system, where high frequencies are buffered by the FET amplifier arrangement, comprising transistors (3),(4), (5! and load components (1.B) and (19) and at low frequencies by the operational amplifier system, (6) and (7).The switch, (1) selects between the direct (xl) range and the attenuated (x10) range where components (8),{9!,(10),(11),(12) and !13) make up the attenuator. Section (2) of the switch is used to control the gain of the LF circuitry by setting the bias on MOSFET, (14). In the unity gain mode, (1o! is turned off by the negative bias applied via high value resistor, (49) and the opamp dc gain is determined by the ratio of resistors (9) and (16). In the x10 mode, the negative bias from resistor (48! is removed and FET (14) is turned hard on hy resistor (15).This effectively places resistor (17) in parallel with (16), where the ratio of values are so arranged to reduce the gain by 10 and hence match that of the HF section.

Transistors (5) and (27) effectively form a differentia] pair amplifier with each half being split between the connecting coaxial cable (25). The termination is determined by resistors (20) and (22) and the supply current by current source (21).

The gain of this amplifier determined by load resistor, (26) and variable resistor (31) anc resistors (20) and (22!. The signal is subsequently buffered by emitter follower stages, (28ì and (3O with associated load resistors, (29) and (42). The opamp, (7! acts as an error amplifier, determininq the difference voltage between the output of opamp (6) via variable resistor (35) and that from the feedback resistor (69! It then feds the required different voltage to the base of transistor (27) to correct the error.

Bias to the FET preamplifier amplifier, (3),(4),(5) is provided by high value resistors (43),(44),(45) giving a lc source impedance of about 15 Megohm. The bootstrap feedback action of emitter follower,(4), via capacitor, (46) increases this impedance at frequencies above 20Hz to about 400 Megohm. This gives two important advantages. Firstly, the bias chain has a low enoua8) dc impedance to provide a stabile bias voltage irrespective of leakage currents, while providing a negligible loading effect on the input attenuator system irrespective of settling.Secondly, it permits- a very low value to be used for input capacitor, (47), while preserving the 20 Hz LF roll of frequency of the HF section. This assists greatly in preventing the FET (3) from being damaged by inadvertent application of high votage or static charge at the input terminal.

In order to minimise input capacitance, extensive use or surface mounted component technology is used in the design with very miniature components being used for the input section.

Also, use is made of the bootstrapping technique to prevent the stray capacitance of the switched attenuator from affecting the input capacitance of the unit. This is particularly important in the xl attenuation mode and is achieved in two ways. Firstly, the bias to the control section of the switch, (2!, is applied as a signal from the emitter of transistor (4! instead of using a fixed dc potential. Although the switching operation of transistor (14) behaves similarly in either case, this method dramatically reduces the stray capacitance between the switch sections (1) and (2! and hence similarly reduces the input capacitance of the probe.

A bootstrap signal is also applied from the source of FET (3) and the emitter oF (4) in the form of printed board tracking along each side of switch (1!,(2) thus reducing the stray capacitance of (1) and its associated components and tracking to the earthed metal body of the probe. (see track pattern Fig.4) In the 1 mode, the HF attenuator is made up of two sections. The capacitative section consists of components (13!, (10), (11) and (5) and the resistive section consists of (8! and (95.

Capacitor, (13) is a very low value, (lpF) in paralle] with approximately 0.5pF stray capacitance from the open contacts of the switch, (1). The capacitance of (1.9! is chosen to match the capacitance in the xl mode to that in the x10 mode. Trimmer,(10) and capacitor (11) plus stray capacitance form the 13.5pF required to give an accurate 10:1 attenuation. Components (43) to (53) are for damping the stray reactance due to path length in the attenuator, switch (1) and associated tracking.

In order to achieve a flat pulse response it is necessary to have at least two decades of frequency crossover between the HF amplifier section and the control opamp section. THe HE response of the opamp section is limited by the phase and frequency error in (6) anc5 also the effect of cable capacitance from (24) seen in parallel with the virtual impedance present at the inverting input of (6). This places a practical limit of about 10Kiiz to this response and hence the LF response of the HF section must extend below 100Hz.The parallel source capacitanco o (10) and (11 ! are significantly smaller than that of (47) which would otherwise increase the LF roll off frequency in xlO mode btzi: for the coupling capacitor, (5) which blocks dc bias from the FE gate (3) entering the resistive attenuator and provides the required LF bypass response. Although this blocking capacitance could equally be placed in series with the FET gate system, this would contribute extra stray capacitance and series inductance.

Because the switch (1), is a miniature type, the contact rating is quite low (typically 30v! although the breakdown rating is 500V. en operated above the contact voltage rating albeit with a negligible current flowing, it is possible to maintain an arc for an indefinite period. By re-connect2 nq the coupling capacitor, (47), in a novel manner in series with the switch, the voltage across the switch terminals is only the bias voltaqe on the gate of (3) and hence no arc can occur.

In Fig. 3 is shown another embodiment of the invention where only a single pole changeover switch is required. This system makes use of a high impedance sense element which may b provided by a variety of means such as, a comparator, opamp or FET to sense the position of the switch without causinq undue loading of the attenuator circuit. The system in the preferred embodiment uses a resistor (71) of high impedance so giving negligible circuit loading to the attenuator circuit.This forms a potential divider with resistor (72) and connects into an inverting opamp, (70). The small negative bias (-Vbias! on the non-inverting input of (70! makes the opamp output move to its maximum negative level when the probe is in xl mode as no current is flowinq in (71). In the xlO mode, the negative bias from the FET is applied to (71) and a t'ne current flowing gives a negative bias at the inverting input of (70).This has greater magnitude than the -Vbias potentialand hence the opamp swings to its maximum positive voltaqe. This control signal is then be used to turn on FET (14 )n the same manner as the circuit of Fig 2. Note that opamp (70) is not affected by signal conditions at the probe input since in normal operation, this has a much smaller magnitude than the -Vbias.

The adx7antage with this arrangement is that the switch is physically smaller, makings for a more compact circuit layout. It also allows tracking lengths to be kept shorter around FET, (3) and thereby considerably improving wideband operation.

Another feature of the design not found in prior art active probes is the use of cold (electronic) switching controls and LED status indication. Although various methods of achieving this function could be used, the preferred embodiment in Fig.2 uses momentary action push button switches (54) and (55) for the AC!DC: and offset selection modems respectively. The output of these switches feeds into pulse forming networks (56) and (58) and then into binary counters (57) and (58). For counter (57) the last significant bit outputs, QO and its complement Q0 drive status LED's (60! and (62).In AC mode, LED (60) is illuminated and in DC mode LED (62? is illuminated. In this mode, the relay 38 is also energised, shorting out capacitor (39). Counter (59) drives an analogue chanqeover switch, (63). The least signi ficant hit, Q0, drives the enable to (63) so that the switch output becomes alternately conducting or non-conducting according to Q0 state.

When (63) is in conducting mode, the next significant bit, Ol from counter (59) toggles the so itch so that the output from it can alternately provide the voltages +Vcc and -Vbb to the offset control (67). The LED's !64) and (65) similarly alternately conduct and illuminate according to the polarity of the applied voltage from (63). The operation of the circuit provides a sequence as follows:- OFF, OFFSET + , OFF, OFFSET - , OFF etc.

Output from the variable offset control (67) is then fed via resistor (68) to provide bias for the opamp (6). This function allows dc input potentials at the probe pin to be suitably nulled so that measurements can be made in the required range.

Claims (6)

1. What is claimed to be novel in this design is detailed in the following paragraphs:1! A switched gain active probe using solid state switch, relay or mechanical switch means to control one or a plurality of impedance input attenuator circuits where said circuits are an integral part of the probe assemb]y.
2. 2) A switched gain active probe as claimed in claim 1 where said switch means uses a two pole switch, one pole to control the high frequency attenuation and the other pole to control the low frequency attenuation.
3. 3) A switched gain active probe as claimed in claims 1 and 2 where said switch means uses a single pole switch to control the high frequency attenuation and a sense circuit means operating from said switch to control the low frequency path.
4. 4) A switched gain active probe as claimed in claim 1 or claim 2 or claim 3 where hiqh input voltages are prevented from being present on said attenuator switch by using dc blocking capacitor means on the input to said switch.
5. 5) An active probe which uses momentary action push button control means.
6. 6) An active probe where the status of controls is shown by illuminated indicators or Liquid Crystal Display panel means.

   6! An active probe where the status of controls is shown by illuminated indicators or Liquid Crystal Display panel means.

   7 ? A switchahle active probe substantially as described herein with reference to the Figures 2-4 of the accompanying drawing.

   Amendments to the claims have been filed as follows What is claimed to be novel in this design is detailed in the following paragraphs:1) A high frequency switched gain active probe using either a two pole mechanical switch, relay or solid state switch means to control one or a plurality of n high input impedance attenuator circuits, with the first pole of said switch being used to control the gain of the high frequency signal path, and the second pole being used to control the gain of the low frequency signal path, where a first dc blocking capacitor is incorporated between the input port and first terminal of first pole of said switch, to prevent high voltages appearing on the switch terminals, thereby allowing a miniature, low voltage switch to be used, the other terminals of the first pole of said switch, apart from the common terminal thereof each being connected to a shunt capacitor associated with each of the n high impedance attenuators in the circuit, where the shunt capacitors in each of the n attenuators are separately connected to the first terminals of n dc blocking capacitors, the second terminals thereof each connect to a different tap in a resistor divider chain connected between the signal input and the virtual earth input of an operational amplifier circuit in the low frequency path, where the ratio in each divider is made to match that ratio of the series and shunt capacitors in each of the n attenuators of the high frequency path, and where the second pole of said switch is used to control n solid state switches selecting n feedback resistors in said operational amplifier circuit, to precisely match the gain provided by the high frequency attenuator, the common terminal of the first pole of said switch feeding the selected high frequency signal from the attenuators to a single high impedance buffer amplifier.

   2) A switched gain active probe as claimed in claim 1 where said switch means uses a single pole switch to control the high frequency attenuation and a sense circuit means operating from said switch to control the low frequency path.

   3) A switched gain active probe as claimed in claim 1 or claim 2 where the preamplifier circuit connecting to the high impedance attenuator uses bootstrap means to raise the input impedance of said high impedance buffer amplifier and thereby reduce the value of the dc blocking capacitor stated in claim 1 so that protection of the circuit against excessive input voltages is enhanced.

   4) A switched gain active probe as in claim 3 where the bootstrap signal from said preamplifier is fed to circuit tracking at the front of said active probe to guard the circuit and switch from stray earth capacitance and thereby reduce the input capacitance of said probe.

   5) An active probe which uses momentary action push button control means.