Active voltage probe

An active voltage probe receiving a DC power voltage via an outer conductor of a coaxial cable is disclosed. The probe includes a differential amplifier with inputs connected to input and reference terminals via DC paths and a follower amplifier with an input connected to the input terminal via an AC path. Outputs currents from the differential and follower amplifiers are applied to an inner conductor of the coaxial cable.

BACKGROUND OF THE INVENTION 
The present invention relates to an active voltage probe for an electronic 
measurement instrument. 
Active voltage probes provide a better method of coupling high speed 
signals to an electronic measurement instrument, such as an oscilloscope 
or a logic analyzer, than passive probes can provide. The active voltage 
probe has higher input impedance with less attenuation than a passive 
voltage probe. Its use extends the measurement capabilities of the 
probe-instrument combination. There are two types of active voltage 
probes: (1) a cathode follower probe and (2) a source follower probe, with 
the source follower version being more popular. 
Conventional active voltage probes have three basic parts; a probe head, a 
cable and termination box. The probe head includes a source follower 
amplifier to receive a signal from a probe tip, and an output from the 
source follower is transmitted to an output amplifier of the termination 
box through the cable. Since the active devices of the probe head receive 
power from the termination box through an additional conductor in the 
cable, the conventional active probe is bulky in construction. Moreover, 
both signal and power voltage connectors are needed on a panel of the 
measurement instrument. When many probes are necessary to simultaneously 
measure a plurality of parameters of components such as integrated 
circuits, it is difficult to connect the required probes to the test 
points or component leads in a compact area, and hence more panel area of 
the instrument is necessary for the power voltage connectors. In addition, 
prior art probes are not isolated from instrument ground and therefore 
cannot measure a voltage (floating voltage) with respect to non-zero 
reference voltage. 
SUMMARY OF THE INVENTION 
According to the present invention, an active circuit of a probe head is a 
floating-voltage circuit which receives a power voltage through an outer 
conductor, for example, a shield member of coaxial cable. At a termination 
box or a measurement instrument, an offset voltage is applied to a signal 
path or an inner conductor of the coaxial cable for compensating a 
reference voltage. The probe head includes a differential amplifier with 
non-inverting and inverting inputs connected to input (probe tip) and 
reference terminals through DC paths for DC and low frequency components 
of an input signal to be measured, and a follower amplifier connected to 
the input terminal through an AC path for middle and high frequency 
components of the input signal. Outputs from the differential and follower 
amplifiers are mixed and applied to the inner conductor of the coaxial 
cable. The follower amplifier is a source follower or a combination of the 
source follower and an emitter follower. 
It is therefore one object of the present invention to provide an active 
voltage probe which does not need an additional power conductor for an 
active circuit in a probe head. 
It is another object to provide an active voltage probe which can detect 
signals from both grounded and floating-voltage circuits. 
It is a further object to provide an active voltage probe allowing 
significant reductions in assembly cost and physical size. 
It is an additional object to provide an active voltage probe which does 
not require a complex circuit and an additional connector to a termination 
box or a measurement instrument. 
This invention is pointed out with particularity in the appended claims. A 
more thorough understanding of the above and further objects and 
advantages of this invention may be obtained by referring to the following 
description taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to the drawing, there is shown a circuit schematic of one 
embodiment according to this invention. An active voltage probe comprises 
probe head 10, coaxial cable 12, and output circuit 14 which may be in a 
termination box or a measurement instrument. Probe head 10 includes 
differential or operational amplifier 16 and source and emitter follower 
amplifier 18. A non-inverting input of operational amplifier 16 is 
connected to input terminal 20 through large resistor 22 and small 
resistor 24, and an inverting input thereof is connected to reference 
terminal 26 through large resistor 28. An output of amplifier 16 is 
connected through large resistor 30 to a gate of field effect transistor 
(FET) 32 which is connected to the common junction of resistors 22 and 24 
through small resistor 34, large resistor 36, and capacitors 38 and 40. A 
drain of FET 23 and a collector of NPN transistor 42 are connected to an 
outer conductor of coaxial cable 12. Resistor 44 is inserted between a 
source of FET 32 and a base of transistor 42 which is connected through 
resistor 46 and capacitor 48 to the common junction of resistor 34 and 
capacitor 38 and further connected through resistors 50 and 52 to an inner 
conductor of coaxial cable 12. Resistor 54 is inserted between an emitter 
of transistor 42 and the common junction of resistors 50 and 52. Capacitor 
56 is inserted between the outer conductor of coaxial cable 12 and 
reference terminal 26, and a series circuit consisting of resistors 58, 60 
and 62 is connected between the outer and inner conductors of coaxial 
cable 12 wherein the common junction of resistors 58 and 60 is connected 
to the common junction of resistors 52 and 54 through Zener diode 64, and 
the common junction of resistors 60 and 62 is connected to the 
non-inverting input of operational amplifier 16. Positive and negative 
power voltage terminals of operational amplifier 16 are connected to the 
outer connector of coaxial cable 12 through resistor 66 and the common 
junction of resistors 50 and 52, respectively. Resistor 68 is inserted 
between the inverting input and negative power voltage terminal of 
operational amplifier 16. The circuit in probe head 10 can be implemented 
as a hybrid. 
In output circuit 14, the outer conductor of coaxial cable 12 is connected 
to voltage source V.sub.1, and the inner conductor thereof is connected to 
voltage source V.sub.2 through termination resistor 70 and further 
connected to a non-inverting input end of comparator 72. Voltage V.sub.1 
is higher than voltage V.sub.2. It should be noted that this embodiment is 
used in a logic analyzer. An inverting input receives a threshold level 
from terminal 74 through resistor 76. A series circuit consisting of 
resistor 78, potentiometer 80 and resistor 82 is inserted between the 
non-inverting and inverting inputs of comparator 72, and a center tap of 
potentiometer 80 receives voltage V.sub.3. A push-pull output of 
comparator 72 is applied to terminals 84 and 86. 
Input terminal 20 detects a signal to be measured from a test point of a 
PUT (product under test), and reference terminal 26 is connected to a 
reference potential of the PUT. Operational amplifier 16, FET 32 and 
transistor 42 receive their operation voltages from voltage source V.sub.1 
through the outer conductor of coaxial cable 12. Since voltage V.sub.1 is 
a DC voltage, there is no problem with regard to the outer conductor 
acting as a shield. At middle and high frequency components of the input 
signal, reference input terminal 26 acts as AC ground, and these frequency 
components pass through resistor 24 and capacitor 40. The high frequency 
component of the input signal is divided by capacitors 38-48 and floating 
and inter-electrode capacitances at the gate of FET 32 and the base of 
transistor 42. The divided high frequency component is amplified by FET 32 
and transistor 42 respectively functioning as source and emitter follower 
amplifiers, and the output currents therefrom are applied to the inner 
conductor of coaxial cable 12 through resistor 52. 
The middle frequency component of the input signal is divided by capacitors 
38-40, floating and inter-electrode capacitances at the gate of FET 32 and 
the base of transistor 42, and resistors 30-36. The divided middle 
frequency component is amplified by source follower/emitter follower 
amplifiers 18, and the output currents therefrom are applied to the inner 
conductor of coaxial cable 12. 
The DC and low frequency components of the input signal are applied to the 
non-inverting input of operational amplifier 16 through input terminal 20 
and resistor 24-22, and a different voltage between the reference 
potential at terminal 26 and the DC and low frequency components is 
obtained at the output of amplifier 16. It should be noted that a general 
operational amplifier is for DC and low frequency signals and is not 
suited for middle and high frequency signals. The output from operational 
amplifier 16 is applied to the inner conductor of coaxial cable 12 through 
resistor 30 and source follower/emitter follower amplifiers 18. The signal 
at the common junction of resistors 50 and 54 is fed back to the inverting 
input of operational amplifier 16 through feedback resistor 68. The DC 
bias and low frequency gain is set by operational amplifier 16, resistors 
22-28-52-60-62-68 and Zener diode 64. This Zener diode is present in order 
to provide the current offset required to bias the probe. Terminals 20 and 
26 can be connected to the floating circuit because of differential 
amplifier 16. Since the negative voltage terminal of amplifier 16 is 
connected to the common junction of resistors 50 and 52, amplifier 16 is 
floated, and the operation characteristic thereof is improved. 
The high, middle and low frequency gains and the DC gain are set to be 
equal. When the difference voltage (V.sub.20 -V.sub.26) between terminals 
20 and 26 is zero, output current I.sub.out flowing through resistor 52 
and the inner conductor of coaxial cable 12 is the bias current I.sub.o 
determined by Zener diode 64 and other components. When the difference 
voltage between terminals 20 and 26 is V (positive or negative), output 
current I.sub.out is I.sub.o +(V/R.sub.o), wherein R.sub.o is determined 
by the gains of the amplifiers in probe head 10. Output current flows to 
resistor 70, and a voltage across resistor 70 is R.sub.70 [I.sub.o 
+(V/R.sub.o)], wherein R.sub.70 is the resistance of resistor 70. Since 
resistor 70 receives voltage V.sub.2 the voltage generated by R.sub.70. 
I.sub.o is cancelled, and only the signal component voltage between 
terminals 20 and 26 is applied to the non-inverting input of comparator 
72. Resistors 78-80-82 comprise a DC balance circuit. In this embodiment, 
the measurement instrument is a logic analyzer, and comparator 72 compares 
the corresponding input voltage with the threshold voltage from terminal 
74. The push-pull output signal at terminals 84 and 86 is applied to a 
main circuit of the logic analyzer. If the measurement instrument is an 
oscilloscope, the common junction of resistor 70 and the inner conductor 
of coaxial cable 12 may be connected to a buffer amplifier. A conventional 
coaxial connector can be used for the connection between coaxial cable 12 
and output circuit 14. 
While I have shown and described herein the preferred embodiment of my 
invention, it will be apparent to those skilled in the art that many 
changes and modifications may be made without departing from my invention 
in its broader aspects.