U.S. patent application number 09/755265 was filed with the patent office on 2001-09-27 for electronic probe for measuring high impedance tri-state logic circuits.
Invention is credited to Draving, Steven D..
Application Number | 20010024116 09/755265 |
Document ID | / |
Family ID | 22672923 |
Filed Date | 2001-09-27 |
United States Patent
Application |
20010024116 |
Kind Code |
A1 |
Draving, Steven D. |
September 27, 2001 |
Electronic probe for measuring high impedance tri-state logic
circuits
Abstract
An electronic probe has a termination portion, a filter, and an
impedance device. The termination portion is connected between a
transmission line end and a common node. The termination portion
has a termination resistor and a termination capacitor connected in
series between the transmission line end and the common node. The
filter has a resistor connected in parallel with a capacitor and an
inductor connected in series with the filter resistor and filter
capacitor combination. The components are connected between the
transmission line end and an output. An impedance device is
connected between the output and the common node. A zero is
associated with the termination portion and a pole is associated
with the filter. The frequency of the zero is approximately equal
to the frequency of the pole. The probe provides a device for
measuring tri-state logic circuits without overloading the
circuits.
Inventors: |
Draving, Steven D.;
(Colorado Springs, CO) |
Correspondence
Address: |
AGILENT TECHNOLOGIES
Legal Department, 51U-PD
Intellectual Property Administration
P.O. Box 58043
Santa Clara
CA
95052-8043
US
|
Family ID: |
22672923 |
Appl. No.: |
09/755265 |
Filed: |
January 5, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09755265 |
Jan 5, 2001 |
|
|
|
09183468 |
Oct 30, 1998 |
|
|
|
6175228 |
|
|
|
|
Current U.S.
Class: |
324/72.5 |
Current CPC
Class: |
G01R 1/06766
20130101 |
Class at
Publication: |
324/72.5 |
International
Class: |
G01R 031/02 |
Claims
What is claimed is:
1. An electronic probe comprising: a probe tip; a transmission line
having a transmission line first end and a transmission line second
end, said transmission line first end being connected to said probe
tip; a termination portion comprising a termination portion first
end and a termination portion second end, said termination portion
first end being connected to said transmission line second end,
said termination portion second end being connected to a common
node, said termination portion comprising a termination resistor
and a termination capacitor connected in series between said
termination portion first end and said termination portion second
end; and a filter comprising a filter first end and a filter second
end, said filter first end being connected to said transmission
line second end, and said filter second end being connected to a
probe output, said filter comprising a filter resistor connected in
parallel with a filter capacitor and a filter inductor connected in
series with said filter resistor and filter capacitor combination;
and an impedance device connected between said probe output and
said common node.
2. The electronic probe of claim 1 and further comprising a tip
resistor connected between said probe tip and said transmission
line first end.
3. The electronic probe of claim 2, wherein said tip resistor in
integral to said probe tip.
4. The electronic probe of claim 2, wherein the value of said tip
resistor is approximately four-hundred twenty-two ohms.
5. The electronic probe of claim 1, wherein said impedance device
is a resistor.
6. The electronic probe of claim 1, wherein said probe output is
electrically connectable to an electronic device having an input
capacitance associated therewith, and wherein the value of said
filter inductor is approximately equal to 0.4 multiplied by the
value of said termination resistor squared multiplied by the value
of said filter capacitor in parallel with said input
capacitance.
7. The electronic probe of claim 1, wherein the value of said
filter inductor is approximately ten nanohenries.
8. The electronic probe of claim 1, wherein the value of said
filter resistor is approximately 33.64 kilohms.
9. The electronic probe of claim 1, wherein the value of said
filter capacitor is approximately one-hundred sixty-eight
picofarads.
10. The electronic probe of claim 1, wherein at least one pole is
associated with said filter in conjunction with said impedance
device, wherein at least one zero is associated with said
termination portion and wherein the frequency of said at least one
pole is substantially equivalent to the frequency of said at least
one zero.
11. The electronic probe of claim 1, wherein said probe has a gain
associated therewith between said probe tip and said probe output
and wherein said gain is approximately uniform over a frequency
spectrum of direct current to a preselected frequency.
12. The electronic probe of claim 1, wherein said probe output is
electrically connectable to an electronic device having an input
capacitance associated therewith, wherein at least one pole is
associated with said filter in conjunction with said impedance
device and said electronic device, wherein at least one zero is
associated with said termination portion, and wherein the frequency
of said at least one pole is substantially equivalent to the
frequency of said at least one zero.
13. An electronic probe comprising: a probe tip; a tip resistor
having a tip resistor first end and a tip resistor second end, said
tip resistor first end being connected to said probe tip; a
transmission line having a transmission line first end and a
transmission line second end, said transmission line first end
being connected to said tip resistor second end; a termination
portion comprising a termination portion first end and a
termination portion second end, said termination portion first end
being connected to said transmission line second end, said
termination portion second end being connected to a common node,
said termination portion comprising a termination resistor and a
termination capacitor connected in series between said termination
portion first end and said termination portion second end; and a
filter comprising a filter first end and a filter second end, said
filter first end connected to said transmission line second end,
and said filter second end connected a probe output, said filter
comprising a filter resistor connected in parallel with a filter
capacitor and a filter inductor connected in series with said
filter resistor and filter capacitor combination; and an impedance
device connected between said probe output and said common node;
wherein at least one zero is associated with said termination
portion; wherein at least one pole is associated with said filter
in combination with said impedance device; and wherein the
frequency of said at least one pole is substantially equivalent to
the frequency of said at least one zero.
14. The electronic probe of claim 13, wherein said tip resistor is
integral to said probe tip and wherein said transmission line first
end is connected to said probe tip.
15. The electronic probe of claim 13, wherein said impedance device
is a resistor.
16. The electronic probe of claim 13, wherein said filter output is
connectable to an electronic device having an input capacitance
associated therewith.
17. An electronic probe comprising: a probe tip; a transmission
means for transferring electromagnetic signals, said transmission
means having a transmission means first end and a transmission
means second end, said transmission means first end being connected
to said probe tip; a termination means electrically connected to
said transmission means second end, said termination means for
terminating said transmission means, said termination means being a
block to direct current and having at least one zero associated
therewith; and a filter means electrically connected to said
transmission means second end, said filter means having at least
one pole associated therewith, said at least one pole being at
approximately the same frequency as said at least one zero; said
filter means comprising at least one inductive means; said filter
means comprising an output being electrically connectable to an
electronic device having in input capacitance; said inductive means
for compensating for said input capacitance of said electronic
device.
Description
[0001] This application is a Continuation-in-Part of U.S.
application Ser. No. 09/183,468 filed on Oct. 30, 1998, which is
hereby incorporated by reference for all that is disclosed
therein.
TECHNICAL FIELD OF THE INVENTION
[0002] The present invention relates to an electronic probe and,
more particularly, to an electronic probe providing a high
impedance input at direct current so as not to lower the impedance
of a high impedance device under test. The probe further provides a
low impedance input at high frequencies in order to provide
improved sensitivity without amplification.
BACKGROUND OF THE INVENTION
[0003] Tri-state logic devices have a high, a low, and a high
impedance mode. In the high mode or logic one mode, a tri-state
logic device outputs a high voltage. In the low mode or logic zero
mode, the tri-state logic device outputs a low voltage or zero
volts. In the high impedance mode, the output of the tri-state
logic device is neither high nor low, but rather is a high
impedance output. Conventional logic devices typically have only
high and low voltage modes.
[0004] Tri-state logic devices are used in many high speed
communications systems because the properties of tri-state logic
are adapted to accommodate multiple bus drivers. The communications
are in the form of binary data consisting of high and low voltages
output by the tri-state logic devices. Communications with
tri-state logic devices also consist of the tri-state logic devices
being in the high-impedance mode so as not to adversely affect data
communications between several tri-state logic devices. For
example, if the outputs of several tri-state logic are electrically
connected together, only one tri-state logic device may be active.
The other tri-state logic devices may be in the high impedance mode
and thus will not affect the output of the active tri-state logic
device.
[0005] Measuring voltages of circuits using tri-state logic devices
presents many problems. For example, many tri-state logic buses
have a plurality of conductors located in very small areas, which
creates high concentrations of conductors in these areas. In order
to measure the voltage of more than one conductor within a data bus
at a time, the probes have to be very small. The probes must also
not load the circuits being measured, which typically occurs when a
conventional resistor divider probe is used to measure a tri-state
logic circuit. For example, a conventional probe may load the
output of a tri-state logic circuit that is in the high impedance
mode.
[0006] Some probes use a plurality of devices in their tips in
order to provide high impedance. For example, the tips may have a
plurality active or passive devices located therein. These probes
have many drawbacks when they are used to measure a plurality of
highly concentrated conductors. Probes with a plurality of devices
in their tips are bulky and may not be small enough to measure
voltages on a circuit having a high concentration of conductors.
Furthermore, probes with active devices or even a plurality of
passive devices tend to be expensive and rather difficult to
manufacture.
[0007] Therefore, a device is needed to overcome all or some of the
above-described problems.
SUMMARY OF THE INVENTION
[0008] The invention is directed toward an electronic probe used to
measure voltage over a broad frequency spectrum. The probe may
comprise a probe tip, a transmission line, a termination portion, a
filter, and an impedance device. The transmission line may have a
transmission line first end and a transmission line second end,
wherein the transmission line first end is connected to the probe
tip. The termination portion may comprise a termination portion
first end and a termination portion second end. The termination
portion first end is connected to the transmission line second end
and the termination portion second end is connected to a common
node. The termination portion may comprise a termination resistor
and a termination capacitor connected in series between the
termination portion first end and the termination portion second
end. The filter may comprise a filter first end and a filter second
end. The filter first end is connected to the transmission line
second end and the filter second end is connected to a probe
output. The filter may comprise a filter resistor connected in
parallel with a filter capacitor and a filter inductor connected in
series with the filter resistor and filter capacitor combination.
The impedance device may be connected between the probe output and
the common node and may serve as a load.
[0009] The electronic probe provides high impedance for direct
current voltages because the termination capacitor provides a
direct current block for direct current and low frequency voltages.
At higher frequencies, the impedance of the termination capacitor
drops to an insignificant value. Thus, the impedance into the
termination portion is substantially equivalent to the termination
resistor. The termination resistor is selected to match the
characteristic impedance of the transmission line, thereby reducing
the incident-wave reflections.
[0010] The filter serves to offset the filtering affects inherent
in the termination portion. Without the filter, the gain of the
probe would vary significantly with the frequency of the measured
voltage. By offsetting the filtering affects of the termination
portion, the gain of the probe remains substantially constant over
a broad frequency spectrum.
[0011] The probe may be electrically connected to a measurement
device having an input capacitance associated therewith. The input
capacitance acts as a filter and reduces the gain of the probe at
high frequencies. The input capacitance also causes an impedance
discontinuity that causes energy to reflect back toward the probe
tip. The filter inductor serves to partially offset the effect of
the input capacitance. Thus, the filter inductor offsets the
capacitive discontinuity caused by the input capacitance and
maintains the gain of the probe constant.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a block diagram of a probe being used to measure a
device under test.
[0013] FIG. 2 is a detailed schematic illustration of the probe of
FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
[0014] A block diagram of a non-limiting embodiment of a probe 100
associated with a measuring device 102 is illustrated in FIG. 1.
The measuring device 102 in conjunction with the probe 100 is shown
measuring a voltage in a device under test 106. The device under
test 106 may, as a non limiting example, be a device using
tri-state logic components and circuitry. The device under test 106
may have circuits 108 that generate voltages on an output line 110
wherein the voltages are reference to a common node, which may be a
ground or, more precisely, an AC ground.
[0015] A simplified and non-limiting embodiment of a tri-state
control 112 is illustrated within the device under test 106 and is
used to model a tri-state logic circuit. The tri-state control 112
may control a switch 113. When the circuits 108 outputs high or low
voltages on the output line 110, the tri-state control 112 closes
the switch 113 and shorts an impedance 114. When the circuits 108
enters the high impedance mode, the tri-state control 112 opens the
switch 113 which causes the device under test 106 to enter a high
impedance mode. The high impedance is represented by the impedance
114. The output line 110 may have a node 116 associated therewith.
The node 116 may, as examples, be a conductive land within the
device under test 106 or a terminal, such as a connector used as an
output terminal of the device under test 106. When the device under
test is in the high impedance mode, the impedance into the device
under test 106 referenced from the node 116 is approximately equal
to the impedance 114. The output line 110 is shown to have zero
resistance when the switch 113 is closed. It should be noted that a
resistance, i.e., a resistor, may be located in the output line 110
so that a resistance is associated with the output line 110 when
the switch 113 is closed.
[0016] When the device under test 106 is in use, the voltage on the
output line 110 relative to ground may be representative of binary
information and thus may vary. For example, the voltage may have a
high state of 3.3 volts and a low state of 1.2 volts and may
alternate between the high state and the low state at a frequency
of several hundred megahertz. In addition, the device under test
106 may enter the high impedance mode wherein the switch 113 is
open and the impedance into the device under test 106 is at least
the value of the impedance 114.
[0017] When the device under test 106 is in the high impedance
mode, the probe 100 must not cause the impedance of the output line
110 to be lowered when the probe 100 is connected to the output
line 110. Lowering the impedance of the output line 110 defeats the
purpose of the device under test 106 being in the high impedance
mode. In addition, the probe 100 should have a constant gain from
direct current through high frequencies. Otherwise, the voltages
measured by the measuring device 102 will not reflect accurate
values over a broad frequency spectrum.
[0018] Having described a non-limiting embodiment of the device
under test 106, the probe 100 will now be described. The probe 100
may have a tip 120 associated therewith. A first side 121 of the
tip 120 may be adapted to electrically contact the node 116 of the
device under test 106. For example, the first side 121 of the tip
120 may be pointed so as to contact a land of a printed circuit
board or it may have a connector attached thereto that mates with a
connector on the device under test 106. A second side 122 of the
tip 120 may be electrically connected to a tip resistor R.sub.t.
The tip resistor R.sub.t may be a discrete device or it may be
intrinsic within the tip 120. The tip resistor R.sub.t may be
electrically connected to a first end 124 of an inner conductor 125
of a transmission line 126. The transmission line 126 may have a
shield 128 that is electrically connected to a common node, such as
ground. The transmission line 126 may have an preselected
characteristic impedance, such as fifty or seventy-five ohms.
[0019] A second end 129 of the inner conductor 125 of the
transmission line 128 may be electrically connected to an input end
or first end 130 of a matching network 132. The matching network
132 may have an output end or second end 134 that is electrically
connected to a connector 140, such as a BNC connector. The
connector 140 may provide an electrical transition between the
probe 100 and the measuring device 102. As described in greater
detail below, the matching network 132 serves to terminate the
transmission line 126. In the situation where the device under test
106 is a tri-state logic device, the matching network 132 must
compensate for the high impedance mode of operation by providing a
high impedance input. In addition, the matching network 132 must
maintain the gain of the probe 100 constant over a broad frequency
spectrum in order to assure that voltages measured by the measuring
device 102 are accurate over the frequency spectrum.
[0020] A more detailed schematic illustration of a non-limiting
embodiment of the matching network 132 is illustrated in FIG. 2. As
shown in FIG. 2, the second end 129 of the inner conductor 125 of
the transmission line 126 is electrically connected to a
termination portion 150 and a filter 154. Both the filter 154 and
the termination portion 150 are components of the matching network
132. The termination portion 150 may serve to match the
transmission line 126 at high frequencies. The filter 154 may serve
to compensate for the filtering effect of the termination portion
150 so as to minimize gain fluctuations of the probe 100 due to the
frequency response of the termination portion 150. This
compensation provides for the gain of the probe 100 at direct
current to be substantially the same as the gain of the probe 100
at high frequency. As described in greater detail below, the flat
gain is achieved by having the time constant of the filter 154
approximately equal to the time constant of the termination portion
150. Accordingly, the gain of the probe 100 remains substantially
flat over a broad frequency spectrum.
[0021] The termination portion 150 may have a resistor R.sub.1 and
a capacitor C.sub.1 connected in series between the second end 129
of the inner conductor 125 and a common node, such as an AC ground.
The capacitor C.sub.1 has a high impedance at low frequencies and a
low impedance at high frequencies. Accordingly, at low frequencies,
the impedance into the termination portion 150 is very high and at
direct current the impedance is ideally infinite. At high
frequencies, the impedance of the capacitor C.sub.1 drops to an
insignificant value relative to the value of the resistor R.sub.1.
Accordingly, the impedance into the termination portion 150 at high
frequencies is approximately equal to the value of the resistor
R.sub.1, which is selected to match with the characteristic
impedance of the transmission line 126.
[0022] The filter 154 may have an inductor L.sub.P, a capacitor
C.sub.2, and resistors R.sub.2 and R.sub.3. The resistor R.sub.2
may be connected in parallel with the capacitor C.sub.2. The
resistor R.sub.3 may be connected between the junction of the
resistor R.sub.2 and the capacitor C.sub.2 and a common node, such
as the aforementioned AC ground. An output node 160 may be located
at the junction of the capacitor C.sub.2 and the resistors R.sub.2
and R.sub.3. The output node 160 may, as a non-limiting example, be
a BNC connector. As described above, the filter 154 serves to
maintain a substantially flat frequency response of the probe 100
over a wide frequency spectrum. The flat frequency response of the
probe 100 is achieved, in part, by matching the frequency of the
pole set by the capacitor C.sub.2 to the frequency of the zero set
by the capacitor C.sub.1. Accordingly, as was described above, the
time constant of the termination portion 150 is the same or
substantially equal to the time constant of the filter 154.
Therefore, the gain of the probe 100 remains substantially constant
over a broad frequency spectrum. As is described below, the
inductor L.sub.P serves to compensate for input capacitance of the
measuring device 102.
[0023] As shown in FIG. 2, the measuring device 102 may be
electrically connected to the output node 160 of the probe 100. The
measuring device 102 may have an input capacitance C.sub.L
associated therewith, wherein the value or approximate value of the
input capacitance C.sub.L is known. The input capacitance C.sub.L
affects the frequency response of the probe 100 by causing an
impedance discontinuity that in turn causes energy to reflect from
the measuring device 102 toward the transmission line 126.
[0024] The inductor L.sub.P is used to offset or otherwise
compensate for the effect of the capacitance C.sub.L. The inductor
L.sub.P thus serves to "peak" the frequency response of the probe
100 so as to extend the relatively flat frequency response of the
probe 100, thus compensating for the attenuation caused by the
capacitance C.sub.L. Accordingly, the impedance discontinuity
caused by the capacitance C.sub.L is minimized and less energy is
reflected from the measuring device 102. In one non-limiting
embodiment, the value of the inductor L.sub.P is selected to be
approximately equal to 0.4 multiplied by the value of the resistor
R.sub.1 squared and again multiplied by the value of the series
combination of the capacitor C.sub.2 and the capacitance C.sub.L.
It should be noted that the value of 0.4 may be varied depending on
the intended use of the probe 100.
[0025] Having described the components of the probe 100, the
operation of the probe 100 will now be described.
[0026] The probe 100 may be used to measure voltages at frequencies
from direct current through relatively high frequencies. At direct
current and low frequencies, the impedance of the capacitor C.sub.1
is very high and may be considered to be an open circuit or
infinite impedance. The inductor LP has a very low impedance at
direct current and low frequency and may be considered to be a
short circuit at direct current and low frequencies. Accordingly,
at direct current and low frequencies, the gain of the probe 100 is
established by resistors R.sub.t, R.sub.2, and R.sub.3.
[0027] At higher frequencies, the impedance of the capacitor
C.sub.1 decreases to where the capacitor C.sub.1 can be assumed to
be a short circuit or close to zero impedance. Accordingly, the
resistor R.sub.1 serves as a terminating impedance for the
transmission line 126. Without the filter 154, the gain of the
probe 100 will not be flat as the frequency is increased due to the
zero established by the termination portion 150, and more
specifically, the zero associated with the capacitor C.sub.1.
Therefore, the filter 154 and more specifically, the capacitor
C.sub.2, establishes a pole to compensate for the zero of the
termination portion 150. Accordingly, the gain of the probe 100
remains relatively constant over a broad frequency spectrum. The
inductor LP has virtually no effect on the poles and zeros of the
matching network 132. As described below, the function of the
inductor LP is primary to compensate for the affect the input
impedance of the measuring device 102, which is represented by the
capacitor C.sub.L.
[0028] The description above assumes ideal conditions of the
measuring device 102 wherein there is no input capacitance C.sub.L
into the measuring device 102. Under more realistic conditions, the
input to the measuring device 102 has the capacitance C.sub.L
associated therewith. The capacitance C.sub.L, in summary, creates
a capacitance discontinuity through the capacitor C2 which inhibits
the ability of the termination portion 150 to terminate the
transmission line 126. Accordingly, the capacitance C.sub.L causes
the gain of the probe 100 to decrease as the input frequency is
increased. The inductor L.sub.P serves to offset the capacitive
discontinuity of C.sub.L. Accordingly, the inductor L.sub.P will
cause the gain of the probe 100 to remain substantially constant
over a greater frequency spectrum.
[0029] Having described the operation of the probe 100,
non-limiting examples of values for the components within probe 100
will now be provided. In a non-limiting embodiment described
herein, the characteristic impedance of the transmission line 126
may be seventy-five ohms. A non-limiting example of values of other
components are listed in Table 1.
1 TABLE 1 Component Value R.sub.t 422 ohms R.sub.1 75 ohms C.sub.1
0.01 microfarads C.sub.2 168 picofarads R.sub.2 33.64 k ohms
R.sub.3 5 k ohms C.sub.L 4 picofarads L.sub.P 10 nanohenries
[0030] The probe 100 described herein is an electronic probe that
does not overload tri-state circuits at low frequency. Thus, the
probe 100 overcomes the low impedance loading limitations of
conventional probes, including resistive divider probes, and may be
used to measure voltages within circuits using tri-state logic
devices. The probe 100 may be limited solely to a having a limited
number of passive component, thus, no relatively expensive active
components are required within the probe 100. The limited number of
components allows the overall size of the probe 100 to be
minimized. Therefore, a plurality of probes 100 may be used to
measure a plurality of voltages within tight confines. In addition,
the use of a single probe tip resistive component, the tip resistor
R.sub.t, serves to further minimize the size of the probe 100.
[0031] While illustrative and presently preferred embodiments of
the invention have been described in detail herein, it is to be
understood that the inventive concepts may be otherwise variously
embodied and employed and that the appended claims are intended to
be construed to include such variations except insofar as limited
by the prior art.
* * * * *