Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2002-12-05
2004-05-11
Pert, Evan (Department: 2829)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S761010
Reexamination Certificate
active
06734689
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention related generally to measurement probes and more particularly to a high frequency measurement probe providing signal control for an electrical over stress (EOS) and electrostatic discharge (ESD) protection module.
Ultra high speed sampling heads used in time domain reflectometry typically dictate extremely low parasitic capacitances. This introduces unique problems. Sampling devices are much more sensitive to static charges residing on a device under test before a test probe touches it. The small geometry of the sampling diodes in the sampling heads often dictate low breakdown voltages. The low parasitic capacitance at the sampling head input means that for a given device under test (DUT) static charge, there will be a higher transient voltage at the sampler input because of the reduced charge sharing effect. It is therefore important to neutralize any static charge on the device under test before the sampling head input is coupled to the device under test.
Conventionally, users are advised to take all anti-static precautions including purchasing and installing antistatic equipment and employing anti-static procedures. Such equipment and procedures include using ionized airflow devices to reduce the DUT static charge on isolated conductors, ground straps on the test bench and the operator, and an anti-static mat around the test bench. Another piece of anti-static equipment that may be used with sampling heads is the SIU600 Static Isolation Unit, manufactured and sold by Tektronix, Inc., Beaverton, Oregon or the Model 1201 Static Isolation Unit, manufactured and sold by Picosecond ATE, Inc. Beaverton, Oreg. Referring to
FIG. 1
, there is representatively shown the static isolation unit
10
that includes an interface box
12
, foot pedal
14
, and a power adapter
16
. The power adapter
16
is connected to a standard electrical outlet to provide DC power to circuitry within the interface box
12
. An RF probe
18
for probing a device under test
20
, such as circuit runs
22
on a circuit board
24
, is connected to the interface box
12
. A coaxial cable
26
couples the interface box
12
to a TDR sampling head
28
mounted in a sampling oscilloscope
30
. The foot pedal
14
is connected to the interface box
12
for coupling the output of the device under test
20
through the interface box
12
to the sampling head
28
. When the foot pedal
14
is in the normal position (not pressed), the input of a buffer circuit is coupled to a TTL logic high that cuts off current flow in a drive circuit to an RF relay in the interface box
12
. The normally open RF relay coupled the probing tip of the RF probe
18
to electrical ground through a 50&OHgr; termination resistor
32
. Positioning the probing tip of the RF probe
18
on the DUT
20
discharges any static charge stored in the DUT
20
. Pressing the foot pedal
14
closes a low resistance switch and allies a TTL active low signal to the buffer circuit that activates drive circuitry in the interface box
12
that energizes the relay and connects the probing tip of the RF probe
18
to the sampling head input, allowing a measurement to be made. The circuitry in the interface box
12
operates under TTL active low logic allowing the foot pedal
14
to be replaced with a TTL external source. The use of TTL active low logic requiring the use of a low resistance switch in the foot pedal
14
for proper operation of the interface box
12
circuitry.
Proper use of the static isolation unit
10
prevents ESD and electrical over stress (EOS) static charge from damaging or destroying the sampling head. The main difference between ESD and EOS is that EOS can occur at a much lower voltage level that ESD. ESD static voltages are typically several hundred to several thousand volts, whereas EOS static voltages may be as low as 15 to 30 volts. The sampling diodes in the sampling head has a breakdown voltage of approximately 9 volts. EOS static discharge causes microscopic damage to the semiconductor layer of the sampling diodes in the sampling head providing a leakage current path around the semiconductor Schottky junction. Over time, the incremental damage of each occurrence of the EOS static discharge continues to degrade the performance of the semiconductor device until the leakage current causes excessive measurement error.
In TDR measurements of a device under test, an operator places the RF probe on the test point with operator's foot off of the foot pedal
14
. The probing tip of the RF probe is coupled to electrical ground through the interface box. Once the operator has properly placed the probe on the test point, the operator depresses the foot pedal with his or her foot and circuitry in the interface box couples the probing tip of the RF probe to the sampling head circuitry. After the measurement is made, the operator removes his or her foot from the foot pedal before removing the RF probe from the test point to disconnect the probe from the sampling head and reconnect the probe to electrical ground. However, in a production environment where repetitive probing is done by the operator, an operator may accidentally keep the foot pedal
14
depressed while repositioning the probe or moving the probe from one test point to another. This allows ESD and EOS voltages on the device under test to be coupled to the sampling head causing damage to the sampling diodes.
One solution is to move the low resistance switch in the foot pedal into the RF probe
18
. This would result in a bulkier probe design requiring the placement low resistance switch in the probe along with a hand operated mechanical actuator to allow an operator to activate the switch for measurements. Such a design does not eliminate the possibility of an operator inadvertently keeping the low resistance switch closed while moving from one test point to another.
What is needed is a fail-safe electrostatic discharge and electrical over stress static discharge solution that prevents electrostatic discharges and electrical over stress static discharges from a sampling head input. Activation of the EOS and ESD protection should be incorporated into the measurement probe thus eliminating the need for a foot pedal. The measurement probe should automatically provide EOS and ESD protection for and signal connectivity to sampling head through the ordinary use of the probe for making measurement.
SUMMARY OF THE INVENTION
Accordingly, the present invention is a measurement probe providing signal control to an electrical over stress and electrostatic discharge protection module for coupling the measurement probe to input circuitry of the measurement test instrument. The measurement probe has a spring loaded coaxial probe assembly formed from a semi-rigid coaxial cable. The semi-rigid coaxial cable has a probing tip at one end and a threaded connector at the-other end for receiving a coaxial cable coupled to the electrical over stress and electrostatic discharge protection module. A compression spring is positioned on the semi-rigid coaxial cable for biasing the spring loaded coaxial probe assembly. A housing having an internal cavity extends the length of the housing and is exposed at opposing ends of the housing. The spring loaded coaxial probe assembly is disposed within the internal cavity with the probing tip extending from one end of the housing and the threaded connector extending from the other end of the housing. The housing is movable from a first position to a second position relative to the spring loaded coaxial probe assembly from pressure applied to the probing tip of the measurement probe in contact with a device under test. A pressure sensor has a first electrically conductive contact secured and electrically coupled to an outer shielding conductor of the semi-rigid coaxial cable and a second electrically conductive contact positioned and secured within the internal cavity of the housing The second electrically conductive contact is coupled to the control module via an electrical conductor. The probing tip is couple
Bucher William K.
Hollington Jermele M.
Tektronix Inc.
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