Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Meter protection or fraud combatting
Reexamination Certificate
2002-12-05
2004-05-04
Karlsen, Ernest (Department: 2829)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Meter protection or fraud combatting
C324S072500, C324S149000
Reexamination Certificate
active
06731104
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention related generally to high frequency probing systems and more particularly to a high frequency probing system having electrical over stress (EOS) and electrostatic discharge (ESD) protection.
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, Oreg. 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 probing system should automatically provide EOS and ESD protection for the sampling head and signal connectivity to sampling head through the ordinary use of the probe for making measurement. The measurement probing system should also allow the use of TTL logic signals for automated measurement applications.
SUMMARY OF THE INVENTION
Accordingly, the present invention is a measurement probing system having electrical over stress and electrostatic discharge protection for input circuitry of the measurement test instrument. The measurement probing system has a measurement probe that provides electrical over stress (EOS) and electro-static discharge (ESD) protection control and a control module providing EOS/ESD protection for the input circuitry of the measurement test instrument. The measurement probe has a housing in which are disposed a spring loaded coaxial probe assembly and a pressure sensor. The housing has an internal cavity extending the length of the housing with the cavity exposed at opposing ends of the housing. The spring loaded coaxial probe assembly is formed from a semi-rigid coaxial cable having a center signal conductor, an intermediate dielectric material surrounding the central signal conductor and an outer shielding conductor surrounding the dielectric material. A portion of the outer shielding conductor and dielectric material is removed from one end of the semi-rigid coaxial cable forming a probing tip. The other end of the semi-rigid coaxial cable receives a threaded connector for coupling the semi-rigid coaxial cable via a coaxial cable to the control module. The coaxial probe assembly is disposed in the housing 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 pressure sensor has first and second electricall
Bucher William K.
Karlsen Ernest
Tektronix Inc.
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