The present invention related generally to signal acquisition probes and more particularly to a multi-channel signal acquisition probe usable for acquiring logic signal from a device under test.
Multi-channel signal acquisition probes are generally used to acquire multiple signals from a device under test. A multi-channel general purpose probe has a single connector one end that couples to a measurement test instrument, such as a logic analyzer, mixed signal oscilloscope or the like. The other end of the multi-channel general purpose probe has individual cables with electrical connectors disposed in housings at the ends of the cables for connecting to separate location on the device under test. High performance multi-channel general purpose probes have a controlled electrical environment the length of the probe that is generally achieved using coaxial signal cables.
Most high performance multi-channel general purpose probes available today have inherent construction expenses and related performance issues. In one design, the high performance multi-channel probe has a ribbon cable having individual coaxial cable ganged together. One end of the ribbon cable is coupled to a connector that interfaces with the measurement test instrument. The other end of the ribbon cable is attaches to a midpoint board. The central signal conductor and outer shielding conductor of each coaxial signal cable is soldered to connections of the midpoint board. Individual coaxial signal lines are also soldered to the midpoint board to breakout the individual coaxial signal lines for connecting to the device under test. The midpoint board may also include a signal ground line that is common to all of the coaxial signal cables to allow a user to connect to a reference ground close to the device under test. The individual coaxial signal cables are electrically coupled to electrical circuitry in the housing at the ends of the cables
A disadvantage of the above design is that there are multiple discontinuities in the signal path due to the soldering of the coaxial signal cables of the ribbon cable and the individual coaxial signal cables to the midpoint board and the soldering of the coaxial signal cable and the ribbon cable to connections at the ends of the cables. Coaxial cables are difficult and labor intensive to attach to a board because there are inner and outer conductors with insulating material in between that need to prepared and connected without shorting the inner and outer conductors together. Attaching ribbonized coaxial signal cables to the board is even more complex and expensive.
Another approach to high performance general purpose probes available in the market has unbroken, individual coaxial cables running from the measurement test instrument connector to the individual channel connectors. This has the advantage of reducing the number of interconnect related impedance discontinuities between the probe input and output. The assembly of the individual probes is easier and the electrical performance is better since there are two coaxial cable lands per cable. However, this design has the disadvantage of longer individual coaxial cables that are difficult to manage and keep from being tangled, caught or damaged in normal use. Existing products using this approach deal with this disadvantage by enclosing the portions of the coaxial signal cables that are normally ganged or grouped together in a loosely fitting sleeve to keep the cables from wandering. However, the sleeve represents extra material and labor expense that diminishes the savings in the electrical assembly. This approach also has the disadvantage in that there is no place to connect a signal ground common to all of the channels unless it is all the way back at the connection with the measurement test instrument which is relatively far from the device under test. The P6417 General Purpose Probe, manufactured and sold by Tektronix, Inc., Beaverton, Oreg., is an example of a high performance multi-channel general purpose probe using this approach.
U.S. Pat. No. 5,223,787 describes a high-speed, low-profile test probe for use in acquiring signals from a device under test. The test probe has a substrate with electrical circuitry thereon and is overmolded with insulating material. A socket connector is electrically coupled to the substrate for connecting to test pins on a device under test. Signal and ground wires are electrically coupled to the substrate for providing electrical ground to the substrate and coupling a signal under test to a measurement test instrument. A notch is formed in the insulating material to expose a ground connection on the substrate. A ground lead may be coupled to the ground connection exposed in the notch. The patent also describes a probe holder for ganging multiple test probes together for multi-channel probing. The probe holder has an electrically conductive clip that electrically couples the exposed ground connections of the individual probes together for the shortest possible ground connections between the ground pins on the device under test and the ground connections of the probes.
The high-speed, low profile probe uses individual signal and ground lines which have poorly controlled signal paths as compared to high performance multi-channel general purpose probes. Further, the probe holder with the electrically conductive ground clip only works when the probes are ganged together in the holder. For applications where the probes are distributed on probing points across the device under test, this solution does not work.
What is needed is a high performance multi-channel general purpose signal acquisition probe that overcomes the limitations of the current high performance general purpose probe designs. The high performance multi-channel general purpose signal acquisition probe should limit the number of signal discontinuities by reducing the number of solder connections of the coaxial signal probes. Further, the high performance multi-channel general purpose signal acquisition probe should provide a common signal ground for all of the coaxial signal probes that is substantially closer than the measurement test instrument.