Ultra-broadband differential voltage probes

Passive balanced probes are disclosed for use with a signal analysis device. The probes are very low cost relative to typical commercially available probes and provide an extremely flat response over a frequency range of approximately 0 to at least 1.5 gigahertz. The probes include a probe body constructed primarily from conventional components, a first surface mount resistor electrically connected between a probe tip and a center conductor, and two surface mount resistors electrically connected and parallel between the center conductor and a conductive shield. The probes further include a coaxial cable for connection to an instrument combiner or other instrument connection device.

TECHNICAL BACKGROUND

The present invention generally relates to high frequency measurement equipment, and, more particularly, the invention relates to wideband differential voltage probes.

BACKGROUND OF THE INVENTION

Oscilloscopes, spectrum analyzers, and other signal analysis devices are generally used to measure a voltage signal between two points with one of the points often being referenced to earth ground. However, in many cases it is advantageous to measure the voltage between two points in a circuit with neither of the points being the ground to which the signal analysis device is referenced.

If the ground of the signal analysis device is connected to the ground of the circuit under test, a ground loop may result. The ground loop may act as a secondary transformer winding, thus producing a magnetic field and creating a signal in nearby conductors, such as the probe used to connect the signal analysis device with the circuit under test. Additionally, current and impedance within the ground loop may produce a signal component in the signal analysis device measurement.

Floating the ground of either the circuit under test or the signal analysis device may open the ground loop; however, floating the ground may result in an electrical shock hazard as one of the devices no longer has a connection to ground through which an electrical short could be carried. Additionally, even with a floating ground, high frequency signals may still act as if they were coupled to ground by creating a ground loop with stray capacitance relative to earth ground, for example, capacitance introduced by the operator's hand holding a probe.

Another problem with probes, including high impedance FET probes, relates to input impedance. While some conventional passive probes utilize a high input impedance, for example 10 megohms in parallel with 10 picofarads, such probes are not high impedance at higher frequencies and generally have significant measurement error at higher frequencies because of the resonance generated between the probe's input capacitance and the inductance of a ground lead. This resonance results in a significant reduction in input impedance near the resonant frequency and often introduces ringing on the measurement waveform.

A typical method of overcoming the above problems while measuring the voltage between two circuit nodes is to replace the conventional probe with a passive, predominantly resistive, balanced differential probe pair. A typical commercially available balanced probe pair includes a pair of coaxial cables having a probe end and an output end. The output end of the coaxial cables are generally fed to a 180° combiner in order to provide a differential input into a single oscilloscope channel or spectrum analyzer. However, parasitic capacitances and inductances are critical to probe performance, and typical commercially available balanced probe pairs and lower capacitance FET probes are very expensive, costing between $250.00 to over $2,400.00.

The lower-cost commercially available probes tend to have a limited useful frequency range because of parasitic electrical characteristics. The more expensive probes having improved wideband response are cost prohibitive for many applications.

SUMMARY OF THE INVENTION

The present invention provides passive balanced probes for use with a signal analysis device, for example an oscilloscope or spectrum analyzer. The inventive probes are very low cost relative to typical commercially available probes and provide an extremely flat response over a frequency range of approximately 0 to at least 1.5 gigahertz. The probes include a probe body constructed primarily from conventional radio-frequency (“RF”) connector components, and a termination resistive input network, including a first surface mount device (“SMD” or “surface mount”) resistor electrically connected between a probe tip and a center conductor, and one or more SMD resistors electrically connected in parallel between the center conductor and a conductive shield end closest to the probe tip. The probes further include coaxial cables for connection to a signal analysis device. For use with signal analysis devices having a single input, such as a spectrum analyzer, the coaxial cables may be connected to a broadband 180° RF combiner.

Each probe in a balanced probe pair includes matched impedance components and is coupled with a matched impedance coaxial cable, for example standard 50 ohm components and coaxial cable; however, other impedance values may alternatively be used, for example, 75 ohms. Additionally, other types of coaxial structures may be substituted for the RF connectors. The outer conductive shields of the coaxial cables are electrically joined along the cable length in order to reduce parasitic inductance. The cables may include a commercially available matched impedance adapter at an end opposite the probes, in order to couple the balanced probe pair to a signal analysis device input or combiner. Use of matched impedance components and component interfaces throughout minimizes standing waves and thus measurement error.

The probe body may also include an outer conductive probe body and a distal probe tip housing supporting a conductive probe tip which protrudes from the probe housing. The probe tip housing may be a non-conductive sleeve housing a center conductor which is electrically coupled through the first SMD resistor to the coaxial cable center conductor. The outer conductive probe body electrically couples the outer shield of the various probe body components to keep impedance low, mechanically stabilizes the probe body, and increases the outer diameter of the probe body in order to minimize stray capacitance introduced by holding the probe. The probe body construction facilitates placement of the termination resistive input network at the very end of the controlled impedance coaxial portion of the probe.

The use of SMD resistors and the location of the SMD resistors minimizes parasitic inductance and capacitance, thereby providing enhanced performance over a wide bandwidth. Specifically, using surface mount components that are soldered to the probe tip and/or center conductor and outer conductor or shield, avoids component leads that typically introduce additional parasitic inductance and capacitance.

Additionally, the use of commercially available connectors, for example coaxial BNC connectors, and other adapters to construct the probe and cable apparatus minimizes the cost of producing an impedance matched and balanced probe pair with a desirable signal-to-noise ratio (“SNR”) while not sacrificing the desired performance across a wide frequency range.

A first exemplary embodiment provides a passive test probe apparatus for use with a signal analysis device, including a conductive probe body, a conductive probe tip supported by and protruding from the probe body, a center conductor supported by and substantially electrically insulated from the probe body, and a first surface mount resistor electrically connected between the probe tip and the center conductor.

Another exemplary embodiment provides a balanced test probe apparatus, including a first probe body having a first surface mount resistor electrically connected between a first probe tip and a first center conductor, a first coaxial cable electrically connected to the first probe at a first end, a second probe having a second surface mount resistor connected between a second probe tip and a second center conductor, and a second coaxial cable of substantially equal length with the first coaxial cable, the second coaxial cable electrically connected to the second probe at a second end, the second surface mount resistor having a resistance value equal to the first surface mount resistor.

Yet another exemplary embodiment provides a passive test probe apparatus for use with a signal analysis device, including a probe body, a conductive probe tip supported by and axially protruding from the probe body, a center conductor supported by the probe body, a first surface mount resistor electrically connected between the probe tip and the center conductor, an outer conductive shield coupled to the probe body, the outer shield insulated from the probe tip and the center conductor, and a second and a third surface mount resistor electrically connected in parallel between the center conductor and a distal end of the conductive shield.

Advantageously, the present invention provides a low-cost ultra-broadband probe having an extremely flat response characteristic over a frequency range of approximately 0 to at least 1.5 gigahertz. The probe may be constructed of commercially available components selected and assembled to minimize parasitic inductance and capacitance and to maximize the amount of power carried from the point of measurement to the signal analysis instrument.

DESCRIPTION OF INVENTION

The embodiment disclosed below is not intended to be exhaustive or limit the invention to the precise forms disclosed in the following detailed description. Rather, the embodiment is chosen and described so that others skilled in the art may utilize its teachings.

Referring toFIG. 3, balanced probe pair assembly10includes probes12, coaxial cables14, and may include ground lead16. Assembly10may be coupled to input channel connector22of signal analysis device18using, for example, combiner20. Referring toFIG. 2A, each probe12includes probe body24, probe end25, body sleeve13, and base connector15.

Referring toFIG. 1, each probe end25includes a probe tip sleeve26, a coupling probe end connector50and a probe tip27. Probe tip27includes an elongate tip28, a threaded sleeve29, a distal tip thread30, and a proximate tip thread31. Threaded sleeve29engages a distal tip thread30and proximate tip thread31engages a distal sleeve thread32of probe tip sleeve26. Probe tip27is constructed of a highly conductive material, for example, nickel-plated brass, and probe tip sleeve26is constructed of a nonconductive material, for example, ABS plastic. Probe tip27is electrically connected to a center conductor34through a wire36and a first surface mount resistor40, for example, by soldering. Wire36may be, for example,30gauge copper wire. Probe tip27and center conductor34are generally electrically isolated from an outer conductor shield38which circumscribes center conductor34; however, the termination resistive input network is electrically connected between center conductor34and outer conductor shield38.

The resistive input network includes first surface mount resistor40, for example a 475 ohm surface mount resistor, which is electrically coupled in series between conductive probe tip27and center conductor34of probe end connector50. Additionally, one or more SMD resistors may be electrically connected between center conductor34and outer conductive shield38of probe end connector50. For example, two 100 ohm resistors, first and second surface mount resistors42and44, may be electrically coupled in parallel between center conductor34and the outer conductive probe body, which includes outer conductive shield38, providing appropriate termination for 50 ohm matched impedance components. The combination of the 475 ohm first surface mount resistor40with the two 100 ohm first and second surface mount resistors42and44provides a probe attenuation factor of approximately 20×; however, other factors may be achieved with values other than 475 ohms. Additionally, other resistor values may be used for first and second surface mount resistors42and44to provide a matched impedance value other than 50 ohms.

The electrical connections between surface mount resistors40,42and44and probe tip27and conductor34and shield38may be, for example, a soldered connection41. Additionally, resistors40,42and44may also be glued, for example epoxyed, in place within outer conductor shield38of probe end connector50. Depending on fit, resistors42and44may also be canted at an angle between center conductor34and shield38. Advantageously, using surface mount resistors for providing input termination at the distal termination of coaxial shielding in a balanced electrical probe pair minimizes parasitic inductance and capacitance, thereby improving response characteristics over a wide frequency range.

The exemplary probe end25includes male BNC connector58and oppositely located probe end connector50having female SMA adapter56forming a portion of outer conductor shield38and center conductor34. Center conductor34in the exemplary embodiment is a hollow cylindrical pin. Insulator37may be located between center conductor34and outer conductor shield38. In the exemplary embodiment, resistors42and44rest against insulator37. Probe end connector50may be, for example, Part No. 16N2740, manufactured by SPC Technology of Chicago, Ill. Probe tip sleeve26and probe tip27may be, for example, Part No. 35N776, manufactured by SPC Technology, Inc.

Assembly of probe tip27and probe end connector50is facilitated by a non-conductive structure, probe tip sleeve26. Sleeve26is secured internally at opposite ends with proximate sleeve thread33engaging female SMA adaptor56and distal sleeve thread32engaging proximate tip thread31. Alternatively, a different or additional fastening structure may be used, for example, epoxy. Wire36transits the hollow interior of sleeve26and couples resistor40and probe tip27.

Referring toFIG. 2B, each probe12may include a plurality of commercially available connectors and adapters that each form a portion of probe body24, are impedance matched, and conduct and shield the electrical signal received at elongate tip28. Specifically, probe body24may include coaxial BNC type connectors50,52and54for coupling probe end25with base connector15. Connectors50,52and54may be constructed primarily of a conductive material such as nickel-plated brass and are impedance matched, for example, 50 ohms.

Finally, base connector15may be, for example, an RF bulkhead adapter, such as Part No. 93F1409, available from Amphenol, of Wallingford, Conn. Exemplary base connector15includes female BNC adapters68and70at opposite ends. Female adapter68of base connector15may be coupled to male adapter66of BNC connector54. Alternatively, types of low cost impedance matched components that minimize parasitic capacitance and inductance may be substituted for connectors15,50,52, and54.

Probe12may also include body sleeve13, for example, a 31/4-inch long and ½ ID metal pipe such as copper which, as shown inFIG. 2A, may be securely received over connectors15,50,52and54. Body sleeve13provides an outer conductive shield as well as structural rigidity and a smooth cylindrical shape for probe body24. The increased diameter of probe body24formed by sleeve13minimizes parasitic capacitance introduced by a user's hand holding probe12. Probe body24may also be insulated, for example by nonconductive heat shrink or another suitable material or coating.

Referring toFIG. 3, coaxial cables14, for example Part No. RG-223, available from Alpha Wire of Elizabeth, N.J., may be used to couple probes12to combiner20, or to an input port of measurement device18. Cables14may include male BNC adapter72for coupling to female BNC adapter70of probe12and female BNC adapter73of combiner20or input channel connector22. Alternatively, other suitable connectors may be used at opposite ends. It is very important that probes12and cables14be virtually the same materials and dimensions in order to provide proper electrical balancing of the electrical characteristics of probe pair assembly10. Variations between probes12or cables14may cause phase errors, signal amplitude errors, and other electrical errors in the measured signal. Between opposite ends of cables14, the insulative coating around the outside coaxial sheath of cables14may be stripped away and the exposed outer shielding conductor can be electrically coupled at adjacent locations, for example, at junctions75, periodically along the cable length, for example every 3 inches. The central but substantial portion of cables14may then be insulated with shrink-wrap74, or another suitable insulating material, for insulating solder junctions75therealong.

The balanced probe pair assembly may be coupled to combiner20or another suitable input device of signal analysis device18. As shown inFIG. 3, combiner20is coupled with input channel connector22, thereby providing a differential input of the circuit nodes probed by probes12of balanced probe pair assembly10. Although ground connector16may be included with probe pair assembly10, ground connector16would generally only be utilized for single probe measurements, very high common-mode voltage measurements, for example, electrostatic discharge testing, or other similarly indicated measurement circumstances.

Referring toFIG. 4A, a typical frequency response for a commercially available balanced probe pair is shown for the purposes of comparison withFIG. 4B, which under the same test conditions and illustrative chart display shows the much flatter response of the lower-cost inventive balanced probe pair assembly10. The frequency range forFIGS. 4A and 4Bis 10 MHz to 500 MHz and the attenuation setting is 10 dB.FIGS. 4C and 4Dalso demonstrate the performance of probe pair assembly10.FIG. 4Cillustrates 10 MHz to 2 GHz for 30 dB andFIG. 4Dillustrates 10 MHz to 3 GHz for 30 dB.FIGS. 4B–4Dillustrate that probe pair assembly10has a flatter response characteristic up to at least 1.5 GHz than prior art probes.

Although described in the exemplary embodiments, it will be understood that various modifications may be made to the subject matter without departing from the intended and proper scope of the invention. Accordingly, it will be understood that other embodiments may fall within the scope of this invention, which is defined by the appended claims.