Abstract:
A signal probe that does not require a direct ground contact is disclosed. The probe consists of a resistive measuring tip, an amplifier and a probe body which acts as an antenna providing a ground reference for the probe. The frequency of the signal measured may be compensated for by adjusting the resistive tip in the probe which varies the shunt capacitance between the resistor and the probe body.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates, in general, to a signal probe, and more particularly, to an indirectly grounded signal probe for use in measuring DC and AC up to several GHz frequency. 
     2. Description of the Background 
     There are many signal probes known in the prior art. However, all of the prior art probes require some type of direct ground connection whether it be by separate connector or by using the body of the person using the probe (U.S. Pat. No. 4,066,953). 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide an apparatus and method that does not require a direct ground to effectively operate. 
     A further object of the present invention is to provide an apparatus and method that is more economical to produce and operate. 
     Still another object of the present invention is to provide an apparatus and method for frequency compensation in measurements. 
     Yet another object of the present invention is to provide an apparatus and method having an economically improved tip design. 
     The above and other objects and advantages of the present invention are provided by an apparatus and method of testing line signals without requiring a direct ground contact. 
     A particular embodiment of the present invention comprises an apparatus and method for testing line signals without requiring the use of a direct ground connection. This is accomplished through the use of a probe having a greatly increased input impedance, thus, the loading effect on the circuit under test is correspondingly reduced. A probe tip that is easily replaceable is inserted in the end of the probe. Contained in the probe is circuitry for compensating and amplifying the signal measurements. In this particular embodiment the body of the probe is used as an antenna to provide a ground reference for the device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the basic concept of a probe embodying the present invention; 
     FIG. 2 is the equivalent schematic diagram of the probe of FIG. 1; 
     FIG. 3 is a graph of the reflection coefficient of a hand held probe embodying the present invention; 
     FIG. 4 is a graph of the measurement error for varying frequencies being measured with a probe embodying the present invention; 
     FIG. 5 is a schematic diagram of a wide band signal probe embodying the present invention; 
     FIG. 6 is a portion of the schematic of FIG. 5 illustrating the tip of the probe; and 
     FIG. 7 is a side view with portions being broken away of a probe body, embodying the present invention, illustrating the tip of the probe. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, a block diagram of the basic concept of a probe, generally designated 10, embodying the present invention is illustrated. The basic concept behind the indirectly grounded signal probe is to make the antenna effect of the probe body serve as the ground reference. Here the device is illustrated as having a probe body 11, an amplifier 12 and a tip 13. A probe cable 14 is coupled to probe 10 and links probe 10 to a scope 15, or other measuring device. Tip 13 is placed on a point 17 to be tested. A ground plane 16 is the ground relative to the signal at point 17. This procedure, as illustrated below, performs satisfactorily when the input impedance of the probe is much greater than the antenna impedance. 
     Referring now to FIG. 2, a schematic diagram of the probe of FIG. 1 is illustrated. Here Z p  is the input impedance of probe 10; A is assumed to be a perfect amplifier with a voltage gain of A; Z a  is the antenna impedance relative to the ground plane; and Z 1  is the load impedance. From this circuit the load voltage, V L , and is defined by the equation; ##EQU1## which reduces to: 
     
         V.sub.L ≈AV.sub.i if Z.sub.a &lt;&lt;Z.sub.p. 
    
     The probe body antenna impedance is affected by several factors: the diameter of the body; the length of the body; the proximity of the ground plane; the frequency of the signal; the loading factor; and the angle of the probe. Although analytical methods to determine the above could be derived the method used in the present case was to design a probe and measure the parameters. In FIG. 3, the measured result of a reflection coefficient of a probe embodying the present invention is illustrated. A reflection coefficient of greater than 6 dB can be maintained over the frequency range of 0 to 4 GHz. The 6 dB reflection coefficient corresponds to an effective antenna impedance of less than 150 ohms over the 0 to 4 GHz range. Using equation (1), above, to calculate the measurement error (ME) caused by a finite amplifier impedance and a non-zero antenna impedance results in the equation: ##EQU2## which reduces to: ##EQU3## 
     Referring now to FIG. 4 a graph of ME for varying frequencies is illustrated for a probe embodying the present invention. In FIG. 4, Z a  is presumed to be 150 ohms and Z p  is varied. Line A utilizes a 1.0 pf probe input impedance with infinite resistance which corresponds to commercially available FET probes, such as produced by Textronic™. This will work satisfactorily as an indirectly grounded unit for signals less than 120 MHz, which is less than or equal to a 10% measurement error, but, is not intended for use in the indirectly grounded probe since it has provisions for direct ground connections near the probe tip. Line B has a 10K input impedance shunted by a 0.20 pf capacitance. Line C has a 10K input impedance shunted by a 0.10 pf capacitance. Line D has a 10K input impedance shunted by a 0.05 pf capacitance. As illustrated in FIG. 4 the measurement error is reduced as the capacitance decreases from 1 pf line A, to 0.05 pf, line D. As illustrated on line D signal waveforms of more than 2.5 GHz may be satisfactorily measured (≦10% ME). The range of measurement may be increased by compensating the error in the 2 to 4 GHz range. This compensation is accomplished by adding a small amount of peaking to the amplifier response. 
     FIG. 5 is a schematic diagram of a wide band signal probe, generally designated 18, embodying the present invention. FIG. 5 is comprised of a resistor 19 coupled to be used as the tip of probe 18; an amplifier 20; a power decoupling 26 which is coupled to a probe cable (not shown); and an antenna ground 27. Probe 18 encompasses the general requirements for the amplifier design which are: easily replaceable tips; not easily burnt out due to transient input voltages; dc coupled; low cost design; 50 ohm output impedance; and a selectable attenuation factor. The burnt out requirement and low cost design indicate that Si bipolar devices are preferable over GaAs FET active components. Probe 18 is basically an operational amplifier with R T  and the 1.5K feedback resistance as the primary gain determining elements. 
     Amplifier 20 has provisions for minor frequency response adjustments. Resistor 19 can be modeled as a resistor in shunt with a small capacitor, this will be illustrated further in conjunction with FIG. 6. The shunt capacitance acts to control the frequency response. The actual value of the shunt capacity is determined by the construction technique used to make the probe. While numerous types and styles of resistors can be used the best type is a standard carbon composition resistor with a 1/4 watt power rating. 
     Referring now to FIG. 6 a portion of the schematic of FIG. 5 is illustrated. This schematic shows probe tip 13 coupled to one end of resistor 19 (R t ). The other end of resistor 19 is coupled to amplifier 20 (not shown). Two shunt capacitances 21 and 22 are illustrated representing the shunt capacitance to the probe body and to the ground respectively. Frequency response compensation is achieved by a proper selection of resistor 19 in addition to the resistor location relative to the probe body and other shunt capacity. FIG. 7 is an exploded view of a probe body, generally designated 23, embodying the present invention. FIG. 7 shows probe body 23 with a holder 25 and a resistor 19 inserted into holder 25. The first shunt capacitance, 21 of FIG. 6, is determined by the distance from a lead wire 24 of resistor 19 to body 23 of the probe. This distance is represented by the number 21&#39;. The second shunt capacitance, 22 in FIG. 6, is determined by the penetration of lead wire 24 into body 23 of the probe. This is represented by the number 22&#39;. 
     The reactive portion of the probe input impedance is primarily the shunt capacitance to resistor 19 (above). The effective value of the shunt capacitance can be determined from a reflection coefficient measurement of the probe. For the present probe a reflective coefficient of less than 1 dB was achieved at 3 GHz indicating that the effective input capacitance is approximately 0.05 pf. 
     Thus, it is apparent that there has been provided, in accordance with the invention, a device and method that fully satisfies the objects, aims and advantages set forth above. 
     It has been shown that the present invention provides an apparatus and method that does not require a direct ground to effectively operate and is economical to produce and operate. It has also been shown that the present invention is capable of frequency compensation in its measurement. 
     While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alterations, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims.