V-Dot Voltage

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Definition:                           

Equation

Parameters

Diagram

Calculate Properties of a Vdot Probe:              top

Parameter        

Value

Units

Special Values

Inputs:

A (probe area)
d (probe distance)   Alternate version uses V1 and E on probe surface
er (dielectrc const.)   1 for vacuum
R (load resistance) usually 50 Ohms
C2 (stray capacitance) enter 0 to ignore
  dV1 (voltage rise)   only needed to calculate V2
  dt (rise time)   only needed to calculate V2
           

Outputs:

V1/V2 (cal. factor)

 
w3dB (corner freq.)

  C1 (probe capacitance)

  V2 (=RC1dV1/dt)    

Output Format:                                                        top

Select Format: Scientific Engineering Fixed

Notes:                                                                       top

bulletIf you know dV/dt then enter 1 second for dt and put dV/dt in for dV1 with volt units (as done for the default values).
bulletThe Vdot probe is essentially a high pass RC filter with a corner frequency equal to (1/R(C1+C2)) as shown in the bode plot below.
bullet
bulletCare must be taken to ensure that the 3dB point is much higher in frequency than the range of interest. 
bulletIn most cases, C2>>C1 so that the 3dB frequency is just 1/RC2
bulletThe phase is +90 degrees for low frequencies, +45 at the 3dB point, and 0 at high frequencies.
bulletFor frequencies above the 3dB point, the probe acts as a capacitive divider.
bulletThe values above are from the design of a typical Vdot probe with a center conductor voltage rising to ~800 kV in about 100 ns.  The peak dV/dt was estimated from a circuit code.  C2 was calculated from a field plotting code.  The actual geometry is cylindrical, but the ID is ~6" and OD is ~12" so the approximate planar gap is 6".  The probe is placed in a recess in the outer conductor so the surface is flush.  The probe is just a 1.8" diameter disk held in a metal cup with plastic support.  The output is through an N connector with center pin attached to the disk as shown below:
bullet
bulletUsually, you'll want the peak output large enough to overcome noise, but much less than the breakdown strength of the connectors.  Also, the 3dB point is usually desired to be >500 MHz (the bandwidth of a lot of the scopes).

Copyright 2006  Raymond J. Allen