V-Dot Probe Applet ## Definition:

#### Diagram    ## Calculate Properties of a Vdot Probe:              top

### 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

• If 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).

•  The 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.

• • Care must be taken to ensure that the 3dB point is much higher in frequency than the range of interest.

•  In most cases, C2>>C1 so that the 3dB frequency is just 1/RC2

•  The phase is +90 degrees for low frequencies, +45 at the 3dB point, and 0 at high frequencies.

• For frequencies above the 3dB point, the probe acts as a capacitive divider.

•   The 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:

• • Usually, 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).