V-Dot Voltage

Goto: Coaxial Section Output Format Notes

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

e_r (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)

w_3dB (corner freq.)

C₁ (probe capacitance)

V₂ (=RC₁dV₁/dt)

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).

Parameter

Value

Units

Special Values

Inputs:

Outputs:

Notes: top

Definition:

Equation

Parameters

Diagram

Calculate Properties of a Vdot Probe: top

Output Format: top

	Parameter	Value	Units	Special Values
Inputs:	A (probe area)
Inputs:	d (probe distance)			Alternate version uses V1 and E on probe surface
	e_r (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)
Outputs:	w_3dB (corner freq.)
	C₁ (probe capacitance)
	V₂ (=RC₁dV₁/dt)

	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).