Coaxial Cable for High Voltage Pulse Measurement
I have to connect a pulse of 500V peak 50uS to an oscilloscope using a coaxial cable. What type of coaxial should I use?
I tried with the RG11, but according to the specifications of the Belden RG11, the maximun voltage is 300Vrms.
The pulse is derived from a capacitive divisor (K=800) in a high voltage impulse test of transformers.
Thanks
300Vrms is average . Your 500V is peak, what is your average? You should find cable can tolerable your peak and average at the same time.
RG11 is specified with 5 kVrms operating voltage for low frequency. RF operation will be additionally restricted by skin effect and dielectric losses. I can't imagine, that restrictions apply for your problem.
You do, of course, realize that a 500V pulse will blow out the front end of most standard oscilloscopes! You will need a resistive voltage divider, and it should be matched to the cable's characteristic impedance.
Yes, I'm using a 100X oscilloscope probe and planned to use a T connector to connect a 75 Ohm terminator and the oscilloscope's probe.
Thanks biff44.
Sorry, too early in the morning i guess, but that makes no sense. Please give us a sketch. The exact details of how you do it determine if it will work or not.
I think the coaxial should terminate in a 75 Ohms terminator to avoid reflections. But I need to connect the oscilloscope probe which has 10M in parallel with some capacitance and BNC connector, then the T connector let me to connect the coaxial with the terminator and the oscilloscope probe, using the BNC to F type adaptors.
The system is working now without terminator, the coaxial (30mt approx) is connected directly to the probe. That connection, in my opinion, produces oscillations in the rise of the pulse.
As biff44 already mentioned , an impedance matched, low resistance voltage divider would be the obvious solution if optimal pulse performance is intended. Using a high impedance passive oscilloscope probe as voltage divider is of course possible, but 75 ohm termination of the transmission line is obligatory. The input capacitance of the oscilloscope proble will still cause some reflections, that's why the this solution can't compete with a resistive divider.