Megaimpulse RC20 User manual

RC20 RESISTIVE COUPLER
FOR NPG SERIES NANOSECOND
PULSE GENERATORS
USER MANUAL
© 2023 Megaimpulse Ltd.
V2.0 of February 07, 2023

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Address: 28 Polytechnicheskaya str., St. Petersburg, 194021 Russia
www: http://www.megaimpulse.com
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CONTENTS
Application, general view, and package content ............................................... 2
Safety manual ................................................................................................... 3
Technical specification ..................................................................................... 4
The drawing of RC20 resistive coupler ............................................................ 5
Assembling and putting into operation ............................................................. 6
Theory of the operation .................................................................................... 8
Energy loss in a cable. Calculation of the energy balance ................................ 14
Evaluation of the pulse voltage on the load and the pulse current .................... 16
Warranty ........................................................................................................... 20
RC20 User Manual 1

RC20 resistive coupler allows the precise measurement of high voltage
nanosecond pulses, including applied to (incident) and reflected from the load,
calculation of their energies and total energy balance of the discharge, as well as
estimation of the pulse voltage on the load and pulse current. Therefore,
comprehensive information about the discharge can be obtained.
Fig. 1. RC20 resistive coupler with two attached HV coaxial cables of
5 meters in length each.
PACKAGE CONTENT
Please check the package for the following items:
-RC20 resistive coupler with two attached HV coaxial cables of 5 meters in
length each;
-6 dB attenuator with N-type connectors;
-20 dB attenuator with N-type connectors;
-3 meters in length semirigid coaxial cable assembly with N-type and SMA
connectors;
-20 dB attenuator with SMA connectors;
-SMA-to-BNC adapter.
MEGAIMPULSE LTD. 2

SAFETY MANUAL
Electrical safety
RC20 resistive coupler is designed for the precise measurement of
nanosecond high-voltage pulses. Please be very careful while working with
high-voltage equipment, and operate by qualified personnel only.
Improper use may result in electric shock, strong electromagnetic
interference, or damage to other electronic equipment.
The signal from the coupler should be registered by an oscilloscope. Please
ground the oscilloscope obligatory, and divide the signal from the coupler
down to a safe level by 46 dB attenuators.
It is strongly prohibited to connect or disconnect the coupler to or from the
system while the HV pulse generator is turned on, as well as leave the N-type
connector of the coupler open, i.e. without attached attenuators. Inevitable
arcing across the open N-type connector may damage it.
Operation safety
Please read this manual before installing and using the coupler.
Make sure that all the cables are applicable and undamaged. All the
connectors should be clean, dry, and free from dust and dirt.
The coupler should operate in normal laboratory conditions. Please, avoid
dust, humidity, and temperature extremes.
Please, if you encounter any technical problems with the coupler, then
contact Megaimpulse Ltd. Do not try to repair it by yourself.
RC20 User Manual 3

TECHNICAL SPECIFICATION
Maximum HV pulse amplitude 20 kV
Maximum average power in HV cable 120 W
Pulse polarity Any
Rise time (transient response) 1.5 ns
Nominal pulse voltage attenuation of
RC20 coupler
RC20 with 46 dB attenuators
1:50 (34 dB) *)
1:10000 (80 dB)
Impedance and length of HV cables 75 Ohm, 5 m + 5 m
Output connector to the oscilloscope N-type, 50 Ohm impedance
The delay between the incident and reflected
pulses
approx. 50 ns, depending on
the coax cable length as well
as the length of the wires to
the load and load dimensions
Pulse amplitude attenuation coefficient:
per 1 meter of HV coaxial cable
per 5 meters of HV coaxial cable
k1 = 1:1.0061
k5= 1:1.031
Energy loss in dB and attenuation coefficient:
per 1 meter of HV coaxial cable
per 5 meters of HV coaxial cable
per 10 meters of HV coaxial cable
0.053 dB / k12= 1:1.0123
0.265 dB / k52= 1:1.063
0.53 dB / k102= 1:1.13
Size (for reference only) 107 х 96 х 46 mm3
*) the exact measured attenuation values are written on the coupler front label and in
the testing protocol.
To prevent damage to the coupler do not exceed the maximum pulse amplitude
and average power in HV coaxial cable.
MEGAIMPULSE LTD. 4

* All the dimensions are in mm and are given for the reference only
Fig. 2. The drawing of RC20 resistive coupler.
1 - HV coaxial cable to the NPG generator. The length is 5 meters; cable impedance
is 75 Ohm. Standard 75 Ohm coaxial connector is installed on the generator's
side.
2 - HV coaxial cable to the load. The length is 5 meters; cable impedance is 75 Ohm.
3 - N-type connector to the oscilloscope. Impedance of the output is 50 Ohm.
RC20 User Manual 5
RC20 User Manual 5

ASSEMBLING AND PUTTING INTO OPERATION
Please follow the next steps. It will help to prevent damage to the equipment
and personnel injury.
Step 1.
Unpack the package and check the presence of the following items:
-RC20 resistive coupler with two attached 5 m HV coaxial cables;
-6 dB attenuator with N-type connectors;
-20 dB attenuator with N-type connectors;
-3 m semirigid coaxial cable assembly with N-type and SMA connectors;
-20 dB attenuator with SMA connectors;
-SMA-to-BNC adapter.
Step 2.
Assemble the attenuators, measurement cable, and adapter in the following order:
-6 dB N-type attenuator;
-20 dB N-type attenuator;
-3 m semirigid coaxial cable assembly with N-type and SMA connectors;
-20 dB SMA attenuator;
-SMA-to-BNC adapter.
Attach them to the output N-type connector of the coupler as is shown in Fig.4. It
is important to keep this order for the best signal-to-noise ratio and to prevent
damage to the coupler in case of a lost connection in the cable assembly.
To prevent contacts damage, please hold the attenuator body fixed and
rotate the nut only when you attach the attenuator to the coupler.
It is strictly forbidden to apply HV pulses without the attenuators attached.
Fig.3. a) when you attach the attenuators please hold the body fixed and rotate the
nut only;
b) it is strictly forbidden to apply HV pulses without the attenuators
attached.
a)
b)
MEGAIMPULSE LTD. 6

Fig.4. RC20 resistive coupler assembling and connection.
Step 3.
Connect the HV cables to the NPG-series pulse generator and to the load as well
as connect RC20 to the oscilloscope.
Set the input impedance of the oscilloscope to 50 Ohm.
Set the external attenuation to 1:10000 or, which is more correct, to the exact
attenuation value specified in the testing protocol. Alternatively, it is possible to
calculate the pulse amplitude manually by setting the external attenuation 1:1. It is
recommended to set the vertical scale to 5 kV/div in the first case or to 500 mV per
division in the second.
Set the horizontal scale to 10 ns per division.
Step 4.
Turn on the generator and register the incident and reflected pulse waveforms by
the oscilloscope.
RC20 User Manual 7

THEORY OF THE OPERATION
RC20 resistive coupler allows the precise measurement of the output pulse
waveform of NPG-series HV pulse generators (incident pulse for the load), as well
as the pulse waveform reflected from the load. To prevent distortion, the nanosecond
pulses, similar to high-frequency signals, should be transmitted from a generator to
load through high-frequency transmission lines, for example, coaxial cables. Most
of NPG generators have 75 Ohm impedance output coaxial connector and operate
with HV coaxial cables having 75 Ohm impedance as well. According to basic
principles of HF transmission lines, the pulse energy can be absorbed by the load
completely in the case of ideal impedance matching only, i.e. if the load impedance
is fixed and equal to the cable impedance. Unfortunately, this is impossible in the
discharge applications. The impedance of the discharge gap changes from a high
before the breakdown down to typically less than one Ohm after it. Part of the pulse
energy inevitably reflects from the load and travels back to the generator. RC20
coupler is a measurement tool of the incident and reflected pulse waveforms, which
allows us to calculate the energy of both pulses, and therefore, determine the energy
balance or energy efficiency of the discharge. In addition, the evaluation of the pulse
voltage on the load and pulse current is possible.
Let us consider the basic relationships between the incident and reflected pulses
as well as RC20 operation principles in more detail. The reflection coefficient 
which is defined as a ratio of the reflected
VR and incident VI signals is equal to:
  
=  [1]
, where
ZLOAD is the impedance of the load;
ZCABLE is the impedance of the cable.
According to equation [1], if the load impedance
ZLOAD is higher than ZCABLE , then
is within 0…1 and the reflected pulse has the same polarity. If the load impedance
is lower than
ZCABLE , then is within -1…0 and the reflected pulse has opposite
polarity. = 1 and = -1 in two extreme cases of open or short load, and the reflected
pulse amplitude is equal to the incident one.
The pulse voltage on the load
VLOAD and pulse current ILOAD are equal to:
 = + [2]
 == () /  [3]
If the load impedance
ZLOAD is higher than ZCABLE , then the reflected pulse has the
same polarity, pulse voltage amplitude on the load is equal to the sum of the incident
and reflected pulses, i.e. higher than the incident one, and the pulse current through
the load is the incident pulse current minus the reflected one, i.e. lower than the
incident pulse current (see Fig.5).
MEGAIMPULSE LTD. 8