RF Generator Troubleshooting
Q: What does it mean if the RF generator will not put out full
power and the reflected power is nearly equal to forward power?
A: This generally means that the generator is in "reflected
foldback." The generator is limiting its forward power output
to protect itself against excessive reflected power. For Comdel
generators, this foldback occurs when the reflected power reaches
about 20% of the full rated power of the generator. This condition
is seldom the fault of the RF generator. The problem might lie in
the coax, the matching network, or a short of open in the RF chain
after the match. In plasma applications, any condition that prevents
the striking of plasma will result in high reflected power.
Q: What if reflected power is higher than desired, but not nearly
as high as forward power?
A: Under these conditions, some energy is being delivered to the
load, but the impedance matching mechanism of the system is not
able to match the load well enough. If the generator is fixed frequency,
then the fault lies elsewhere. If the generator adjusts its frequency
to accomplish impedance matching, then this may be suspect, but
so might the impedance matching network that works with the generator.
Other causes include chamber conditions resulting in plasma impedance
outside the range of the matching network.
Q: What does the blinking red light on the right front of my
CX Series RF generator mean?
A: This means the safety interlock loop is not satisfied. Internal
to the generator there are cover interlocks and an interlock switch
associated with the RF output(s) that is actuated by the cover that
goes over the coax cable connection. In addition to these there,
are two pins on the external interface that must be connected together
in order to satisfy the interlock chain. Typically these two pins
are connected together using switches on associated external parts,
such as the matching network cover or plasma chamber.
here for the standard CX Series troubleshooting guide.
Q: What should we do if the lights for the generator and display
will not go on?
A: First, check to see if breaker on the back of the generator is
on (circuit breaker should be in the up position). Next, verify
that there is AC power to the generator (verify AC power with a
voltage meter). If these conditions have been verified and there
are still no lights lit and no display present, the generator will
need to be replaced.
Q: What do I need to know about troubleshooting my RF power supply?
Test Equipment for Troubleshooting:
To properly test your supply, you need a standard portable digital
multimeter (DMM), a power-SWR meter, and a dummy load.
Multimeter: It measures AC/DC voltages, amperage, and ohm resistance
of pots and pickups. It is used to measure line voltage and, when
set as an ohm meter, it can be used to check the coax cable.
Power-SWR Meter: Our generators have built-in power meters that
show forward and reverse power, but for setting up and testing power
supplies, an external meter is useful.
Dummy Load: Dummy loads are necessary for troubleshooting the generator
at medium and high power levels. Commercial dummy loads are available
with a 50-ohm resistive, noninductive impedance, with power levels
ranging from a few hundred to many thousands of watts. The dummy
load is usually connected to the output of the RF generator with
a short 50-ohm coaxial cable and allows you to put maximum power
into a standard load while making tests on the generator. When testing
RF generators using impedance-matching networks, a dummy load that
more accurately represents the load in the system is highly desirable.
Oscilloscope: This is used to observe the signals from the crystal
oscillator through the various driver, buffer, and power amplifier
stages. The scope will show any signal distortion, noise, or other
characteristics that cannot be detected with a voltmeter.
Frequency Counter: The primary use of this piece of equipment is
to check the operational frequency of the RF generator and to verify
that the crystal oscillator is on frequency. RF generators are regulated
by the FCC and must operate on specific frequencies within a given
tolerance range. If the actual frequency deviates too much, adjustments
may be necessary. Frequency adjustments are madeonly after the equipment
has warmed up for at least an hourits important that
the generator be at a stable normal temperature before adjusting
Spectrum Analyzer: This Oscilloscope-like equipment is useful in
diagnosing unusual troubles in a generator that may be related to
distortion and harmonics or spurious signals. Most spectrum analyzers
are used in RF communications systems where they can show the sidebands
developed during modulation as well as harmonic distortion and for
detecting and isolating interference.
RFI-EMI Tester: This test equipment is a highly sensitive radio
receiver designed to detect stray RF radiation, including harmonics
and intermodulation products. The more sophisticated test sets incorporate
a spectrum analyzer.
Field Strength Meter: This is a simple radio receiver used to measure
the amount of signal radiated from an RF source.
Troubleshooting Safety Tips:
- Remove all jewelry before working on RF generators and related equipment. Metal jewelry can cause serious harm or even death when you are working on high-voltage RF equipment.
- To minimize accidentally touching a voltage source when working inside the equipment on power supplies or high-voltage connections, use only one hand and keep your other hand in your pocket.
- Do not disable any equipment interlocks. With power removed, you can usually troubleshoot in safety. The low voltage (24 V) safety interlock circuit is designed to disable the unit in the event of an interlock fault condition. Low voltage power for the interlock circuit is supplied by a step down transformer located inside the unit. This transformer is designed to provide safe low- voltage operation and provide isolation from the main AC line.
- To avoid an accidental shock, keep your clothes and work area free of water or moisture.
- To prevent electrical shock and/or RF burns, never operate equipment with the covers removed. Never operate without an appropriate cable connected between the RF output connector on the rear panel and the load.
- Equipment must be bonded to Protective Earth (safety ground) prior to operating the unit. Safety ground connection must be made at the units rear panel designated 1/4"- 20 threaded ground stud. The ground wire should be a #14 awg or equivalent (minimum) green/yellow lead.
- Prior to performing system maintenance, repair
or other service operations, the generator must be locked out
and tagged out to prevent accidentally energizing the system.
First disconnect AC input power to the generator, then mount a
suitable Clamshell type lockout device to the AC input
plug such as a Hubbell # HLD2 or equivalent. Follow all manufacturers
directions for the lockout device.
Troubleshooting First Steps:
WARNING: Equipment must be installed, operated and serviced only by trained, qualified persons.
CAUTION: Breaking the seal or removing the warranty decal from any unit will void the warranty. If internal damage is suspected, contact factory for assistance.
- If a problem with the generator has been identified, verify it by attempting to duplicate the problem.
- Inspect cables and connectors.
- Check for the correct settings of front panel controls, observe any meter readings or display readouts, and re-enter commands from equipment connected to a personal computer or terminal. For instance, zero forward power and maximum reflected power might indicate an open coax cable between the generator and matching unit, or a bad matching unit.
- One of the most obvious problems is simply lack of power to the equipment. Check circuit breakers and reset, if necessary. Verify that there are no bad fuses. If a fuse was blown, replace it with one of the same value and type. Then recheck the equipment.
- If the system is still malfunctioning, in most cases, the next step should be to replace the generator with a functioning spare to get the system back in operation.
If your equipment is still under warranty, the
best course at this point is to request an RMA number and return
the unit to the factory, or you can call 800-468-3144 and ask for
Q: What's S-Technology?
A: S-Technology is a patented, unique amplifier design that makes
the gain of the power amplifiers in Comdel "S" generators
constant rather than dependent upon load conditions. As plasma impedance
changes, especially the large change that occurs at the instant
of plasma ignition, non "S" generators may display erratic
power gain characteristics that result in amplitude modulation of
the RF envelope. This modulation can cause matching networks to
malfunction completely or, more subtly, can shorten matching network
life. This is the reason that Comdel does not supply matching networks
to be used with non "S" generators.
Q: Why is the 13.56 MHz frequency used for RF plasma applications?
A: 13.56 MHz is one of several I.S.M. frequencies mandated by the
FCC. These are frequencies set aside for industrial, scientific,
and medical purposes and have relaxed emissions limitations because
they are so positioned in the radio wave spectrum as to not interfere
with safety, emergency, and military communications. It just happens
that this particular ISM frequency has the most plasma research
Q: Whats the difference between a linear and a non-linear
A: A linear amplifier is one in which the output current is directly
proportional to the voltage input. Linear amplifiers do not distort
the signal or cause hamonics to be introduced. A class A amplifier
is an example of a linear amplifier. A nonlinear amplifier is one
that does not operate over the linear operating region of the transistor
or tube. Instead, the device is used primarily as a switch to turn
the current off and on. Nonlinear amplifiers generate pulses or
square waves at the frequency of operation.
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