User Manual
NOTE: This is a copy of the users manual that was delivered with the RF generators made for the University of Utah and as such contain information that is specific to these units. Use this manual as a reference only and keep in mind that references to part locations, colors, etc. are specific to the University of Utah RF generators.
RF Generator Basics
THE CONVENTIONAL APPROACH
The conventional approach to generating a high voltage RF signal is to take the output of a low voltage oscillator (such as a sine wave function generator) and amplify it with a high voltage RF amplifier. The output of the amplifier is connected to the load (an octapole, hexapole, quadrupole, etc.) Unfortunately, however, the amplifier can not be connected directly to the load but must be connected to the load through a special impedance matching circuit. This circuit may be a conventional "antenna tuner" and is housed in chassis separate from the amplifier. There must also be circuits converting the amplifiers unbalanced output (single wire and ground) to a balanced signal (two wires with 180 degree out of phase signals). In addition, circuits for adding any DC bias voltages may be required.
Although this type of system is awkward in that it requires a number of separate pieces of equipment with interconnecting cables plus it can be difficult to adjust, it has one more undesirable characteristic that makes it difficult to work with. The voltage delivered to the load is difficult to predict and more important, can change with changes in the load and tuning. This is due to the fact that the load (the octapole, hexapole, quadrupole, etc.) appear to the circuit to be only a capacitor with a small amount of resistive losses. The voltage that gets applied to this type of load is a strong function of the coupling circuit. Since the voltage across the load is the critical parameter, this type of system is awkward to work with.
A DIFFERENT APPROACH
The current high voltage RF generator takes a totally different approach. Instead of using a low voltage RF source, an amplifier and an impedance matching circuit, the new RF generator IS a high voltage oscillator. It is, in its simplest form, an oscillator where a tank circuit (a coil and capacitor is parallel) determines the frequency. The coil is built in the chassis and the capacitor is the external load (the ion guide/trap). The concept of impedance matching is meaningless. No matching circuit is required. The RF voltage delivered to the load is almost totally determined by the high DC voltage applied to the generator and can be easily adjusted over a wide range simply by adjusting the high DC voltage supply voltage. Since a commercial DC supply can be used, it is straightforward to computer-control the RF amplitude.
MORE DETAILS:
DC power supply:
High voltage DC to power the oscillator circuit is provided by an external high voltage DC supply. This needs to be variable from 50V to whatever maximum RF amplitude is required for your experiment (600 VDC max). See RSI article for suggestions.
Frequency adjustments
The above description is conceptually correct, however there are a few details that are important. The tank circuit with its coil and capacitor actually has a built in variable capacitor in parallel with the load. This is for fine frequency adjustments. The coil is constructed so it can be tapped at various positions to vary the inductance, giving a coarse frequency control (WARNING: turn off the DC before touching the coil, it has high voltage RF and DC). The coil is removable to allow different coils to be installed to further increase the total range of frequencies available.
Output DC float voltage:
There is a circuit for coupling an externally supplied DC float voltage to the RF output. This is a DC voltage relative to ground. It may be positive or negative and up to about 150 VDC. This float voltage allows, for example, the collision energy to be controlled in a ion guide experiment.
Output differential voltage
A built in low voltage supply may be coupled to the RF output to provide a low DC differential voltage between the two RF output phases. This voltage is adjustable both in amplitude and in polarity. The differential voltage allows calibration of trapping potentials and adjustment of focusing properties.
RF voltage
The output is a differential output, that is to say, there are two output connections where the RF signal on the output connections are 180 degrees out of phase. These are connected directly to the load (ion guide/trap). The maximum positive voltage the signal will attain is approximately equal to the applied DC high voltage. The maximum negative voltage the signal will attain is approximate equal to the same amplitude of the applied DC high voltage but will be negative. This means that the peak voltage between the outputs will be 2X the applied DC voltage. For example, if the DC voltage is 200V, each output will swing between +200 V and -200V and the maximum voltage between the outputs will be 400 volts and that condition will occur 2 times each RF cycle. The tubes are limited to a maximum of 600 V applied to them, so the high voltage DC supply should be limited to 600V. This will give a maximum of 1200V between the outputs. The minimum voltage for the tubes to operate properly is approximately 50 VDC. Generally 1200V is plenty, but if not, you could use some higher voltage tubes.
RF balance
There are provisions for adjusting the RF balance between outputs so the RF level delivered to each output is equal. There are fixed capacitors for coarse adjustments and a variable capacitor for fine balance adjustments. The output is coupled to the load through large value capacitors. The value of these capacitors must be large relative to the load in order to maximize the voltage delivered to the load. RF balance is attained by selecting fixed capacitors for a coarse adjustment and using the variable capacitor as a fine adjustment.
RF Test Points
The RF output many be monitored from the front pane RF test point. This test point has a built in 100:1 attenuator, meaning that a 400V output signal will appear as 4V on this test point. There is a calibration adjustment associated with the test point. The test point is designed to be used with a 4 foot length of RG-58 coax and the high input impedance of an oscilloscope. If other than a 4 foot length of RG-58 coax is used, the calibration will not be correct. These test points can not be used to measure the DC float or differential voltages.
DC Test points
There are DC test point which can be used to measure the DC float and differential voltage on the output. These test points have RF filters to block the RF signal from reaching the test point so only the DC value can be measured. This is the exact DC value and not a divide by 100 value as is the RF test point.
ON/OFF keying
There are provisions for keying on and off the RF either with an external TTL signal or with an built in 10 Hz keying circuit. Also the rise time for the RF as it is keyed on and off can be controlled from the front panel. The RF can be off, on all the time (CW), keyed on and off internally by the 10 Hz circuit or it can be keyed on and off by an external TTL signal. When the oscillator is keyed, a TTL output signal is available to trigger an oscilloscope.
OPERATION
AC Power
The generator must be plugged into a standard 115VAC outlet. The AC powers a 6.3V transformer which in turn powers the filaments in the two oscillator tubes. It also provides power for the 5V power supply which powers the keying circuit board.
High Voltage
The RF generator does not have a built in high voltage power supply. High voltage must come from an external power supply connected to the side panel MHV connector. This power supply must be able to provide about 100 mA of current. When adjusting the DC voltage keep in mind that the peak to peak RF voltage between the two output phases will be about 2X the applied DC voltage. The maximum voltage, limited by the tubes, is 600 VDC. The minimum to make the tubes work properly is about 50 VDC.
DC Float Voltage
IMPORTANT: if no external DC float voltage is to be used, it is important that the float input BNC connector on the side panel be grounded.
The float DC is from a separate external supply. This voltage is applied to BOTH output phases. This voltage can be of either polarity, however keep in mind that one of the wires from the power supply will be grounded. If a negative voltage is required, it is not necessarily sufficient to simply reverse the leads from the power supply. If a negative float voltage is required, caution must be taken that the float supply can deliver a negative voltage relative to ground. This voltage can be a maximum of about 150 VDC.
DC Offset voltage
Offset or differential voltage is from an internal power supply. This supply is adjustable from the front panel and the voltage can be measured on the front panel DC test points. The supply can be adjust to deliver up to about a 24 VDC voltage difference of either polarity between the two output phases. It is adjusted with a single knob. The clockwise direction is for one polarity and counterclockwise for the other. The voltage adjustment is non-linear with knob rotation which makes it easier to adjust at the low voltage settings that are most commonly needed.
ON/OFF Keying
If the RF generator is to not be keyed on and off with an external TTL keying signal, the MODE switch should be set to CW to cause the RF generator to always be on or to OFF to turn off the RF output.
If TTL keying is to be used, connect the TTL source to the side panel BNC connector. This connector is electrically isolated meaning the ground of the RF generator does not ground the equipment supplying the TTL signal. The TTL signal should be such that it goes high to turn on the RF and low to turn off the RF. The rise time of the keyed RF envelope is adjustable from the front panel RISE TIME knob. To help set up the equipment, an internally generated 10 Hz signal is activated when the key source switch selects internal. To help trigger a scope on the keying signal, a scope trigger output is available on the front panel. This is a trigger for the keying circuit, not for the actual RF wave. By triggering a scope on this signal one can more easily adjust the rise time.
Frequency Adjustments
The front panel frequency adjustment has a frequency range of about 2:1 To set the approximate operating frequency it may be necessary to change the taps on the internal coil. Before you change the taps, be sure to unplug the high voltage and float supply. Unplug them from the RF generator, don't just turn them off!!!!
The frequency of operation is determined not only by the placement of the taps but also by the capacity of the load, therefore some trial and error will be necessary in determining the required tap location.
Move the taps in toward the center to increase the frequency, move them out to decrease the frequency. It is important that the thin wire be a turn or two inside the fat wire. Also, keep the red thin and fat wire on the same side of the coil. Keep the placement of the taps symmetrical about the center of the coil. Other than those basic rules, it will require some trial and error to get the frequency range proper, but keep in mind that the main frequency adjust capacitor has a range of about 2:1 which should make the placement of the taps fairly uncritical.
Balance adjustment
The balance of the level of the RF output signal on both phases can be adjusted to some extent from the front panel. If there is a severe RF unbalance, there may be an unbalance in the load and an indication of a problem with the load. Be sure the load is truly RF balanced by measuring the capacity to ground on each line. If they are not equal, this is an indication of a potential problem which must be first addressed before the RF generator is connected.
To attain RF balance, if the RF balance control will not balance the outputs, some additional capacity may be required across the balance capacitor. To best accomplish this, one should understand how the balance circuit works. See the main schematic below. The two output phases from the RF generator are each coupled to the output connectors through large value capacitors (2000 pF or so). These capacitors have a capacity that is large compared to typical loads, thus most of the output voltage will be across the loads and very little across the coupling capacitors. However, the coupling capacitors must be matched in value. To this, one of the capacitors has a variable capacitor in parallel with it. This is the balance control capacitor. High voltage, large valued variable capacitors are very large and expensive, therefore a compromise size capacitor is used and the result is that only a small adjustment range is available. By selecting fixed value capacitor for the coupling capacitors and by adding parallel capacitor to the variable, a good balance can be attained. It should not normally be required to do this if the load is in good balance.
Links to:
RF section schematics and description