U.S. patent application number 11/482900 was filed with the patent office on 2007-01-25 for waveform generator electronics based on tuned lc circuits.
This patent application is currently assigned to Ionalytics Corporation. Invention is credited to Yves Baribeau, Lucien Potvin.
Application Number | 20070018629 11/482900 |
Document ID | / |
Family ID | 35446667 |
Filed Date | 2007-01-25 |
United States Patent
Application |
20070018629 |
Kind Code |
A1 |
Potvin; Lucien ; et
al. |
January 25, 2007 |
Waveform generator electronics based on tuned LC circuits
Abstract
Disclosed is an apparatus for generating a periodically varying
electrical signal for creating a periodically varying electrical
field between electrodes of an ion mobility spectrometer. The
apparatus includes an output port. A first tuned circuit is
provided for being electrically coupled to an external power source
and for, in isolation, providing a first periodically varying
electrical signal having a first frequency. The first tuned circuit
is coupled to the output port for providing an output electrical
signal having a component at the first frequency thereto. A second
tuned circuit is provided for being electrically coupled to an
external power source and for providing a second periodically
varying electrical signal having a second frequency different from
the first frequency. The second tuned circuit is coupled to the
first tuned circuit for varying the output electrical signal about
the first periodically varying electrical signal.
Inventors: |
Potvin; Lucien; (Kanata,
CA) ; Baribeau; Yves; (Orleans, CA) |
Correspondence
Address: |
FREEDMAN & ASSOCIATES
117 CENTREPOINTE DRIVE
SUITE 350
NEPEAN, ONTARIO
K2G 5X3
CA
|
Assignee: |
Ionalytics Corporation
Ottawa
CA
|
Family ID: |
35446667 |
Appl. No.: |
11/482900 |
Filed: |
July 10, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10529309 |
Mar 25, 2005 |
7078678 |
|
|
PCT/CA03/01444 |
Sep 23, 2003 |
|
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11482900 |
Jul 10, 2006 |
|
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|
60413162 |
Sep 25, 2002 |
|
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Current U.S.
Class: |
323/304 |
Current CPC
Class: |
H01J 49/42 20130101;
H01J 49/022 20130101 |
Class at
Publication: |
323/304 |
International
Class: |
G05F 3/00 20060101
G05F003/00 |
Claims
1. An electromagnetic transformer comprising: a secondary winding
comprising a plurality of turns of a first wire wound defining a
core and having an approximately uniform spacing between adjacent
turns; a first primary winding comprising at least one turn of a
second wire wound around the core and spaced apart from both the
core and the secondary winding; and, a second primary winding
comprising at least one turn of a third wire wound around the core
in parallel with the first primary winding and spaced apart from
both the core and the secondary winding.
2. An apparatus according to claim 1, wherein at least some of the
core comprises a core material other than air.
3. An apparatus according to claim 2, wherein the turns of the
first wire of the secondary winding are wound proximate the core
material other than air.
4. An apparatus according to claim 2, wherein the core comprises a
substantially toroid-shaped core having a gap.
5. An apparatus according to claim 2, wherein the core material has
a magnetic permeability similar to the magnetic permeability of
air.
6. An apparatus according to claim 3, wherein the turns of the
first wire of the secondary winding are wound tightly around the
core.
7. An apparatus according to claim 1, wherein the approximately
uniform spacing between adjacent turns of the first wire of the
secondary winding is approximately equal to the diameter of the
first wire.
8. An apparatus according to claim 1, wherein the space between the
first wire of the secondary winding and either one of the second
wire of the first primary winding and the third wire of the second
primary winding defines an air gap.
9. An apparatus according to claim 1, wherein the first wire of the
secondary winding is wound around the core over a substantial
portion of a length thereof between the first end and the second
end.
10. An apparatus according to claim 1, comprising: a third primary
winding comprising at least one turn of a third wire wound around
the core and spaced apart from both the core and the secondary
winding; and, a fourth primary winding comprising at least one turn
of a fourth wire wound around the core in parallel with the third
primary winding and spaced apart from both the core and the
secondary winding.
11. An apparatus according to claim 10, wherein at least some of
the core comprises a core material other than air.
12. An apparatus according to claim 11, wherein the turns of the
first wire of the secondary winding are wound proximate the core
material other than air.
13. An apparatus according to claim 11, wherein the core comprises
a substantially toroid-shaped core having a gap.
Description
FIELD OF THE INVENTION
[0001] The instant invention relates generally to high field
asymmetric waveform ion mobility spectrometry (FAIMS), more
particularly the instant invention relates to waveform generator
electronics based on tuned LC circuits.
BACKGROUND OF THE INVENTION
[0002] High sensitivity and amenability to miniaturization for
field-portable applications have helped to make ion mobility
spectrometry (IMS) an important technique for the detection of many
compounds, including narcotics, explosives, and chemical warfare
agents as described, for example, by G. Eiceman and Z. Karpas in
their book entitled "Ion Mobility Spectrometry" (CRC, Boca Raton,
1994). In IMS, gas-phase ion mobilities are determined using a
drift tube with a constant electric field. Ions are separated in
the drift tube on the basis of differences in their drift
velocities. At low electric field strength, for example 200 V/cm,
the drift velocity of an ion is proportional to the applied
electric field strength, and the mobility, K, which is determined
from experimentation, is independent of the applied electric field.
Additionally, in IMS the ions travel through a bath gas that is at
sufficiently high pressure that the ions rapidly reach constant
velocity when driven by the force of an electric field that is
constant both in time and location. This is to be clearly
distinguished from those techniques, most of which are related to
mass spectrometry, in which the gas pressure is sufficiently low
that, if under the influence of a constant electric field, the ions
continue to accelerate.
[0003] E. A. Mason and E. W. McDaniel in their book entitled
"Transport Properties of Ions in Gases" (Wiley, New York, 1988)
teach that at high electric field strength, for instance fields
stronger than approximately 5,000 V/cm, the ion drift velocity is
no longer directly proportional to the applied electric field, and
K is better represented by K.sub.H, a non-constant high field
mobility term. The dependence of K.sub.H on the applied electric
field has been the basis for the development of high field
asymmetric waveform ion mobility spectrometry (FAIMS). Ions are
separated in FAIMS on the basis of a difference in the mobility of
an ion at high field strength, K.sub.H, relative to the mobility of
the ion at low field strength, K. In other words, the ions are
separated due to the compound dependent behavior of K.sub.H as a
function of the applied electric field strength.
[0004] In general, a device for separating ions according to the
FAIMS principle has an analyzer region that is defined by a space
between first and second spaced-apart electrodes. The first
electrode is maintained at a selected dc voltage, often at ground
potential, while the second electrode has an asymmetric waveform
V(t) applied to it. The asymmetric waveform V(t) is composed of a
repeating pattern including a high voltage component, V.sub.H,
lasting for a short period of time t.sub.H and a lower voltage
component, V.sub.L, of opposite polarity, lasting a longer period
of time t.sub.L. The waveform is synthesized such that the
integrated voltage-time product, and thus the field-time product,
applied to the second electrode during each complete cycle of the
waveform is zero, for instance V.sub.Ht.sub.H+V.sub.Lt.sub.L=0; for
example +2000 V for 10 .mu.s followed by -1000 V for 20 .mu.s. The
peak voltage during the shorter, high voltage portion of the
waveform is called the "dispersion voltage" or DV, which is
identically referred to as the applied asymmetric waveform
voltage.
[0005] Generally, the ions that are to be separated are entrained
in a stream of gas flowing through the FAIMS analyzer region, for
example between a pair of horizontally oriented, spaced-apart
electrodes. Accordingly, the net motion of an ion within the
analyzer region is the sum of a horizontal x-axis component due to
the stream of gas and a transverse y-axis component due to the
applied electric field. During the high voltage portion of the
waveform an ion moves with a y-axis velocity component given by
v.sub.H=K.sub.HE.sub.H, where E.sub.H is the applied field, and
K.sub.H is the high field ion mobility under operating electric
field, pressure and temperature conditions. The distance traveled
by the ion during the high voltage portion of the waveform is given
by d.sub.H=v.sub.Ht.sub.H=K.sub.HE.sub.Ht.sub.H, where t.sub.H is
the time period of the applied high voltage. During the longer
duration, opposite polarity, low voltage portion of the asymmetric
waveform, the y-axis velocity component of the ion is
v.sub.L=KE.sub.L, where K is the low field ion mobility under
operating pressure and temperature conditions. The distance
traveled is d.sub.L=v.sub.Lt.sub.L=KE.sub.Lt.sub.L. Since the
asymmetric waveform ensures that
(V.sub.Ht.sub.H)+(V.sub.Lt.sub.L)=0, the field-time products
E.sub.Ht.sub.H and E.sub.Lt.sub.L are equal in magnitude. Thus, if
K.sub.H and K are identical, d.sub.H and d.sub.L are equal, and the
ion is returned to its original position along the y-axis during
the negative cycle of the waveform. If at E.sub.H the mobility
K.sub.H>K, the ion experiences a net displacement from its
original position relative to the y-axis. For example, if a
positive ion travels farther during the positive portion of the
waveform, for instance d.sub.H>d.sub.L, then the ion migrates
away from the second electrode and eventually will be neutralized
at the first electrode.
[0006] In order to reverse the transverse drift of the positive ion
in the above example, a constant negative dc voltage is applied to
the second electrode. The difference between the dc voltage that is
applied to the first electrode and the dc voltage that is applied
to the second electrode is called the "compensation voltage" (CV).
The CV prevents the ion from migrating toward either the second or
the first electrode. If ions derived from two compounds respond
differently to the applied high strength electric fields, the ratio
of K.sub.H to K may be different for each compound. Consequently,
the magnitude of the CV that is necessary to prevent the drift of
the ion toward either electrode is also different for each
compound. Thus, when a mixture including several species of ions,
each with a unique K.sub.H/K ratio, is being analyzed by FAIMS,
only one species of ion is selectively transmitted to a detector
for a given combination of CV and DV. In one type of FAIMS
experiment, the applied CV is scanned with time, for instance the
CV is slowly ramped or optionally the CV is stepped from one
voltage to a next voltage, and a resulting intensity of transmitted
ions is measured. In this way a CV spectrum showing the total ion
current as a function of CV, is obtained.
[0007] In FAIMS, the optimum dispersion voltage waveform for
obtaining the maximum possible ion detection sensitivity on a per
cycle basis takes the shape of an asymmetric square wave with a
zero time-averaged value. In practice this asymmetric square
waveform is difficult to produce and apply to the FAIMS electrodes
because of electrical power consumption considerations. For
example, without a tuned circuit the power that is required to
drive a capacitive load of capacitance C, at frequency f, with a
peak voltage V and a 1:1 duty cycle square wave, is V.sup.2fC.
Accordingly, if a square wave at 750 kHz, 4000 V peak voltage 1:1
duty cycle is applied to a 20 picofarad load, the theoretical power
consumption will be 480 Watts produced by the sum of the squares of
the voltage changes on the capacitive load of 4000.sup.2+4000.sup.2
multiplied by f*C. If, on the other hand, a waveform is applied via
a tuned circuit with Q factor (Bandwidth 3 dB/Frequency) of 200,
the power consumption is reduced to less than 2.5 Watts.
Theoretically the power is P(cos.THETA.)) where .THETA. is the
angle between the current and the voltage applied to the capacitive
load, and P is 2V.sup.2fC. This power consumption approaches zero
if the current and voltage are out of phase by 90 degrees, as they
would be in a perfectly tuned LC circuit with ideal components.
Similarly, if the waveform is asymmetrical with duty cycle of 2:1,
as for example in a FAIMS application, then the theoretical power
consumption is reduced to 333 Watts, produced by the sum of squares
of the voltage changes on the capacitive load of
4000.sup.2+2000.sup.2+(2000.sup.2-1333.sup.2) times f*C.
[0008] Since a tuned circuit cannot provide a square wave, an
approximation of a square wave is taken as the first terms of a
Fourier series expansion. One approach is to use: V(t)=2/3D
sin(.omega.t)+1/3D sin(2.omega.t-.pi./2) (1) where V(t) is the
asymmetric waveform voltage as a function of time, D is the peak
voltage (defined as dispersion voltage DV), and .omega.0 is the
waveform frequency in radians/sec. The first term is a sinusoidal
wave at frequency .omega., and the second term is a sinusoidal wave
at double the frequency of the first sinusoidal wave, 2.omega..
Alternatively, the second term is represented as a cosine, without
the phase shift of .pi./2.
[0009] In practice, both the optimization of the LC tuning and
maintenance of the exact amplitude of the first and second applied
sinusoidal waves and the phase angle between the two waves is
required to achieve long term, stable operation of a FAIMS system
powered by such an asymmetric waveform generator. Accordingly,
feedback control is required to ensure that the output signal is
stable and that the correct waveform shape is maintained.
[0010] In U.S. Pat. No. 5,801,379, which was issued on Sep. 1,
1998, Kouznetsov teaches a high voltage waveform generator having
separate phase correction and amplitude correction circuits. This
system uses additional components in the separate phase correction
and amplitude correction circuits, thereby increasing complexity
and increasing the cost of manufacturing and testing the devices.
Furthermore, this system cannot be implemented in the control
software, making it difficult to vary certain operating parameters
during use.
[0011] It is an object of the instant invention to provide an
asymmetric waveform generator based on LC tuning electronics that
overcomes the limitations of the prior art.
SUMMARY OF THE INVENTION
[0012] In accordance with an aspect of the instant invention there
is provided an apparatus for generating a periodically varying
electrical signal for creating a periodically varying electrical
field between electrodes of an ion mobility spectrometer,
comprising: an output port; a first tuned circuit for being
electrically coupled to an external power source and for, in
isolation, providing a first periodically varying electrical signal
having a first frequency, the first tuned circuit coupled to the
output port for providing an output electrical signal having a
component at the first frequency thereto; and, a second tuned
circuit for being electrically coupled to an external power source
and for providing a second periodically varying electrical signal
having a second frequency different from the first frequency, the
second tuned circuit coupled to the first tuned circuit for varying
the output electrical signal about the first periodically varying
electrical signal.
[0013] In accordance with another aspect of the instant invention
there is provided an electromagnetic transformer comprising: a
secondary winding comprising a plurality of turns of a first wire
wound defining a core and having an approximately uniform spacing
between adjacent turns; a first primary winding comprising at least
one turn of a second wire wound around the core and spaced apart
from both the core and the secondary winding; and, a second primary
winding comprising at least one turn of a third wire wound around
the core in parallel with the first primary winding and spaced
apart from both the core and the secondary winding.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Exemplary embodiments of the invention will now be described
in conjunction with the following drawings, in which similar
reference numbers designate similar items:
[0015] FIG. 1 shows a plurality of cycles of an asymmetric waveform
that is formed as a combination of first and second sinusoidal
waves of frequency .omega. and 2.omega., respectively;
[0016] FIG. 2a shows a simplified circuit diagram of an electronic
circuit for adding two waves of different frequencies according to
an embodiment of the instant invention;
[0017] FIG. 2b shows a simplified circuit diagram of an electronic
circuit for adding two waves of different frequencies according to
another embodiment of the instant invention;
[0018] FIG. 3a shows a simplified circuit diagram of an electronic
circuit for adding two waves of different frequencies according to
yet another embodiment of the instant invention;
[0019] FIG. 3b shows a timing diagram for applying pulses to the
electronic circuit of FIG. 3a;
[0020] FIG. 4a is a simplified diagram of an inductor suitable for
use with the electronic circuits of FIG. 2a, FIG. 2b, and FIG.
3a;
[0021] FIG. 4b shows a timing diagram for applying pulses to the
inputs of the inductor of FIG. 4a;
[0022] FIG. 5 shows a simplified diagram of another inductor
suitable for use with the electronic circuits of FIG. 2a, FIG. 2b,
and FIG. 3a; and,
[0023] FIG. 6 shows a simplified diagram of yet another inductor
suitable for use with the electronic circuits of FIG. 2a, FIG. 2b,
and FIG. 3a.
DETAILED DESCRIPTION OF THE INVENTION
[0024] The following description is presented to enable a person
skilled in the art to make and use the invention, and is provided
in the context of a particular application and its requirements.
Various modifications to the disclosed embodiments will be readily
apparent to those skilled in the art, and the general principles
defined herein may be applied to other embodiments and applications
without departing from the spirit and the scope of the invention.
Thus, the present invention is not intended to be limited to the
embodiments disclosed, but is to be accorded the widest scope
consistent with the principles and features disclosed herein.
[0025] As is noted above, the waveform that is applied in FAIMS is
a combination of two sinusoidal waves of frequency omega (.omega.)
and two times omega (2.omega.). The two sinusoidal waves are of
amplitudes that differ by a factor of two and that are offset by a
phase shift of .pi./2, resulting in a waveform that is defined by,
for example, Equation 1, below: V .function. ( t ) = 2 3 .times. D
.times. .times. sin .function. ( .omega. .times. .times. t ) + 1 3
.times. D .times. .times. sin .function. ( 2 .times. .omega.
.times. .times. t - .pi. / 2 ) ( 1 ) ##EQU1## This simple equation
is the equivalent of the first two terms of a Fourier series, which
describes a square wave with a 2:1 duty cycle.
[0026] In practice the application of two sinusoidal waves of
frequency .omega. and 2.omega. is used to generate a waveform with
the shape shown at FIG. 1. The sinusoidal wave of frequency
2.omega. is applied with a 90 degree phase shift and amplitude that
is 50% of the amplitude of the sinusoidal wave of frequency
.omega.. The peak voltage D, which is equal to the dispersion
voltage, DV, is normalized to one as shown at FIG. 1.
[0027] Referring now to FIG. 2a, shown is a simplified circuit
diagram of an electronic circuit for adding two waves of different
frequencies according to an embodiment of the instant invention,
and for delivering the voltage through a conductive port 5 to one
of the electrodes of FAIMS. Four inductances, IN1, IN2, IN3 and IN4
in a novel arrangement are used to drive a capacitive load FAIMS
including inner electrode 2 and outer electrode 4. Power is
supplied to each one of the inductances IN1, IN2, IN3 and IN4
through a primary winding by a pulsed input signal. In the
embodiment of FIG. 2a, pulsed input drive signals are used because
they can accurately be generated by digital circuits, providing a
high degree of control of the timing, frequency, and phase
relations between the signals. For instance, PP1 is a pulse in the
positive polarity (plus) applied to the primary of IN1 and PM1 is a
pulse in the negative polarity (minus) also applied to the primary
of IN1 but out of phase with PP1. For illustrative purposes, each
of the positive-going and negative-going pulses is applied to a
separate primary winding wound onto inductor IN1. Similarly, a
series of positive inputs PPn and negative inputs PMn are applied
to inductors INn, where n=1 to 4 in FIG. 2a. Each of the inductors
IN1, IN2, IN3 and IN4 is wound with a secondary winding that
becomes part of an LC tuned circuit along with an output capacitive
load. Since the LC tuned circuit is not a perfect oscillator, some
energy is required to sustain oscillations. The loss rate is
matched by the supply of power introduced through the primary
windings on inductors IN1 through IN4.
[0028] Inductors IN1 and IN2 are arranged in series with each
other, and the input pulses of a first frequency are approximately
identical and in phase. In other words, in this example, PP1 and
PP2 are identical, and PM1 and PM2 are identical, however the
positive-going (PPn) and negative-going (PMn) pulses are applied
alternatively in a push-pull manner, not simultaneously to the
inductors. The combined inductances of IN1 and IN2 are selected to
oscillate in tuned resonance with a capacitance of C3 combined in
parallel with the capacitance of the rest of the circuit attached
to the secondary windings of IN3 and IN4, namely C4, and FAIMS load
plus all other stray capacitances throughout the circuit. C1 and C2
do not contribute to the tuning as they are bypass capacitances for
the DC voltages B1 and B2. C5 does not contribute to the tuning of
IN1 and IN2 as it is balanced across IN3 and IN4. For example, if
the combined inductance of IN1 and IN2 is 0.45 mH then the circuit
will oscillate at 750 kHz if the capacitance of C3 in parallel with
the rest of the circuit is 100 pF.
[0029] The secondary windings of IN3 and IN4 are in series, but the
center tap between these inductors is attached to the secondary of
IN1 and IN2. This means that the combined oscillation of the IN3
and IN4 is around the floating voltage provided from IN1 and IN2.
It is therefore possible for IN3 and IN4 to oscillate at a second
frequency that is independent of the first frequency of oscillation
of IN1 and IN2. The secondary windings of inductors IN3 and IN4 are
coupled with three capacitors in a symmetrical arrangement. One
capacitor, C5, is parallel to the inductors IN3 and IN4, whereas
the other two capacitors, C4 and the FAIMS load, are each in series
with ground or with some other dc potential, for example B1 in FIG.
2a. Since capacitors C4 and the FAIMS load are referenced to the AC
ground potential, their respective values must be equal for the
resonant circuit at IN3 and IN4 to be balanced, i.e. for the same
instantaneous and opposite polarity voltage to appear at the
terminals of C5 relative to the center tap between IN3 and IN4. For
example, IN3 and IN4 oscillate at 1500 kHz if the total
capacitance, including stray capacitance, is 25 pF and the
inductance is 0.45 mH. Note that the series arrangement of the
FAIMS load and C4 in FIG. 2a minimizes the apparent capacitance of
the FAIMS load. For example, if the FAIMS load, electrodes 2 and 4
in FIG. 2a, is approximately 25 pF, and C4 is approximately 25 pF
(the total series combination is 12.5 pF), with appropriate
selection of C5 (to 12.5 pF) the net capacitance coupled to the
inductors IN3 and IN4 is 25 pF.
[0030] In FIG. 2a there are two inputs for dc bias voltages B1 and
B2. B1 establishes the dc offset voltage applied to the outer
electrode 4 of FAIMS. The dc bias voltage may be used to establish
a desired voltage difference between FAIMS and some other detector
device such as the input plate of a not illustrated mass
spectrometer. The dc bias voltage B2 is used to establish the dc
offset voltage applied to the inner electrode 2 of FAIMS. The
asymmetric waveform is superimposed upon this dc bias voltage.
Also, the compensation voltage, defined by the difference between
the dc voltages applied to the inner and outer electrodes of FAIMS,
is established by the difference in dc voltage of B1 and B2.
[0031] Tuning of IN3 and IN4, in concert with their capacitive load
including C5, C4 and the FAIMS load, is made possible through
adjustment of C5. Simultaneously, adjustment of C3 is required to
ensure that the tuning of IN1 and IN2 with the remaining circuit is
retained. Advantageously the computer control of this circuit is
possible by using adjustable capacitors whose capacitance is
changed by motors activated electronically.
[0032] Advantageously, the two frequencies applied to the two tuned
circuits may be adjusted independently with the input signal
provided to the other of the two tuned circuits disabled or fixed.
In other words, if the inputs PP1, PP2, PM1 and PM2 are all reduced
to zero, the application of PP3, PP4, PM3, and PM4 activates the LC
oscillation at a frequency defined by the values of the inductances
and capacitances attached to IN3 and IN4. The tuning of this part
of the circuit is adjusted by changing the input frequency and
voltages applied to PP3, PP4, PM3, and PM4, as well as by adjusting
the variable capacitor C5. Similarly, with the inputs PP3, PP4,
PM3, and PM4 set to zero, the oscillator defined by IN1, IN2 and
their capacitive load, is activated by applying PP1, PP2, PM1 and
PM2. Adjustment of this LC oscillation is achieved by changing the
voltage and frequency applied to PP1, PP2, PM1 and PM2, and by
adjusting variable capacitor C3. If both oscillators are
independently optimized to maximum efficiency, quality value Q, the
phase shift between the oscillations are adjusted by digital
control of the phase difference between the PP1, PP2, PM1, PM2
relative to PP3, PP4, PM3, PM4 inputs.
[0033] Optionally, in a microprocessor controlled system it is not
necessary to zero the other frequency to tune each resonant
circuit. In this case, the data processing system extracts the
amplitude of each frequency from the combined waveform.
[0034] Of course, a person skilled in the art will appreciate that
optionally the sinusoidal drive waveforms are applied to a not
illustrated conventional version of primary coil on the inductors
IN1, IN2, IN3 and IN4. For maximum control over the drive
waveforms, additional electronics, optionally including digital
synthesis of the sinusoidal waveforms, may be also utilized.
[0035] Referring now to FIG. 2b, shown is a simplified circuit
diagram of an electronic circuit for adding two waves of different
frequencies according to another embodiment of the instant
invention, and for delivering the voltage through a conductive port
5 to one of the electrodes of FAIMS. Elements labeled with the same
numerals have the same function as those illustrated at FIG. 2a. In
FIG. 2b, a single inductor IN1b replaces the two inductors IN1 and
IN2 running at a first frequency. Similarly, a single inductor IN2b
replaces the duplicated series inductors IN3 and IN4 operating at a
second frequency. The secondary windings on the inductors IN1b and
IN2b are analogous to those described with reference to FIG. 2a.
The associated capacitors and the FAIMS load electrodes 2 and 4 are
the same in both figures.
[0036] Referring now to FIG. 3a, shown is a simplified circuit
diagram of an electronic circuit for adding two waves of different
frequencies according to yet another embodiment of the instant
invention. Elements labeled with the same numerals have the same
function as those illustrated at FIG. 2a. In FIG. 3a, a single
inductor IN1b replaces the two inductors IN1 and IN2 running at a
first frequency. Similarly, a single inductor IN2b replaces the
duplicated series inductors IN3 and IN4 operating at a second
frequency. The secondary windings on the inductors IN1b and IN2b
are analogous to those described with reference to FIG. 2a. The
associated capacitors and the FAIMS load electrodes 2 and 4 are the
same in both figures. FIG. 3a also illustrates an optional approach
for application of the driving currents to initiate and maintain
the oscillation in the tuned LC circuit. In FIG. 3a the primary
winding consists of a center-tapped winding. The center tap is
coupled to a dc power supply (for example +28 volts is shown at
FIG. 3a). The voltage available at this terminal affects the
amplitude of the wave generated by the particular oscillator. The
two portions of the primary winding are alternately connected to
ground potential through switches. The primary winding of inductor
IN1b is operated at a first frequency by alternately grounding the
primary winding through switches Sa and Sb, only one of which is
closed at any time, as shown in the timing diagram at FIG. 3b. When
Sa is closed, current runs through one half of the primary winding
in a first direction. At a later time, Sa is opened and Sb is
closed so as to drive current through the other half of the primary
winding, but in a second direction. In a practical implementation,
a dead zone is required between the opening of one switch and the
closing of the next one, i.e. break-before-make operation. As a
result of the changing magnetic fields thus induced in IN1b, an
oscillating high voltage potential appears on the secondary
winding, assuming that the drive frequency is such that an LC
oscillation takes place. The primary winding of IN2b is operated in
a similar manner to that of IN1b except that the frequency of
oscillation is different and a phase difference exists between the
oscillations induced in IN1b and IN2b. The switches Sa, Sb, Sc, and
Sd shown in FIG. 3a are preferably electronic. Optionally, another
circuit is used to generate an input signal to IN1b and IN2b
similar to that generated through the use of the electronic
switches shown in FIG. 3a. For example, a conventional primary
coil, with a sinusoidal voltage applied can be used, if appropriate
control of voltage, frequency and phase is implemented. The example
in FIG. 3a uses digitally controlled switches as an illustration of
a simple interface to a digital control circuit. Optionally,
another known method of delivering an input driver oscillation for
the primary coils on IN1b and IN2b is used.
[0037] FIG. 4a illustrates the fundamentals of the windings of
inductors IN1, IN2, IN3 and IN4 that were discussed in relation to
the circuit shown at FIG. 2a. A similar approach is taken for IN1b
and IN2b in FIG. 3a but using one center-tapped primary winding
rather than two completely independent primary windings as shown in
FIG. 2a and FIG. 2b. Referring still to FIG. 4a, primary winding 20
is coupled to an input 10 and primary winding 22 is coupled to an
input 12, to which are applied a primary positive pulse (PP1 in
FIG. 2a) and a primary negative pulse (PM1 in FIG. 2a),
respectively. Referring also to FIG. 4b, the primary positive pulse
is composed of a square wave with a low side 47 near zero volts and
a high side 45 at an adjustable value (for example +5 V as shown at
FIG. 4b). The primary negative pulse is composed of a square wave
with a high side 50 near zero volts and negative side 55 at an
adjustable voltage (for example -5 V as shown at FIG. 4b). As shown
in the timing diagram of FIG. 4b, the pulses are applied in an
alternating fashion, wherein the voltage 45 is applied on the
primary positive pulse while voltage 50 is applied on the primary
negative pulse. Similarly, the voltage 47 is applied on the primary
positive pulse while voltage 55 is applied on the primary negative
pulse. The effect is to create magnetic fields in the inductive
core 14, which alternately changes direction during application of
pulses in the positive polarity through input 10 and negative
polarity through input 12. The pulses are driven through load
resistors 16 and 18 on the positive and negative sides,
respectively. The load resistors 16 and 18 ensure a minimum source
impedance for the driver circuit. This source impedance multiplied
by the square of the transformer turns ratio appears as a load in
parallel with the secondary tuned circuit. This extra load
reflected from the primary source impedance is driven by the LC
tuned circuit, thereby reducing the real voltage amplitude output
of the combined LCR circuit. If one chooses the circuit parameters
so that R is equal to the tuned impedance of LC, the output voltage
is one half of a similar free running (or unloaded) LC tuned
circuit. The currents in primary windings 20 and 22 result in
magnetic fields in core 14 that also induce electrical currents in
the secondary winding 24. The voltage induced in the secondary
winding is related to the number of times the secondary winding 24
is wrapped around the core 14 relative to the number of times that
primary winding 20 or 22 is wrapped around core 14. Referring again
to FIG. 2a, the secondary windings 24 of inductors INn are linked
to a capacitive load. Preferably, the inductance of the secondary
winding 24 wrapped around core 14 is suitable for a tuned LC
oscillation with the capacitive load.
[0038] While FIG. 4a illustrates schematically the concepts used in
the present invention, a novel approach was discovered to reach the
performance required for the FAIMS application. The FAIMS
application requires a high voltage (for example 4000 Volt peak)
into an approximately 20 pF load. Minimization of the power
consumption also benefits from an LC oscillator with a high quality
factor (Q) of over 200.
[0039] FIG. 5 illustrates two improvements of the embodiment shown
at FIG. 4a. First, the secondary winding 30 is wrapped along a
significant portion of the core 32. This permits an increased
number of turns of the secondary winding 30 to be placed on the
core 32, relative to the arrangement illustrated at FIG. 4a. Each
turn of the secondary winding 30 is spaced-apart from adjacent
turns of the secondary winding 30 in a direction along the length
of the core 32. The set of parallel primary windings 34 and 36 from
the drive circuit are wrapped external to the turns of the
secondary winding 30, and are spaced away from the core 32 and from
the secondary winding 30 by an air gap to prevent electrical
discharge and capacitive coupling between the primary windings
(either 34 or 36) and the secondary winding 30. The second
improvement is a modification of the core 32. A segment of the core
32 is removed to leave a gap 38. Alternatively, the core 32 is
formed initially into a substantially C-shape, leaving a space
between opposite ends of the core 32 that defines the gap 38. This
gap 38 is required in order to prevent electrical discharge and
electric field leakage through the core material between the two
ends of the secondary windings 30 which may have significant
voltage differences between them. The gap 38 also minimizes the
heat generated in the core material in the region between the two
ends of the secondary windings. Heat is generated by electrical
leakage and power losses in the material between the two ends of
the secondary windings and through the core. The gap 38 minimizes
this power loss. The core material is chosen not to have a high
magnetic permeability, this is necessary for the number of turns
and the inductance requirements of the application. The material
also exhibits low losses at the frequencies of interest. Therefore
the gap 38 does not significantly change the inductance of the core
32 and the secondary winding 30.
[0040] FIG. 6 illustrates an additional improvement of the
schematic shown at FIG. 5. In particular, the system shown at FIG.
5 is limited in effectiveness because the ability of the primary
windings 34 and 36 to induce magnetic fields in the core 32 is
limited by (i) the small number of turns and (ii) the limited
coverage of the core 32. The improvement is realized by running a
second, or optionally more, set of parallel primary windings at
different locations around the core 32. In FIG. 6, the primary
windings are 60, 62, 64 and 66. The primary positive pulse is
applied to the primary windings at 68 in FIG. 6. After load
resistance 70, the primary windings 60 and 64 are wound in parallel
around the core 32. Similarly, the primary windings for the
negative pulse input are wound in parallel at 62 and 66 after load
resistance 72 and are powered from the primary negative pulse
applied to wire 74. This achieves two purposes. First, the magnetic
field induced in the core 32 is higher, and therefore the coupling
between the primary and the core is more efficient, in part because
there are more primary windings around the core 32. Since the
magnetic permeability of the core material is low (due to circuit
requirements as mentioned above) the magnetic lines are not
contained easily within the core material and the coupling factor
between the primary windings and the core material is poor, adding
a number of parallel primary windings significantly improves the
coupling factor. Secondly, it is an advantage that this efficiency
of applying the input pulse is increased without the need to wrap
further turns of the secondary winding 30 around the core 32. In
some cases additional turns on the secondary is impractical because
of the small size of the core 32 and it is important to achieve a
high ratio of output turns to input turns to induce as high output
voltage as possible on the secondary winding 30. In other words,
using the approach of parallel application of the primary winding
shown in FIG. 6, the voltage induced in the secondary winding 30
approaches the theoretical limit of the transformer design due to
the improved coupling factor in the primary circuit. The number of
turns of the primary winding appears to have been increased
significantly without having changed the transformer turns ratio.
Each of the primary windings, because they are wound in parallel,
continues to behave as a small number of turns and the ratio of
turns on the primary winding to the turns on the secondary winding
is unchanged.
[0041] Advantageously, a plurality of primary windings 60, 62 and
64, 66 as shown at FIG. 6 increases the efficiency of coupling to
the core 32. The large diameter turns in the primary windings
minimize the possibility of discharge from the primary winding to
either the core 32 or to the secondary winding 30. Furthermore, the
large diameter turns of the primary windings shown at FIG. 6
minimize the capacitance between the primary windings and the turns
of the secondary winding on the core 32. It would appear that this
decreased capacitance would be achieved at the expense of the
ability of the primary winding to induce a magnetic field in the
core 32. In fact, this expected effect is minimized. Although the
turns of the primary windings 60, 62, 64 and 66 in FIG. 6 are
further from the core 32, the wire of the primary winding is
longer, the enclosed area is larger but the total magnetic flux
remains the same (1 turn carrying the same current). In the instant
example, the chosen core magnetic permeability for this application
is .about.8 times that of air so the magnetic flux is mostly
concentrated in the core material rather than the surrounding air
space. Accordingly, a high efficiency of coupling the primary
windings and secondary winding is achieved by providing a plurality
of parallel primary windings, and a low capacitance is maintained
by providing large diameter primaries that are wound at various
locations around the core 32.
[0042] Beyond three or four parallel sets of primary windings wound
around the core 32, the efficiency of coupling does not further
increase significantly since the coupling is over 90% with three
sets of parallel primary windings. Additional sets of parallel
primary windings (beyond three or four) also have the detrimental
effect of increasing the stray capacitance between the primary and
secondary windings.
[0043] Advantageously, the cut toroid-shaped core results in a
small instrument package. Optionally, the core is provided in the
form of a bar, or another suitable shape.
[0044] The entire contents of U.S. patent application Ser. No.
10/529,309 filed on Mar. 25, 2005, are hereby incorporated by
reference.
[0045] Numerous other embodiments may be envisaged without
departing from the spirit and scope of the instant invention.
* * * * *