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CROSS-REFERENCE TO RELATED APPLICATIONS
My commonly owned, concurrently filed U.S. Patent application entitled "AC
To AC Power Converter With A Controllable Power Factor" and identified as
Ser. No. 293,045.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to methods and apparatus for synthesizing a single
or multiphase AC output waveform of a substantially constant frequency
from a multiphase input waveform of varying frequency. More particularly,
it relates to control of the timing of the operation of the solid state
switches which sequentially gate segments of each phase of the input
waveform to the output as a function of the frequency of the input
waveform.
2. Description of the Prior Art
In several applications, the production of a constant-frequency power
output from a variable-frequency power source is required. One, and
presently the most important, application in this category is aircraft
power conversion. Here the prime source of electrical power is a rotating
generator that receives its mechanical power input from the engine of the
aircraft. Since the engine speed varies, usually over a 2 to 1 range, it
is not possible for the generator to produce constant frequency output if
coupled directly to the engine. Heretofore the general practice has been
to insert a hydraulic constant-speed coupling device between the engine
and the generator, thereby enabling the generator to be driven at a
constant speed and hence to deliver a constant frequency power. Such a
system has several disadvantages, not least of which is the relatively
frequent and costly maintenance required.
An alternative system approach to aircraft power generation is to couple
the generator directly to the aircraft engine, allowing it to produce a
variable-frequency output power, as dictated by the engine speed. This
variable frequency power is then converted to accurately regulated
constant-frequency output power by means of a static frequency converter.
This type of arrangement is generally referred to as a
variable-speed-constant-frequency (VSCF) power generating system.
Two basic types of frequency converters have been proposed for VSCF
applications. In one type of converter arrangement, the alternating
voltage of the generator is converted first into a direct voltage by a
(phase-controlled) rectifier circuit, then the direct voltage is converted
back to alternating voltage (of the desired frequency) by a static
inverter circuit. In the other type, a static frequency changer, which is
capable of converting the variable-frequency alternating voltage of the
generator directly into constant-frequency output voltage, is employed.
The first type of arrangement is generally referred to as a DC link
converter, while the second type is called a direct AC to AC frequency
changer or frequency converter. Since the direct AC to AC frequency
changer is capable of converting the variable-frequency generator power
into constant-frequency output power in one stage, its operating
efficiency is generally higher and its weight and size are usually lower,
than those of its DC link type counterpart. For these reasons, the direct
AC to AC frequency changer appears presently the best solution for VSCF
power conversion.
Various types of direct AC to AC frequency changers have been proposed for
aircraft VSCF applications. These include the naturally commutated
cycloconverter (NCC), the unrestricted frequency changer (UFC), and the
unity displacement factor frequency changer (UDFFC). For a detailed
explanation of these frequency changers refer to pages 384 to 395 of the
book "Static Power Frequency Changers" by L. Gyugyi and B. R. Pelly, John
Wiley and Sons, Inc., 1976. These frequency changers have different
operating and performance characteristics resulting in often mutually
exclusive benefits and penalties when used in a VSCF power generating
system. The major operating and performance features of these frequency
changers in a VSCF power generating system can be summarized as follows.
The naturally commutated cycloconverter (NCC) employs controlled-rectifier
type semiconductors (SCRs) with no intrinsic turn-off capability. These
devices are commutated (turned-off) by the process of "natural
commutation", by which the current is transferred without external forcing
between the controlled-rectifier type circuit elements. This is achieved
by proper selection of the switching instants relative to the
instantaneous polarities of the input voltages, when the output voltage
waveform is synthesized. Natural commutation is desirable because
controlled-rectifier type devices are presently available with
sufficiently high rating in small physical sizes. However, the
restrictions in the output waveform construction to satisfy the conditions
for natural commutation result in a lagging input power factor (at any
load power factor) and in the generation of harmonic components in the
output that are difficult to filter. The lagging power factor increases
the rating and size of the generator; the harmonics necessitate a
relatively large output filter.
The unrestricted frequency changer (UFC) requires switching devices with
intrinsic turn-off capability (e.g., transistors) or an external
commutating circuit. The generated output voltage waveform of the UFC is
optimized for harmonic content; therefore, only a minimum amount of output
filtering is needed. The phase angle of the current drawn from the
generator is the negative of the load phase angle. Thus, a lagging load is
seen by the generator as a leading load, and vice versa, a leading load is
seen as a lagging load. In aircraft VSCF power systems, the load power
factor is usually in the lagging (inductive) domain. Thus, the power
factor seen by the generator is normally leading. This helps to keep the
rating of the generator relatively low, close to the output rating of the
UFC. However, at high generator speeds and under heavy inductive output
loads, the generator may become overexcited. This may require undesirably
high voltage rating for the semiconductors in the UFC or some form of
external overvoltage protection. Another potential problem with the UFC is
that at high generator frequencies, the switching rate of the
semiconductors is rather high (that is, f.sub.switching =f.sub.generator
+f.sub.out), which may result in undesirably high losses.
The Unity Displacement Factor Frequency Changer (UDFFC) requires two
complete converter circuits with devices having an intrinsic turn-off
capability (or an external commutating circuit). The two converters are
operated in a complementary fashion so that the input displacement (power)
factor remains unity under all output load conditions. Thus, the generator
has to supply only the real load power demand. This results in a generator
rating that is minimum for a given output rating. The distortion of the
output waveform is low, and thus the filtering requirement is also
relatively low. The switching rate of the devices in at least one of the
converter circuits is the same as in the UFC, which may cause some concern
for efficiency at high generator frequencies. The greatest disadvantage of
the UDFCC is the requirement for two complete power circuits, which make
it unattractive in most airborne applications, except possibly in those
applications where the high output requirements would make device
paralleling necessary.
SUMMARY OF THE INVENTION
In all of the frequency changers above described, a reference waveform
having the frequency of the desired output waveform can be used to
determine the switching instants of the solid state switches. In
conventional operation of an unrestricted frequency changer (UFC), the
switching occurs at instants which result in switching to input phases
which are more positive during periods when the reference waveform, and
therefore the fundamental component of the output waveform is positive
going and at instants which result in switching to input phases which are
more negative during periods when the reference waveform is negative
going. The unrestricted frequency changer can also be operated in what can
be called a complementary mode. That is, switching can be caused to occur
at instants which result in switching to input phases which are more
negative during periods when the reference waveform in positive going and
in switching to input phases which are more positive when the reference
waveform is negative going. The complementary mode of operation results in
the phase angle of the current drawn from the generator being the same as
the load phase angle, and in the switching rate of the bidirectional
switches being as low as possible (that is, f.sub.switching
=f.sub.generator -f.sub.out).
The basic concept of the invention is to operate the power converter as a
UFC in the lower part of the generator frequency range, and to operate it
in the complementary mode in the upper end of the generator frequency
range. With this operating scheme, the generator supplies power at a
leading (capacitive) power factor when the generator speed and therefore
frequency is low. This reduces the excitation requirement and helps to
minimize the size of the generator. The UFC type of operation also
provides the best frequency spectrum attainable with an AC to AC frequency
changer at low generator frequencies where the filtering of the output is
the most difficult. This permits the use of an output filter of minimum
size and weight. The switching rate of the bidirectional switches is also
moderate because the generator frequency is relatively low.
At the upper end of the generator frequency range where the complementary
mode of operation is used, the generator supplies power at a lagging
(capacitive) power factor which eliminates the potential problem of
self-excitation and the consequent high generator terminal voltages. The
switching rate (f.sub.switching =f.sub.generator -f.sub.out) of the
switching devices in the converter is only moderately higher than that
under conventional UFC operation (f.sub.switching =f.sub.generator
+f.sub.out) used in the lower part of the generator frequency range.
Although the components in the output waveform obtained with the
complementary operating mode have lower frequencies than those obtainable
with conventional UFC operation at the same frequency, the filtering
requirements essentially remain the same because of the increased
generator frequency in the upper part of the speed range.
In an aircraft power generating system to which the invention may be
applied, the generator speed, and therefore frequency, varies over about a
2 to 1 range. Actually, the aircraft engine runs at the two extremes most
of the time: at the minimum speed on the ground, and at or close to
maximum speed in the air. The intermediate speeds are generally
transistional. It is therefore proposed that the converter be operated as
a UFC up to a convenient, predetermined mid-frequency and that it be
operated in the complementary mode above the predetermined mid-frequency.
The switching instants are determined by generating timing waveforms which
are phase locked to the phases of the input waveform. When the input
frequency is less than or equal to the predetermined frequency, switching
occurs at instants when the reference waveform is equal to a timing
waveform but is opposite in slope. Under these conditions the converter
operates as a UFC. When the input frequency is above the predetermined
frequency, switching occurs at instants when the reference waveform is
equal to a timing waveform and has a slope of the same sense as the timing
waveform. This causes the converter to operate in the complementary mode.
The reference waveform and timing waveforms can be generated in various
interrelated forms. For instance, the timing waveforms and the reference
waveform can all be triangular. Or, the timing waveforms can be the well
known cosine waves and the reference waveform sinusoidal. As another
example, the timing waveforms can be ramp functions and the reference
waveform triangular. In the later two cases, two complementary reference
waveforms, 180.degree. out of phase with each other, are required. During
UFC operation, the portions of the reference waveforms which have a slope
opposite in sense to the slope of the timing waves are alternately
compared to the timing waves. During the complementary mode of operation,
the portions of the reference waveforms having a slope of the same sense
as the timing waveforms are used to determine the switching instants.
In the preferred embodiment of the invention, a first train of pulses
generated at the switching instants required for UFC operation is applied
to a first sequencer which sequentially turns on the switches when the
input frequency is equal to or less than the predetermined frequency. When
the frequency of the input waveform is above the predetermined frequency a
second train of pulses generated at the switching instants required for
complementary operation is applied to a second sequencer which turns on
the switches one at a time. The pulses are generated at the instants when
the reference waveform is equal in magnitude to, and either of opposite
slope or the same slope, respectively, as a timing waveform.
A three phase output can be generated by simultaneously switching to
separate output lines at each switching instant segments of phases of the
input waveform which are angularly displaced the same number of electrical
degrees as the phases of the desired multiphase output waveform. This can
be accomplished by having the sequencers simultaneously turn on the
required switches.
The invention relates both to the methods and apparatus for performing the
above described operations.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1(a) through (d) are waveform diagrams illustrating unrestricted
frequency changer (UFC) operation and complementary UFC operation;
FIG. 2 is a schematic diagram in block diagram form of a frequency changer
incorporating the invention;
FIG. 3 is a schematic diagram of power frequency converter suitable for use
in the frequency changer of FIG. 2 in which the switches are arranged in
two three-pulse groups;
FIGS. 4a, b, c are schematic diagrams illustrating combinations of
transistors and diodes forming bidirectional switches which may be used
alternatively for the switches in the power frequency converter of FIG. 3;
FIG. 5 is a schematic diagram of the Pulse Timing and Gating Circuits of
the frequency changer of FIG. 2 according to one embodiment of the
invention;
FIGS. 6(a) through (m) are waveform diagrams illustrating the operation of
the circuits of FIG. 5;
FIG. 7 is a schematic diagram illustrating circuitry for generating a
three-phase output waveform from the circuit of FIG. 5; and
FIG. 8 is a schematic diagram of the Pulse Timing and Gating Circuits of
the frequency changer of FIG. 2 according to another embodiment of the
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates the relationship between the output voltage waveforms of
a 6-pulse unrestricted frequency converter operated in the conventional
mode, waveform (b), and in the complementary mode, waveform (d). Waveforms
1(a) and (c) illustrate one way in which the switching instants for
waveforms 1(b) and (d) respectively can be generated and will be discussed
in more detail below.
The 6-pulse converter sequentially switches to an output line segment of
each phase of a six-phase sinusoidal input waveform. The switching
instants are selected such that the filtered output is a sinusoidal
waveform having a fundamental component of a preselected frequency. As
shown by waveform 1(b), under UFC operation the converter switches
successively to input phases which are more positive during those periods
when the fundamental output component is positive going and switches to
input phases which are more negative when the fundamental component of the
output waveform is negative going. Conversely, under complementary
operation, the converter switches successively to input phases which are
more negative while the fundamental component of the output waveform is
positive going and to input phases which are more positive when it is
negative going as shown by waveform 1(d).
The output waveform in both UFC and complementary UFC operation is produced
by causing the switching devices in the converter to conduct, and
therefore connect the generator voltages to the output, in sequence for a
fixed period of time. Thus, the output voltage waveform is constructed
from equi-length segments of the input voltages. The length (duration) of
the segments is different, however for the two operating modes, and can be
expressed in terms of the generator or input frequency, f.sub.I, the
desired output frequency, f.sub.o, and the pulse-number of the converter,
P, in the following way:
T.sub.UFC =l/P(f.sub.I +f.sub.o), (1)
T.sub.CUFC =l/P(f.sub.I -f.sub.o), (2)
where the subscripts UFC and CUFC indicate UFC and complementary UFC
operating modes, respectively. The pulse number of the converter is the
number of switching intervals per cycle of the input voltage. In the
example given, the pulse number 6 corresponds to the number of input
phases. However, as will be seen, this is not always so as where the
phases are divided into two three-pulse groups.
The frequency, f.sub.SW, of switching one generator voltage to the output,
that is, the rate at which each switching device in the converter is
operated, can be expressed for the two modes of operation as follows:
f.sub.SW(UFC) =f.sub.I +f.sub.o, (3)
f.sub.SW(CUFC) =f.sub.I -f.sub.o, (4)
In the example illustrated in FIG. 1 which we will consider to represent
the minimum generator frequency, f.sub.I =3f.sub.o and therefore the
minimum switching frequencies are:
f.sub.SW(UFC) =4f.sub.o, (5)
f.sub.SW(CUFC) =2f.sub.o, (6)
If we now consider the maximum generator frequency for an aircraft
electrical power system wherein as previously discussed the maximum
frequency is typically twice the minimum, then f.sub.I =6f.sub.o and the
switching frequencies using equations (3) and (4) are:
f.sub.SW(UFC) =7f.sub.o, (7)
f.sub.SW(CUFC) =5f.sub.o, (8)
Since in accordance with the invention the converter is operated as a UFC
at and around the minimum generator frequency (see equation 4) and is
operated in the complementary manner at and around the maximum generator
frequency (see equation 8), the switching frequency of the devices in the
converter at maximum generator frequency is only about 25% higher than at
minimum generator speed. If the transition from UFC to complementary UFC
operation is made just below the mid-point of the generator frequency
range, that is where:
f.sub.Imid =(f.sub.Imax +f.sub.Imin)/2, (9)
the switching frequency would be:
f.sub.SW(max) =5.5f.sub.o, (10)
which is about 37% higher than the minimum switching frequency (see
equation 4).
As illustrated by the waveforms of FIGS. 1(b) and 1(d), the fundamental
component of the output voltage waveform generated by the UFC and its
complementary counterpart are identical if the time instants of the
switching in the converter are properly related. This ensures that the
transition between the two operating modes can be accomplished without any
appreciable transients.
FIG. 2 illustrates in block diagram form a
variable-speed-constant-frequency (VSCF) power generating system
incorporating the present invention. The generator 1 is a 6 phase AC
generator driven by a prime mover, such as an aircraft engine, at variable
speed. The six phases of generator voltage are each connected to a switch
in a power frequency converter 3. A reference voltage wave generator 5,
generates a reference waveform at the frequency of the desired output
waveform. Pulse timing and gating circuits 7, generate a set of timing
waves phase-locked to the generator voltages and sequentially turn on the
switches in the power converter 3 at instants determined by a comparison
of the reference waveform to the timing waveforms. The sequential
operation of the switches in the power converter 3 produces a composite
waveform made up of selected segments of each phase of the generator
output. This composite waveform is passed through an L-C output filter 9
to produce the converter output having the desired frequency. The
frequency of the converter output is exactly equal to that of the
reference waveform. The amplitude of the output voltage can be regulated
by controlling the generator voltage through a voltage regulator 11 as
shown in FIG. 2. It is also possible to control the amplitude of the
output voltage internally in the converter by using, for example, the
technique of pulse width modulation which is described in U.S. Pat. No.
3,493,838.
It is common to arrange the switches used in the power frequency converter
3 of FIG. 2 in pulse groups. For instance, as shown in FIG. 3, the six
switches used to switch the six input phases V.sub.g1 through V.sub.g6 of
the converter can be arranged in two 3-phase groups. In this arrangement,
the switches 11, 13 and 15 associated with phases 1, 3 and 5 are operated
as one group 23 and switches 17, 19 and 21 associated with phases 2, 4 and
6 are operated as the second group 25. The waveforms generated by each 3
pulse group are combined through an interphase transformer 27 to produce a
composite output waveform V.sub.o. The arrangement of the switches in
pulse groups reduces the switching frequency of the individual switches.
The switches are shown in FIG. 3 as symmetrical field effect transistors
(FETs) with forward and reverse blocking capability. Actually any
bidirectional switch of suitable power, frequency, and voltage blocking
capability can be used. Thus such switches can also be realized with
arrangements of transistors and diodes as for example those shown in FIGS.
4a, b and c.
The gating signals for the switches of the power frequency converter 3 are
generated by the pulse timing and gating circuits 7 of FIG. 2 in
conjunction with the reference voltage wave generator 5. As previously
mentioned, one way of generating the gating signals is to generate a
triangular reference waveform and a set of triangular waveform timing
signals. A suitable arrangement for gating the switches in this manner is
shown in FIG. 5. The pulse timing and gating circuits 7 include a timing
wave generator 29 which generates 6 triangular timing waves phase locked
to the generator voltages. These signals can easily be generated by
techniques well known by those skilled in the art such as by shaping the
sinusoidal generator voltages into essentially square waves which are then
integrated.
In the interests of clarity, only the remaining portions of the pulse
timing and gating circuits 7 associated with the first pulse group 23 are
shown in FIG. 5. Each of the timing waves V.sub.T1, V.sub.T3 and V.sub.T5
is applied to a Slope Detector 31, 33 and 35 respectively which compares
the slope of the tuning wave to that of the reference waveform generated
by the Reference Voltage Wave Generator 5. If the slope of the timing wave
V.sub.T1 is opposite in sense to that of the reference waveform, V.sub.T1
is gated to a Zero Detector 37. If on the other hand, the slope of the
reference waveform and the timing waveform are of the same sense, V.sub.T1
is gated to a second Zero Detector 39. Similarly, the timing waveforms
V.sub.T3 and V.sub.T5 are gated to either Zero Detectors 41 and 45 or to
Zero Detectors 43 and 47 respectively depending upon whether they have a
slope which is of the opposite sense or the same sense as the reference
waveform.
The zero detectors compare the amplitude of the applied timing waveform to
that of the reference waveform and when they are equal a pulse is
generated. Pulses generated by the Zero Detectors 37, 41, and 45 are
applied through an OR element 49 to a three-state UFC Ring Counter 51
while those generated by Zero Detectors 39, 43 and 47 are applied to
another three-state CUFC Ring Counter 55 through OR element 53. The ring
counter 51 has three outputs each of which is connected to a gate drive
circuit for one of the FETs 11, 13 or 15 through an AND element 57 and an
OR element 59. In a similar manner, the three outputs of ring counter 55
are each connected to the gate drive circuit for one of the FETs of the
group through the OR element 59 and a second AND element 61. A Generator
Frequency Discriminator 63, which monitors the frequency of the voltages
generated by the generator 1, supplies a second gating signal B to AND
elements 57 and B to AND elements 61. The signal B is high when the
generator frequency is at or below a predetermined frequency while the
signal B is high when the frequency exceeds the predetermined value. Thus,
when the generator frequency is below the predetermined value, the
switches 11, 13 and 15 are controlled by the UFC Ring Counter 51 which
sequentially turns these switches on one at a time as the counter is
pulsed. On the other hand, sequential operation of these switches is
controlled by the CUFC Ring Counter 55 when the generator frequency is
above the predetermined value and B is high. The counter 51 is designated
the UFC Ring Counter since it is pulsed when the reference signal and a
timing waveform are equal in amplitude but opposite in sense and this
condition produces conventional UFC operation. Ring counter 55 on the
other hand produces complementary UFC operation and is therefore
designated the CUFC Ring Counter.
Operation of the circuit of FIG. 5 can be better understood by reference to
FIG. 6 which is a waveform diagram illustrating the signals that are
generated at various points in the circuit. Waveform 6(a) illustrates the
relationship between the reference waveform V.sub.REF generated by the
Reference Voltage Wave Generator 5 and the timing waveforms V.sub.T1,
V.sub.T3 and V.sub.T5 generated by the Timing Wave Generator 29. The
portions of these waveforms with a negative slope are shown in dotted line
form only for clarity. For purposes of illustration, the frequency of the
generator and therefore of the timing waveform signals is higher in the
right side of the diagram.
The waveform shown at 6(b) is the pulse train PUFC which is applied to the
UFC Ring Counter 51 as also indicated in FIG. 5. Similarly, waveform 6(c)
represents the pulse train PCUFC applied to CUFC Ring Counter 55. Waveform
6(d) illustrates the signal B which is high on the left side of the Figure
indicating that the generator frequency is below the predetermined mode
switching frequency, but goes low to initiate a transfer to complementary
operation in the right side of the figure.
Waveforms 6(e), (f) and (g) represent the UFC operation drive signals,
DUFC, appearing at the three outputs of the UFC Ring Counter 51. In like
manner, waveforms 6(h), (i) and (j) represent the complementary operation
drive signals, DCUFC, generated by CUFC Ring Counter 55. The portions of
these waveforms which are selected for driving the converter switches are
shown in full line while the unselected portions are shown in dashed
lines. Thus during UFC operation shown in the left side of the figure, the
DUFC waveforms are shown in full line while the DCUFC waveforms appear in
dashed lines with the representations reversed during complementary
operation shown in the right side of the figure.
Waveform 6(k) illustrates the three-pulse unfiltered output waveform,
V.sub.DUFC, generated during UFC operation. Portions of the generator
phase voltages V.sub.g1, V.sub.g3 and V.sub.g5 are switched to the output
in accordance with the pattern of the drive signals DUFC shown in
waveforms (e), (f) and (g). Similarly, waveform 6(e) illustrates the
unfiltered three-pulse waveform generated during complementary UFC
operation again with the selected portion shown in full line and the
unselected portion in dashed line.
Waveforms 6(m), (n) and (o) represent the three drive signals D.sub.1,
D.sub.3 and D.sub.5 generated by the circuit of FIG. 5 at the outputs of
the OR elements 59. They illustrate that with the signal B high indicating
low generator frequency, the DUFC signals are selected to drive the
bidirectional switches of the converter and that the DCUFC signals become
the drive signals during complementary operation when the B signal is low
and generator frequency is high. The final waveform (p) in FIG. 6,
illustrates the three pulse output waveform V.sub.3PO generated by the
three pulse group 23 shown in FIG. 3. A similar three pulse waveform
V.sub.3PO ' derived from generator voltages V.sub.g2, V.sub.g4 and
V.sub.g6 is simultaneously generated by three-pulse group 25 with these
two signals being combined by the interphase transformer 27 to produce the
output signal V.sub.o. V.sub.OFUND appearing in waveforms 6(k), (l) and
(p) represents the fundamental component of this output waveform V.sub.o.
The drive signals for the switches of three-pulse group 25 are generated by
a circuit which uses the reference waveform from the Reference Voltage
Generator 5, the B and B signals from the Generator Frequency
Discriminator 63, the V.sub.T2, V.sub.T4 and V.sub.T6 timing signals from
the Timing Wave Generator 29 and circuitry identical to the remaining
components in FIG. 5.
FIG. 7 illustrates a circuit for generating a three-phase output waveform
in accordance with the invention. In this arrangement, the Drive signals
D.sub.1, D.sub.3 and D.sub.5 generated by the OR elements 59 of FIG. 5
simultaneously turn on through drive circuits 65 bidirectional switches in
three, three-pulse groups 23a, b and c each associated with one of the
output phases.
Returning to FIG. 1, the timing waveforms illustrated are ramp functions
which are phase locked to the phase voltages of the generator. In using
this type of timing waveform, two triangular reference waveforms A and B
must be generated. These waveforms have the same frequency but are
180.degree. out of phase. During UFC operation the positive sloped
portions of the two reference waveforms are compared with the timing
waveforms to determine the switching instants. During complementary UFC
operation the negative slopes are used. FIG. 8 discloses a circuit for
generating the switching signals using this pattern of timing and
reference waveform. The components in FIG. 8 which are identical to those
in FIG. 5 use the same reference characters. Those that are related but
modified utilize primed reference characters. The Reference Voltage Wave
Generator 5 generates the two reference waveforms A and B and applies the
positive sloping segments of these waveforms to Zero Detectors 37, 41 and
45 and the negative sloping segments to Zero Detectors 39, 43 and 47.
Timing Wave Generator 29' generates the negatively sloped, ramp timing
signals V.s | | |
