CTD LINE HAVING A PLURALITY OF CTD ELEMENTS

LIGNE A DISPOSITIFS A TRANSFERT DE CHARGE MULTIPLES

English Abstract

ABSTRACT
In prior art filters using CTD (charge transfer device)
lines and having relatively narrow band widths, the capacitance
ratios of the CTD lines employed differ significantly, which
leads to difficulties with respect to tolerances in the manu-
facturing process. The present invention provides a CTD line
consisting of a plurality of CTD elements with which one obtains
any random division ratio without the influence of the tolerances
having a negative influence at the same time. The CTD line has
a plurality of CTD elements with separating ridges for charge
division oriented in the direction of charge transport, the
ridges being applied as thick oxide ridges or channel-stop
regions. The separation ridges for successive stages are
disposed to yield a charge division in the ratio of approximately
1:1. A plurality of summation sections are provided between the
division stages, all remaining partial charges with the
exception of only one partial charge being summed in the
summation sections.

Claims

WHAT IS CLAIMED IS:
1. In a CTD line having a plurality of CTD elements in
with separating ridges for charge division oriented in the
direction of the charge transport, such ridges being applied as
thick oxide ridges or channel-stop regions, the combination
comprising: a plurality of successive charge division stages,
the separation ridges for successive stages being disposed to
yield a charge division in the ratio of approximately 1:1, and
plurality of summation sections provided between the division
stages, all remaining partial charges with the exception of only
one partial charge being summed in said summation sections.
2. Apparatus according to claim 1, including a channel
expansion provided for the channel carrying the smaller charge
component.
3. Apparatus according to claim 2, wherein said channel
is increased at each stage to provide a substantially constant
surface charge density.
4. Apparatus according to claim 2, including a narrowing
of the other channel, whereby a substantially constant charge
density is obtained.
5. Apparatus according to claim 4, wherein said other
channel is narrowed by the factor K, where K is
<IMG>
6. The method of arranging a CTD line for charge
division which is substantially independent of manufacturing
tolerances, comprising the steps of,

providing each of a plurality of charge division stages
with a separation ridge oriented in the direction of the charge
transport, disposed to yield a charge division in the ratio of
approximately 1:1;
providing summation sections between the division stages
for summing all remining charges with the exception of only one
partial charge.
7. The method according to claim 6, including the step
of expanding the channel carrying the smaller charge component.
8. The method according to claim 7, including the step
of narrowing the other channel, to obtain a substantially
constant charge density.

Description

~lS71~j8
S P E C I F I C A T I O N
T I T L E
"CTD LINE HAVING A P~URALITY OF CTD ELEMENTS~
BACRGROUND OF THE INVENTION
Field of the Invention
The invention relates to a CTD line having a plurality of
CTD elements in which separating ridges are oriented in the
direction of the charge distribution, the ridges being
specifically applied in the manner of thick oxide ridges or
~.~
channel-stop regions.
CTD elements and CTD lines of the aforementioned type are
known, and are described in the periodical "Elektronikn, 1974, No.
1, pages 3 through 8. The article "Ladungsverschiebeschaltungen"
in such periodical treats the manner of functioning of such CTD
circuits in detail and it also describes that so-called bucket
brigade circuits (BBD circuits) and so-called charge-coupled
circuits (CCD circuits), are also to be understood by that term.
The abbreviations CTD and CCD are abbreviations taken from
English-language technical literature which have the meaning
"charge transfer devices", "bucket brigade devices" and "charge
coupled devices", respectively.
Such CTD lines are also described in the book "Charge
Transfer Devices" by Sequin and Tompsett, Academic Press, New
York, San Francisco, London, 1975. On pages 43 and 44 of that
book, i~ is described that so-called "channel-stop-diffusion
regions" can be provided in addition to thick oxide ridges as
separating ridges. Combinations of these two methods are
likewise possible. It is also known that layers consisting of
thin oxide and thick oxide may be employed for the insulation of
conducting surfaces.
Such charge transfer circuits can be employed for the
realization of integrated filter circuits. See German Letters
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11571~8
Patent 2,453,669 and 2,555,835. Circuits are specified in these
patents which require a firmly prescribed capacitance ratio to
one another. Given the filter circuits desired there, when
filters with relatively narrow band width are described, then the
capacitance ratios of the CTD lines to be employed differ signi-
ficantly, which in turn leads to difficulties with respect to the
tolerances in the technological manufacturing process. This is
because in the manufacturing process, the tolerances which occur,
for example, in the masks to be employed as well as in the
technological process, lead to changes in the channel width and
the electrode length of such CTD arrangements. Scatterings also
derive in the active electrode surfaces. The charge distribution
affects the ratio of the electrocapacitances so that such toler-
ances have a direct effect on the filter characteristic as well.
The aforementioned references are only to be considered as
examples, because the task of distributing charges in a definite
ratio, occurs over and over even in other such CTD lines.
SU~ARY OF TE~E INVENTION
The object of the present invention is to specify a CTD
line consisting of a plurality of CTD elements with which one
obtains any random division ratio without the influence of the
tolerances having a negative influence at the same time.
This object is achieved, in achieving a prescribed charge
division ratio, because the charge division takes place in more
than one step, with the separation ridges for successive division
steps being disposed in such manner that a charge divsion in the
ratio of approximately 1:1 results for the individual step, and
summation sections are provided between the division stages, all
remaining partial charges (with the exception of only one partial
charge) being summed in said summation sections.
In one advantageous embodiment of the present invention,
one can provide a channel expansion for the partial branch con-

~57~8
ducting the smaller charge component.
In the present invention, any random division ratio canbe realized by means of a plurality of division ratios of respec-
tively approximately 1:1 which have a cumulative effect.
BRIEF DESCRIPTION OF THE DRAWING
Reference will now be made to the accompanying drawing,
which shows a four-stage charge division with a division ratio
A:B per stage.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In the apparatus shown, separation of the charge packets
is provided by means of thick oxide ridges. In order to avoid an
increase of the surface charge density-, the channel width is
increased with each thick oxide ridge. The charge division and
channel expansion is undertaken under different electrodes. The
charge component Ai is divided at the next stage in, for example,
the ratio A:B = Ai+l:Bifl, whereby the charges are subsequently
summed under the B-surfaces.
The following relation applies in general for random
division ratios Ai:Bi: n
qA = A1 x A2 x .................. x An = ~ Aj
qges (Al+Bl) (A2+B~ ( n Bn) j =l (Aj+Bj)
K = qB = 1 - qA ; K = 1 ~ ~ r
qges qges j=l LAj+Bj~
In the above, n indicates the number of division steps,
i.e., of stages and j denotes a counting or numerical variable.
qA qges (A+B)
applies for the resultant charge qA at the output for n stages
with a respectively identical division ratio A/B.
Analogously, the relation
K = _B = 1 _( A
qg~ s

11571~8
is valid for the division factor K.
Given narrow channels or large division ratios, i.e.,
many stages, the CCD channel for the last stages must be expanded
so that the channel width is not less than the technologically
conditioned channel width. This expansion is subsequently
reversed in channel B. A constant surface charge density can be
obtained in such manner. The influence of the poorer
transmission coefficient E of the channel expansion is slight,
since only a small charge component is affected by it for a few
elements.
It can be understood from the Figure that the charge qges
flows into the CTD line at the input of the CTD line and that a
channel width W is set there which is limited by means of thick
oxide layers DOX. The surfaces referenced with ~poly-Sil" and
"poly-Si2" are electrode surfaces which mutually alternate and
which, if need be, can overlap. The individual partial charges
are referenced with A and B, and the corresponding indices in
each stage show how the individual partial charges are divided
or, respectively, add up. The charge division takes place in
more than one step, and thick oxide ridges, oriented in the
direction of the charge transport, are provided for this purpose,
such ridges being likewise referenced with DOX. The ridges are
disposed in such manner that, for instance, a charge distribution
in the ratio of 1:1 ensues for successive division steps. The
summation sections Cl through C3 can also be seen in the Figure,
all remaining partial charges 81, Bl+B2, Bl+B2+B3 with the
exception of only one partial charge being summed in such
summation sections. As already explained above, the channel A
can then be expanded in the area 3 if the division ratio to be
achieved is particularly large, whereby the output 6, at which
the charge qa* appears, can be realized as a CTD line whose
manufacturing tolerances can still be well governed. A correc-
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;8
tion of the channel expansion for the charge components B isagain undertaken in the area 5, in such manner that the channel
obtains a smaller width W' = K W in said area in accord with the
equation already cited for K. A constant surface charge density
can be achieved by means of this feature. As shown in the
Figure, the charge qB* appears at the output 7. Accordingly, R =
qB*/qges likewise applies.
By means of the measures described above, the described
CTD lines are produced in such a way that scatterings in the
manufacture can be minimized by means of a successive charge
division. In accord with the generally prescribed charge divi-
sion ratio, a certain minimum number of CTD elements is required
for this purpose; however, this requirement can be accepted in
view of the precision to be achieved in the practical realization
of such arrangements.
It will be understood that various modifications and
additions may be made in the subject matter of the invention
without departing from the essential features of novelty thereof,
which are intended to be defined and secured by the appended
claims.
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