Monolithically integrated charge transfer circuit

A plurality of MIS charge transfer systems are connected to the output of a first MIS charge transfer system and are operated in parallel with each other. The plurality of charge transfer systems form filters of the same kind, or of a different kind, and charge packets emitted thereby have different delay times and are subsequently converted into a voltage which is sampled at a higher frequency than the sampling frequency for the signal which is applied to the input of the first MIS charge transfer system. This voltage is smoothed by a simple low pass filter.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a monolithically integrated charge 
transfer circuit having a signal scanner, a first clock pulse controlled 
metal-insulator-semiconductor (MIS) charge transfer system which is 
supplied with charge packets via the signal scanner and a clock pulse 
controlled output stage which is connected to the MIS charge transfer 
system and which serves for the restoration of a signal which is constant 
in time. 
2. Description of the Prior Art 
Such monolithically integrated charge transfer circuits are, for example, 
described in "Der Elektroniker", No. 3/1978, pp. EL7-EL15, whereas the 
essentials concerning MIS charge transfer systems having clock pulse 
control may be found in "Der Elektroniker", No. 2/1978, pp. EL3-EL6. The 
embodiment of such an output stage for MIS charge transfer systems is to 
be represented in the following for charge coupled circuits (CCD 
circuits), and nevertheless it is applicable in the same manner to bucket 
brigade device circuits (BBD circuits). 
In the case of the transmission of signals by means scanning systems, the 
scanning brings about a periodicity of the frequency spectrum of the 
scanned signal, which must be done away with in the case of a conversion 
into signals which are constant in time by means of suitable filters, 
since the higher spectral components interfere in the case of multiples of 
the scanning frequency for many cases of application. This means that one 
must solve the problem of regaining a band-limited signal from the output 
signal of a charge transfer circuit, which output signal is formed by the 
scanning values, whereby the higher spectral components which arose by 
means of scanning are adequately attenuated. For the interpolation of the 
scanning values, as a rule, a scan holding element and a subsequently 
connected analog low pass filter are provided. In many cases, a RC filter 
is insufficient for the low pass filter which is to be used, so that one 
must use LC filters. However, with such structure, one loses the 
possibility of integrating the filter monolithically upon a common 
semiconductor chip with the remaining parts of a charge transfer circuit. 
A completely integratable solution of the problem is nevertheless of great 
significance for CCD circuits which, for example, form filters or time 
delays. 
There are two possibilities for reducing the selection requirements placed 
upon a low pass filter: (1) increasing the scanning frequency f.sub.A of 
the CCD; or (2) post-connecting a further CCD low pass filter which 
functions with the scanning frequency f'.sub.A to the first CCD, whereby 
the scanning frequency f'.sub.A of the low pass filter is larger than the 
scanning frequency f.sub.A of the first CCD. Then the following analog low 
pass filter can be designed very simply, or perhaps can be completely 
omitted. 
However, a higher scanning frequency of a CCD installation [case (1)] in 
the case of the same system requirements, increases proportionally the 
number N of the charge transport elements and, therefore, increases the 
surface requirement for the semiconductor chip and, as a rule, leads to 
poorer system characteristics by means of raised charge transport losses 
(.epsilon.N). Also, the expense of a following CCD low pass filter of 
higher scanning frequency f'.sub.A is relatively high, conditioned by the 
clock pulse generation. 
SUMMARY OF THE INVENTION 
It is therefore the object of the present invention to provide a 
monolithically integrated charge transfer circuit without raising the 
scanning frequency f.sub.A and without a CCD low pass filter which 
operates with an increased scanning frequency, and, on the other hand, 
which permits the use of a simple analog low pass filter and, therefore, 
permits the complete integratability of the circuit without requiring this 
advantage at a sacrifice in the quality of reproduction. 
According to the present invention, the above object is achieved in that at 
least two MIS charge transfer systems are coupled in parallel to the 
output of a charge transfer system, that the charge packets which are 
transferred to the output of the first charge transfer system having 
different weight are supplied to the parallel charge transfer systems of 
the output stage, of which there are at least two. In addition, for 
controlling the MIS charge transfer system, the same transfer clock pulses 
are provided and, in addition, the MIS charge transfer systems in each 
case represent a filter. Further, these filters, in total, display filter 
characteristics which are different from one another and display different 
time delays and all of the MIS charge transfer systems are placed on a 
common charge output stage which functions with a whole number multiple of 
the scanning frequency of the first charge transfer system. In addition, a 
simple low pass filter follows the charge output stage for smoothing the 
remaining higher spectral components. 
The method which forms the basis for the device which is proposed herein 
executes an interpolation of the scanning values of the CCD output signal 
in an extended CCD output circuit. The effect is that of a CCD low pass 
filter having a higher clock pulse frequency, so that the interpolating 
output stage releases scanning values having an increased scanning rate. 
The first charge transfer system which the interpolating output stage 
follows is to be a m-phase CCD having a scanning frequency f.sub.A (with, 
for example, m=3 or 4). At the input of the output stage, the charge 
packet of the m-phase CCD is split up weighted and is supplied to m 
different CCD circuits, which in each case possesses filter 
characteristics according to the principle of the weighted charge 
splitting and the different time delays of the charges in these branches. 
After these filters, the charges are delayed differently, whereby these 
delays amount to a multiple of 1/mf.sub.A in the case of the m-phase CCD. 
The charges, in each case, displaced by 1/mf.sub.A, proceed into the 
customary diffusion region. The voltage which arises is scanned by means 
of a scan holding member having the frequency mf.sub.A. Since in the case 
of m-phase CCD's, the clock pulse phases .phi..sub.1 . . . .phi..sub.n are 
advantageously derived from the frequency mf.sub.A by means of division, 
the additional expense for the clock pulse generation being low. Also, a 
delay in each case by the amount i/mf.sub.A (where i=1, 2 . . . m) is 
easily possible by means of the use of i electrodes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, if a signal x.sub.e (t) is transmitted by means of a 
CCD and if a clock pulse frequency f.sub.A is employed, then the output 
signal x.sub.i comprises scanning values x.sub.i (nT), where 
T=f.sub.A.sup.-1. The spectrum X.sub.i (.omega.) is periodic with the 
scanning frequency f.sub.A. In order to recover a band-limited signal 
x.sub.a (t) which is constant in time, the scanning values are generally 
supplied to a scanning-holding circuit, which attenuates higher spectral 
components of X.sub.i (.omega.) by means of the transmission function 
H.sub.SH (.omega.), whereby 
EQU H.sub.SH (.omega.)=sin (.omega.T/2)/(.omega.T/2). 
As a rule, also a low pass filter must be connected to the output in order 
to attain a sufficient attenuation of the spectrum X.sub.ao (.omega.) for 
frequencies f, which are larger than f.sub.A /2. 
In this manner, the scheme of a scanning system which is represented in 
FIG. 1 results, comprising a scanner, a CCD connected to the scanner, a 
following sample and hold circuit S+H, and a low pass filter TP. 
The high attenuation of the higher spectral frequencies of X.sub.i 
(.omega.), which is generally required in the case of the use of CCD 
circuits cannot be attained automatically in the case of the customary 
design as an integrated circuit. There exists only the possibility which 
was already indicated of allowing a CCD low pass filter, which functions 
at a higher scanning frequency f'.sub.A f.sub.A and which delivers 
interpolated scanning values x.sub.A (1T'), where T'=f.sub.A '.sup.-1 from 
the scanning values x.sub.i (nT) to follow the first CCD with the 
transmission function H (.omega.), so that one arrives at the design which 
can be seen from FIG. 2. Then, the low pass filter TP which is connected 
to the output of the sampling and holding circuit S+H can be designed as a 
simple low pass filter, therefore, only with resistances and capacitances. 
However, the high expense which was indicated above arises for the 
generation of the clock pulse frequency f'.sub.A. 
The invention provides a method which unites the characteristics of a CCD 
low pass filter of the scanning frequency f.sub.A ' mentioned above, in an 
output circuit of the first CCD circuit. This principle will be described 
with reference to FIGS. 3 and 4. 
The scanning values x.sub.i (nT) are supplied to the m parallel filters 
H.sub.j (z), where j=1, 2 . . . m. The output signals x.sub.j (nT) of each 
filter are delayed by a time interval (j-1) T', where T'=T/m. The sum of 
these delayed scanning values x.sub.j " produces the output signal X.sub.A 
(1T'), which now displays a higher scanning rate f.sub.A '. FIG. 5 
illustrates these signals for an example where m=4. 
The derivation of the transmission function H.sub.A of the output stage is 
made easier if one represents the signals x.sub.j (nT) as scanning values 
x.sub.j '(1T') of the scanning frequency f.sub.A ', that is, that one uses 
a mathematical model according to FIG. 4, which is equivalent to that of 
FIG. 3. This leads to the relationships 
EQU x.sub.j '(1T')=x.sub.j (nT) (1a) 
with l=m.multidot.n and n=0,1,2, . . . and 
EQU x.sub.j '(1T')=0, (1b) 
with l.noteq.m.multidot.n 
whereby T'=T/m. 
Therefore, the following relationship applies for the Z transformations of 
the signal x.sub.j, or respectively, x.sub.j ' 
EQU X.sub.j '(z')=X.sub.j (z'.sup.m)=X.sub.j (z), (2) 
with z=e.sup.i T and z'=e.sup.i T'. 
Therefore, one obtains the representation for X.sub.A (Z'), to wit 
##EQU1## 
The transmission function of the output stage is, therefore, 
EQU H.sub.A (z')=H.sub.J (z'.sup.m) z'.sup.-j+1. (4) 
If the transmission function H.sub.A (z') is formed by a non-recurrent 
filter, the function of which is expressed as 
##EQU2## 
then one obtains for the filter H.sub.j (z'.sup.m)=H.sub.j (z), the 
following representation 
EQU H.sub.j (z'.sup.m)=a.sub.(j-1) +a.sub.(m+j-1) z'.sup.-m +a.sub.(zm+j-l) 
z'.sup.zm + (6a) 
EQU H.sub.j (z)=a.sub.j-1 +a.sub.m+j-1 z.sup.-1 +a.sub.am+j-1 z.sup.-2 +(6b) 
Therefore, according to equation (5), one can prescribe a transmission 
function H.sub.A (z') of the interpolating output stage which is 
preferably that of the low pass filter of the scanning frequency f.sub.A 
'=mf.sub.A. From this, according to equation (6b), the transmission 
functions of the m filter H.sub.j (z) can be provided, which can be 
realized as a CCD filter having the scanning frequency f.sub.A. 
With respect to a practical embodiment, it is first to be determined that 
one in many cases can select the non-recurrent transmission function 
H.sub.A (z') such that the coefficients a.sub.i are exclusively positive 
according to equation (5). Then, the filters H.sub.j (z'.sup.m) can be 
realized by means of branching circuits. 
The electrode pattern of an exemplary CCD branching filter is represented 
in FIG. 6, and has a transmission function which is expressed as 
##EQU3## 
The quantity k is that portion of the charge quantity which is displaced 
from the left to the right which is crossing over at the branching 
location of the CCD filter onto the one parallel branch and the quantity 
(1-k) is that portion which is crossing over onto the other parallel 
branch. The factor k or, respectively, (1-k), is equal to the ratio of the 
surface of a divided electrode to the sum of the surface of all divided 
parallel disposed electrodes. 
The output signal x.sub.i of the CCD circuit is a charge q.sub.i (nT), 
which, in the case of a m-phase CCD, is fed to the CCD branch filters 
H.sub.j (z) by means of weighted splitting off in m branches. The output 
signals q.sub.j (nT) are delayed by an interval (j-1)T' by means of (j-1) 
electrodes of the m-phase CCD. The charges, in each case displaced by the 
time interval T', arrive in a diffusion region D, the potential of which 
customarily released by means of a source-follower as an output signal 
u.sub.A (t). In this manner, one arrives at the embodiment of the device 
constructed in accordance with the invention as illustrated in FIG. 7. The 
diffusion region D, the potential of which is generally released as an 
output signal by means of a source-follwer SF, as already noted above, is 
reset to a defined potential with a frequency f.sub.A '=T'.sup.-1. The 
signal u.sub.A (t) is scanned with a frequency f.sub.A ' by means of a 
sample and hold circuit S+H which is designed in a conventional manner and 
which can be favorably realized in MOS technology and which has connected 
thereto a simple low pass filter (for example, an RC circuit with f.sub.g 
.apprxeq.f.sub.A /2) for providing an analog output signal x.sub.ai (t). 
Since the m clock pulse signals of an m-phase CCD can be derived from the 
frequency f.sub.A '=mf.sub.A, no large additional expense arises for 
generating the reset clock pulse and the scanning or sampling clock pulse. 
The exemplary embodiment now to be discussed concerns a four-phase CCD low 
pass filter with a frequency f.sub.3dB =4 kHz, which is dimensioned for a 
clock pulse frequency f.sub.A =24 kHz, and which has a sample and hold 
circuit S+H and a post-connected RC low pass filter TP with f.sub.g =10 
kHz. The resulting transmission function H.sub.o (2.pi.f) shows that the 
output signal x.sub.ao (t) possesses noticeable spectral components at 
f.sub.A, 2f.sub.A. . . , as can be seen from FIG. 9. 
An interpolating CCD output stage according to the present invention has 
for this CCD filter the following advantageous transmission function 
EQU H.sub.A (z')=0.0653+0.1285z'.sup.-1 +0.1867z'.sup.-2 +0.239z'.sup.-3 
+0.1867z'.sup.-4 +0.1285z'.sup.-5 +0.0653z'.sup.-6 
This low pass function can be determined with known synthesis methods of 
digital filters and can be optimized for specific applications. 
From this, for the filter H.sub.j (z), one obtains the expressions 
EQU H.sub.1 (z)=0.0653+0.1867z.sup.-1 
EQU H.sub.2 (z)=0.1285+0.1285z.sup.-1 
EQU H.sub.3 (z)=0.1867+0.0653z.sup.-1 
EQU H.sub.4 (z)=0.239. 
FIG. 8 illustrates a structure for the interpolating CCD output stage. The 
coefficients are determined by means of the surface relationships noted on 
FIG. 8. 
The total transmission function of the CCD low pass filter having an 
interpolating output stage, a sample and hold circuit and an RC low pass 
filter (f.sub.g =10 kHz) is illustrated in FIG. 10. 
With respect to a post-connected CCD low pass filter of higher clock pulse 
frequency, the invention leads to a significantly lower surface 
requirement in the case of monolithic integration, it requires no further 
input and output converter, it needs only a simple clock pulse generation, 
and, in addition, it is suitable for higher clock pulse frequencies 
because no amplifier is necessary and because in such a case the charge 
transport proceeds with the frequency f.sub.A instead of the frequency 
f.sub.A '. 
In contrast to customary (LC or RC active) low pass filters, by means of 
the interpolating CCD output stage, the requirement on the edge steepness 
of the post-connected low pass filter can be reduced by a factor of 10-20 
at m=4, and an integratable RC circuit which is tolerance-insensitive 
suffices for the low pass filter TP, as a general rule. 
Although I have described my invention by reference to particular 
illustrative embodiments thereof, many changes and modifications of the 
invention may become apparent to those skilled in the art without 
departing from the spirit and scope of the invention. I therefore intend 
to include within the patent warranted hereon all such changes and 
modifications as may reasonably and properly be included within the scope 
of my contribution to the art.