Multichannel surface acoustic wave encoder/decoder

A multichannel surface acoustic wave encoder/decoder having a common electrical input is designed so that the insertion losses of the channels differ despite the apparent design constraint of closely matched channel output impedances. The input transducing device or SAW transmitter is divided into individual transducers (or into subtransducers) having different numbers of finger overlaps, thus achieving different acoustic output levels which in turn result in different insertion losses for different channels. The finger spacing of the SAW transmitters can differ between the respective input transducers or subtransducers so as to optimize the center frequency of each channel.

This invention relates to improvements in surface acoustic wave (SAW) 
devices. It is particularly directed to SAW devices for encoding and 
decoding scrambled broadcasts in the subscription television industry. 
BACKGROUND OF THE INVENTION 
Surface acoustic wave (SAW) devices are used as delay lines and band-pass 
filters in a variety of applications. Recently they have been employed in 
encoding and decoding circuits for subscription TV systems of the type 
described in copending U.S. patent application Ser. No. 711,947, filed on 
March 15, 1985. 
An encoder SAW device with a plurality of electrical bandpass filter 
channels is used there to introduce frequency-dependent amplitude and 
phase distortion into the video output of a TV transmitter, as a means of 
encoding the video signal and making it unintelligible to receivers which 
are not equipped with suitable decoders. For additional encoding, the 
transmitter output is switched in an unpredictable way between alternative 
SAW filter channels having different frequency response characteristics. 
At the TV receiver a decoder SAW device unscrambles the video signal by 
passing it through corresponding SAW channels which have frequency 
characteristics complementary to those of the SAW channels in the 
transmitter. The decoder has a switching circuit which selects the proper 
SAW channel to match the encoder. This switching circuit takes its 
switching control input from the video signal, after processing by still 
another SAW channel. 
In an encoder/decoder of this type, all of the SAW channels must be matched 
to each other with respect to delay time and various other electrical 
characteristics. This need for electrical matching creates certain 
problems, which it is the object of the present invention to solve. 
Matching the electrical characteristics of the encoder/decoder SAW channels 
requires the designer to meet several seemingly contradictory criteria. On 
one hand, the output impedances of these SAW channels must match each 
other closely, or the performance of the scrambling/unscrambling system 
will be quite sensitive to variations of the electrical characteristics of 
other components of the system. For example, if the output impedances are 
not closely matched, the decoded TV picture noticeably deteriorates as the 
output is switched from one alternative channel to another. To avoid such 
picture deterioration, the other system components must then be subjected 
to unacceptably high tolerance requirements. On the other hand, there must 
be significant insertion loss differences between the channels or they 
will not differ in their frequency response characteristics as required by 
their encoding/decoding function. These insertion losses, however, must be 
kept at an overall minimum in order to avoid degrading the quality of TV 
reception. 
Each of the SAW channels is of the type having a uniform interdigital input 
SAW transducer (or acoustic transmitter) and an apodized output 
interdigital SAW transducer (or accustic receiver) which are acoustically 
coupled to each other. All the SAW channels of a given enccder/decoder 
conventionally are mounted upon a common piezoelectric substrate, and 
share a common input transducer. But they have individual output 
transducers (one for each SAW channel), and these output transducers have 
different apodization envelopes in order to achieve the different 
frequency response characteristics required for a subscription TV 
encoding/decoding system. 
For a given input transducer, both the insertion loss and the output 
impedance of each SAW channel are approximately inversely proportional to 
the same parameter, i.e. to the aperture of its apodized output 
transducer. Thus, if the SAW channels are to have closely matching output 
impedances, their respective output transducers must all have the same 
aperture width, whereas the aperture widths of the output transducers must 
be different if the SAW channels are to have specified different insertion 
losses. Both requirements evidently cannot be met at the same time, so 
long as each SAW channel shares a common input transducer which has the 
same acoustical coupling characteristics relative to the respective output 
transducers of each SAW channel. 
It has now been discovered, however, that these conflicting design 
requirements can be met if the SAW channels are provided with different 
input transducers, or at least with common input transducer which presents 
different acoustical coupling characterstics to the output transducers of 
different SAW channels. 
The amplitude of the acoustic signal transmitted from a uniform input 
transducer of the interdigital type is proportional to the number of 
overlaps between its electrically oppositely poled digits (fingers). By 
providing a distinct input transducer for each channel, the amplitude of 
the acoustic signal transmitted from the input transducer of each channel 
can be made different from that of the others by selecting a different 
number of finger overlaps for each distinct transducer. To make the input 
transducers distinct, each can be a separate uniform transducer component 
with its electrical input terminals connected in parallel with the others. 
Or better yet, these input transducers can be combined into a common 
structure which couples differently to each channel. 
By transmitting different acoustic signal strengths to each of the output 
transducers, the output signal levels of the various output transducers 
can be modified independently of their output impedances. This permits the 
insertion loss of each channel to be chosen largely independently of the 
channel's output impedance. 
By means of this technique, the output impedances of the output transducers 
can be matched even if the insertion losses of the individual channels 
differ significantly. This avoids the problem of signal distortion as the 
subscription TV encoder/decoder switches between SAW channels. 
The use of different input transducers, or a common input transducer with 
distinct coupling to each outut tranducer, enables each channel to have a 
different insertion loss. This permits the aperture width of each channel 
to be a best compromise to achieve fine adjustment of that channel's 
insertion loss while maintaining a close match between the channel's 
output impedance and that of the other channels. 
Furthermore, when using separate input transducer sections for each 
channel, or a combined input transducer with distinct coupling 
characteristics to each channel, it is also possible to employ different 
interdigital spacings for each distinct input transducer section. This 
enables the center frequency of the input transducer of each channel to be 
independently selected. 
The circuit designer can then choose a different center frequency for use 
with each output transducer. That, in turn, permits the choice of a 
different optimal center frequency for each channel, instead of forcing 
the designer to accept a single compromise center frequency for all 
channels. The use of an optimal center frequency for each SAW channel 
keeps the overall insertion loss level to a minimum, improving the quality 
of reception in a subscription TV system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a multi-channel SAW encoder/decoder 10 for coupling a common 
input to a plurality of SAW channels in a subscription TV receiver. The 
encoder/decoder is formed on the surface of a piezoelectric substrate 12. 
A uniform interdigital type SAW input transducer 14, having terminal pads 
M1 and M2 acting as electrical input terminals, is printed on the 
substrate surface. Terminal pad M1 is connected to an upper bus bar U, 
from which electrode fingers f protrude downward. Terminal pad M2 is 
connected to a lower bus bar L, from which electrode fingers F protrude 
upward between fingers f. An electrical signal applied to terminals M1, M2 
causes input transducer 14 to launch two surface acoustic waves on the 
substrate surface, a forward wave 26 and a backward wave 28. 
It is known that the surface acoustic waves so generated will have a center 
frequency with a half wavelength equal to the center to center spacing b 
of adjacent "active" fingers, also called the "interdigital spacing." A 
finger f (or F) lying parallel and adjacent to at least a portion of an 
oppositely electrically poled finger F (or f) is referred to as "active." 
All the fingers f and F of uniform input transducer 14 are active. 
Both waves launched by transducer 14 have the width of the aperture a.sub.M 
of the uniform transducer, and propagate away from it as shown by the 
arrows 26 and 28. The forward acoustic signal wave 26 generated by 
transducer 14 propagates toward four output transducers 16, 17, 18, and 
19. These respond by generating electric signals across corresponding 
pairs of terminal pads A1-A2, B1-B2, C1-C2, and D1-D2. An acoustical 
absorber 34 is provided to absorb the forward wave 26 after it passes the 
receiving transducers, preventing any unwanted reflection of the forward 
wave from the substrate's forward edge 30. 
The backward wave 28 generated by transducer 14 is not used, and an 
acoustical absorber 36 is provided to prevent its reflection from the 
substrate edge 32. Also, a grounded conductive shield 24 is provided on 
the substrate between the input transducer 14 and the output transducers 
16-19 to reduce capacitive coupling between their respective electrode 
structures. 
The output transducers 16-19 have respective characteristic frequency 
response curves suitable for their respective channels, by virtue of their 
respective apodization envelopes, i.e. their multi-lobed finger overlap 
patterns schematically illustrated in the drawing, as is well understood 
in the SAW art. 
It is known that the output impedance of any of the output transducers 
16-19 is inversely proportional to its respective aperture a.sub.A 
-a.sub.D. The proportionality factor depends on the frequency response of 
the output transducer, which may be different for each channel. However, 
in the coding/decoding applications considered here, the frequency 
responses of the various output transducers are close enough so that, to a 
good approximation, sufficiently close output impedance matching is 
obtained when transducers 16-19 all have the same aperture width a.sub.A 
=a.sub.B =a.sub.C =a.sub.D. 
However, substantial differences in output signal levels at terminals 
A.sub.1 -A.sub.2, B.sub.1 -B.sub.2, C.sub.1 -C.sub.2 and D.sub.1 -D.sub.2 
are also needed. These differences in signal levels are obtained by 
corresponding differences between the insertion losses for each channel. 
It is known, however, that each SAW channel will have an insertion loss 
that is approximately inversely proportional to the aperture a.sub.A, 
a.sub.B, a.sub.C, a.sub.D, of the apodized transducer for that channel. 
For example, channel A will have an insertion loss inversely proportional 
to the aperture width a.sub.A of transducer 16, and so on. Therefore, 
specifying predetermined relative differences in insertion losses among 
the channels normally requires their receiving transducers to have 
different aperture widths. 
As a consequence, the encoder/decoder filter of FIG. 1 has the serious 
design limitation that prescribing closely matched output impedances for 
channels A through D rules out the possibility of simultaneously having 
different insertion losses for each channel. 
FIG. 2 shows a simplified top plan view of a first embodiment of an 
inventive multi-channel SAW encoder/decoder 11 which is in all respects 
similar to device 10 except that a novel transducer stack 14x composed of 
four uniform transducers 14A, 14B, 14C,and 14D is substituted for the 
single uniform transducer 14 of FIG. 1. To share a common input, these 
transducers are electrically connected in parallel by leads 72 and 74. The 
upper bus bars U.sub.A through U.sub.D of each transducer are 
interconnected by lead 72, and their lower bus bars L.sub.A through 
L.sub.D are interconnected by lead 74. 
The transducers 14.sub.A -14.sub.D of the embodiment of FIG. 2 are shown as 
having the same interdigital spacing b and aperture a.sub.m, but the 
number of finger overlaps between their respective top fingers f and their 
respective bottom fingers F are different. 
This difference in the number of fingers, and hence of finger overlaps, 
among transducers 14A-D results in a corresponding difference in their 
acoustic output signal amplitudes. Thus the acoustic signal strength 
varies among channels A through D, and thereby provides a means for 
varying the insertion losses of the individual SAW channels. This enables 
the SAW encoder/decoder of this invention to achieve the different 
insertion losses for each channel required by the encoding/decoding 
application, and at the same time maintains a close match between the 
impedances of the output transducers. This impedance match prevents 
visible distortion of the TV picture as the alternative channels are 
switched in and out in the course of subscription TV encoding/decoding 
operation. 
Thus, the respective numbers of fingers in transducers l4A-14D are chosen 
to reflect the required insertion loss differentials between channels, 
enabling each channel's insertion loss to be selected substantially 
independently of the output impedance of its output transducer. In 
particular, it permits output transducers 16'-19' all to have matched 
output impedances, without sacrificing the required differences in the 
insertion losses between the channels A through D. 
The surface acoustic waves generated by the transducer stack l4x will 
differ from those generated by transducer 14 of FIG. 1 in that there will 
be four separate forward signal waves traveling in different tracks across 
substrate 12 toward their respective output transducers 16'-19'. 
FIG. 2 may be modified so that the interdigital distance b between the 
fingers f, F is not the same for each of the uniform transducers 14A, l4B, 
14C, 14D. It can be independently chosen for each transducer to fit the 
design of the individual channels. Adjusting the interdigital dimension b 
enables the center frequency of each channel to be chosen individually, 
which has significant advantages for reasons which will be discussed 
further below. 
FIG. 3 shows an alternative encoder/decoder 11' which is similar to the 
encoder/decoder 11 of FIG. 2, but incorporates an inventive single 
interdigital SAW transducer 14y that can be substituted for the transducer 
stack 14x. It represents another way of varying the acoustic output signal 
strength among channels A-D, but is a simpler structure since it has only 
one set of bus bars U, L and does not require the leads 72 and 74 needed 
in FIG. 2 to connect the pairs of bus bars U.sub.A -L.sub.A, U.sub.B 
-L.sub.B, U.sub.C -L.sub.C, U.sub.D -L.sub.D of the transducers 14A, 14B, 
14C, 14D. 
The transducer 14y has upper and lower bus bars U, L to which the fingers 
(f, F) of its upper and lower combs are respectively attached at a regular 
interdigital spacing b. The fingers f of the upper bus bar U include both 
active fingers (f.sub.-7, f.sub.-5, f.sub.-3, f.sub.-1, f.sub.+1, 
f.sub.+3, f.sub.+5, f.sub.+7) and inactive "filler" fingers (f.sub.-6, 
f.sub.-4, f.sub.-2, f.sub.+2, f.sub.+4, f.sub.+6) to maintain a uniform 
surface acoustic wave velocity in the transducer. All the fingers 
(F.sub.-3, F.sub.-2, F.sub.-1, F.sub.0, F.sub.+1, F.sub.+2, F.sub.+3) of 
the lower bus bar L, however, are active substantially along their entire 
lengths. 
The active fingers f.sub.-7, f.sub.-5 . . . f.sub.+5, f.sub.+7 of the upper 
bus bar U are interspersed with the fingers F.sub.-3, F.sub.-2 . . . 
F.sub.+2, F.sub.+3 (all active) of the lower bus bar L. The latter fingers 
are of nonuniform length; while the upper active group f.sub.-7, f.sub.-5 
. . . f.sub.+5, f.sub.+7 are of length, extending across nearly the entire 
aperture of transducer 14y. The lengths of the non-uniform finger set 
increase from short finger F.sub.-3 to long finger F.sub.0, and also from 
short finger F.sub.+3 to long finger F.sub.0. Consequently, finger F.sub.0 
is the only lower finger which is long enough to overlap with any fingers 
of the upper bus bar U within the portion of the transducer aperture 
bounded by lines 50 and 52; fingers F.sub.-1, F.sub.0 and F.sub.+1 are the 
only lower fingers long enough to overlap with any of the fingers of the 
upper bus bar U within the portion of the transducer aperture bounded by 
lines 52 and 54; and fingers F.sub.-2, F.sub.-1, F.sub.0, F.sub.+1 and 
F.sub.+2 are the only lower fingers long enough to overlap with any of the 
fingers of the upper bus bar U within the portion of the transducer 
aperture bounded by lines 54 and 56; while all of the lower fingers 
F.sub.-3 through F.sub.+3 are long enough to overlap with the upper bus 
bar fingers in the area bounded by lines 56 and 58. As a result, the 
number of finger overlaps increases monotonically, from a minimum in 
subaperture 50-52, to a larger number in subaperture 52-54, a still larger 
number in subaperture 54-56, and the largest number in subaperture 56, 58. 
Subaperture 50-52 corresponds to Channel A; i.e. it is the only portion of 
transducer 14y which is acoustically coupled to output transducer 16'. 
This is because subaperture 50-52, like input transducer 14A of FIG. 2, to 
which it is analogous, is oriented to transmit its highly directional 
acoustic beam in the direction of output transducer 16'. Similarly, 
subaperture 52-54 corresponds to Channel B because it is acoustically 
coupled to output transducer 17', subaperture 54-56 corresponds to Channel 
C because it is acoustically coupled to output transducer 18', and 
subaperture 56-58 corresponds to Channel D because it is acoustically 
coupled to output transducer 19'. This means that the number of active 
finger overlaps in the input transducer 14y increases monotonically from 
Channel A to Channel D, and consequently the strength of the acoustic 
signal it produces also increases monotonically from Channel A to Channel 
D, or from the upper bus bar U to the lower bus bar L. 
Connecting the upper bus bar U of transducer 14y to ground will reduce any 
electrical crosstalk between the input transducer and the individual 
channel output transducers. This crosstalk reduction arises from the 
shielding effect of having groups of grounded fingers at both the left 
(f.sub.-7, f.sub.-6, f.sub.-5) and right (f.sub.+5, f.sub.+6, f.sub.+7) 
sides of the structure 14y. 
Other embodiments of the composite input transducer can be substituted for 
transducer 14y of FIG. 3 when it is not desired that the strength of the 
acoustic signal increase monotonically from the channel nearest the upper 
bus bar U to the channel nearest the lower bus bar L. For example, FIG. 3A 
shows an embodiment of a composite input transducer 14yy which produces 
its strongest acoustic signal in channel A' nearest upper bus bar U. 
Continuing across the transcuder, the acoustic signal falls to its weakest 
in channel B' and then increases somewhat in channel C'. It reaches its 
second greatest strength in channel D', nearest lower bus bar L. To 
produce this effect, the number of finger overlaps increases in the 
channel order B', C', D', A'. In this more complicated transducer, channel 
A' has its own local lower bus bar LBB, connected to the composite 
transducer's lower bus bar L via finger 100. Likewise, channels B', C' and 
D' have common local upper bus bars UBB1 and UBB2, respectively connected 
to the composite transducer's upper bus bar U by fingers 101 and 102. 
FIG. 4 shows another encoder/decoder 11" incorporating an inventive input 
transducer 14z which is a modification of transducer 14y of FIG. 3. The 
two are similar except that the transducer 14z has different interdigital 
spacings (b.sub.A, b.sub.B, b.sub.C, b.sub.D) for each of the individual 
subapertures of channels A through D. This enables each subtransducer to 
have a center frequency individually selected for best performance of its 
own channel. The resulting transducer 14z has finger spacing transition 
regions (T.sub.1, T.sub.2, T.sub.3) containing slanted finger segments 
which provide connections between the fingers of the individual 
subtransducers. For a comparison with a prior art multichannel transducer 
that also uses slanted transition segments, see FIGS. 5 and 6 of U.S. Pat. 
No. 4,379,274 (Hansen). 
When the center frequencies of adjacent channels do not differ by much, the 
transition regions of slanted segments may be entirely avoided, thus 
saving substrate material. FIG. 4A shows an enlargement of some of the 
fingers of a composite input transducer having such construction at the 
boundary between channels B and C. The effect of the different 
interdigital spacing (b.sub.B= b.sub.C) between the fingers of the 
channels B and C can be absorbed because of the different widths (b.sub.B 
/2.sub.= b.sub.C /2) of the fingers of each channel. In the embodiment 
shown, in each channel the interdigital spacing is a half wavelength, and 
the fingers have a width of a quarter wavelength, of the channel's 
respective center frequency. The fingers from the upper channel are offset 
from the fingers from the lower channel, but join up with them at 
junctions . . . J.sub.-2, J.sub.-1, J.sub.0, J.sub.+1, J.sub.+2 . . . , 
thus providing electrical contact between the corresponding fingers of 
each set. 
The embodiment of FIG. 4A employs solid-strip, half-wavelength-width 
fingers. To reduce acoustic reflections it may be advantageous to use 
split fingers each formed of two spaced one-eighth-wavelength-width 
strips. Thus FIG. 4B shows an enlargement of the split fingers of yet 
another composite input transducer in which the boundary between adjacent 
channels B and C is formed without slanted transition segments. Here 
again, compare FIGS. 5 and 6 of Hansen U.S. Pat. No. 4,379,274, which also 
employs split fingers 
In the embodiment of FIG. 4B of the present invention, in each channel the 
interdigital spacing is a half-wavelength, and each split finger is formed 
from a pair of individual parallel electrode strips, each having a width 
of an eighth of a wavelength. The space between the individual strips 
forming each finger also has a width of an eighth of a wavelength. Scme 
fingers from each upper channel join up with fingers from a lower channel 
at their ends, thus providing electrical contact between the fingers of 
each set. Fingers (f.sub.-4, f.sub.+4) that do not join up with those of 
another channel are spaced by a gap from opposing fingers (F.sub.-3, 
F.sub.+3) of the other channel. 
Prior art input transducer designs can only have a single compromise center 
frequency, chosen to provide the best overall performance for the 
multichannel encoder/decoder 10 as a whole. A single center frequency 
cannot be optimal for all channels. 
In contrast, the input transducer stack 14x of FIG. 2, the transducer 14z 
of FIG. 4, and the embodiments of FIGS. 4A and 4B, can all use different 
interdigital spacings (b.sub.A, b.sub.B, b.sub.C, b.sub.D) for the 
individual transducers 14A-D or the subtransducers of channels A-D, 
enabling the center frequency of each input transducer or subtransducer to 
be selected to equal the optimum center frequency of its respective 
channel. 
The particular application of this SAW device as an encoder or decoder 
requires insertion loss differences between the individual channels. The 
channel with the least insertion loss fixes the insertion losses for the 
other channels, and must have the lowest possible insertion loss for best 
system performance. The selection of the optimum center frequency for this 
channel insures minimum insertion loss. As a result of the freedom to 
choose different center frequencies for the uniform transducer sections of 
the different channels, the apodized transducers are simpler and easier to 
construct. 
The composite transducer structures 14y and 14z of FIGS. 3 and 4 are 
preferable to the stacked transducer structure 14x of FIG. 2, since the 
composite structures 14y and 14z do not have to provide parallel 
electrical connections between the individual transducers 14A-D. These 
connections are an inherent part of the composite structure 14y or z. 
Another advantage is that the composite structures 14y and z each have a 
total of only one pair of bus bars, whereas the stacked structure 14x has 
one bus bar pair per transducer, or a total of four pairs. It has been our 
experience that multiple bus bar pairs impair the frequency response of 
the encoder/decoder 11 on the upper slope of its passband and in the 
rejection band as well. It is believed that the bus bars act as waveguides 
and thus form a multitude of wave modes in the acoustic beam. The higher 
order wave modes can distort the frequency responses of the SAW channels 
in undesirable and uncontrollable ways. 
In carrying out the design of encoder/decoders according to this invention, 
there is a practical limit to the number of finger overlaps that can be 
used in the input transducers. A uniform transducer's frequency bandwidth 
is known to be inversely proportional to the number of its finger 
overlaps, so the maximum number of finger overlaps for the input 
transducer is fixed for each channel A-D by the desired bandwidth. 
Furthermore, only an integral number of finger overlaps can be used, which 
makes the channel insertion losses adjustable only in quantized steps. 
However, in practice, fine adjustment of insertion losses can be made by 
slight modifications to the apertures of the apodized receiving 
transducers 16'-19'. Although an apodized transducer's output impedance 
depends on its aperture, the impedance is relatively insensitive to slight 
aperture modifications. 
While the principles of the invention have been described above in 
connection with specific apparatus and applications, it is to be 
understood that this description is intended only by way of example and 
not as a limitation on the scope of the invention. Therefore, the 
following claims are to be construed to cover all equivalent structures.