US2173894A

US2173894A - Variable band width piezoelectric filter

Info

Publication number: US2173894A
Application number: US188034A
Authority: US
Inventors: Atwood John Boyd
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1938-02-01
Filing date: 1938-02-01
Publication date: 1939-09-26
Anticipated expiration: 1956-09-26

Description

Sept. 26, 1939. J. B. ATWOOD 2,173,894

VARIABLE BAND WIDTH PIEZOELECTRIC FILTER Filed Feb. 1, 1938 I5 L SECT/O/VA SECTION B *G *5 v 1 1 .2 VAR/ABLE F/XED LOSS 9 O r :2 Q [ARMS Y 1 g I t FREQ m I g I 55/2/55 ARMS I K I l k Q 1 4 .3

+ FIXED VAR/ABLE v) LOSS LA F g I 1 E5 ARMS 9 k l g INVENTOR i J. 5. ATWOOD ATTORN EY Patented Sept. 26, 1939 UNITED STATES PATENT OFFICE FILTER John Boyd Atwood, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application February 1, 1938, Serial No. 188,034

5 Claims.

This invention relates to piezo electric crystal filter systems and more particularly to a filter employing quartz crystal resonators in which variability of the band width is obtained.

It is an object of my invention to provide a filter system of the type mentioned in which fixed peaks of attenuation at the edges of a desired pass band are obtained.

It is another object of my invention to provide a filter system having two attenuation regions, one extending from zero frequency to the low frequency edge of the pass band, and the other extending from the high frequency edge of the band to infinite frequency.

Another object of my invention is to provide a piezo electric filter system having two variable attenuation bands which may be caused to vary in extent thruout the region of the pass band and which produce a variable band width feature.

Still another object of my invention is to provide a piezo electric filter system having a variable pass band width which exceeds the limit of 13.5% heretofore considered a maximum.

Other objects and advantages of my invention will be made apparent in the description to follow. This description is accompanied by a drawing, in which Figure 1 shows a schematic wiring diagram of a filter system having two sections the elements of which are suitably designed and co-ordinated for carrying out my invention,

Fig. 2 shows diagrammatically a system of reactance curves which correlate the reactance or loss in one section of a lattice network with frequency; and

Fig. 3 shows a system of reactance curves which correlate reactance with frequency in the second section of the lattice network of my invention.

When dealing with piezo electric crystal filters of the prior art it has been noted that there is a maximum limit to the fixed band width of approximately 13.5% of the mid-frequency. Once the filter has been designed for a fixed band width this width cannot be varied. The reason for this is that any attempt to vary the width of the band results in spurious pass bands in the desired attenuation region or undesired loss bands in the desired pass band, or both. In carrying out my invention I have found it possible to overcome these difficulties by a suitable arrangement of the circuit elements themselves.

Referring to Fig. 1, it will be seen that the filter consists of a two section lattice arrangement connected by an impedance matching transformer I which merely eliminates such losses as would be caused by the impedance mismatch at the juncture of the two sections. Each section comprises two series-arms and two crossarms; hereinafter referred to as lattice-arms.

The series arms of section A are made up of the matched quartz crystals l4 and I5 shunted by the fixed condensers 6 and l. The lattice arms are made up of two identical resonant circuits,

one being 2 and Ill and the other being 3 and H. 3

The series arms of section B are made up of the matched quartz crystals 16 and I! shunted by the fixed condensers 8 and 9. The lattice arms are made up of two identical anti-resonant circuits, one being 4 and I2 and the other being 5 I and 13.

In Figure 2 are shown the reactance curves and the loss Vs. frequency characteristics of section A. It will be seen that the reactance curve for the series arms of the section is that of a resonant circuit shunted by a capacity. This is produced by the quartz crystals l4 and l 5. The condensers 6 and l shunting the crystals are for the purpose of making the curve extend further to the left below the axis so that the reactance curve of the lattice arms will intersect it at only a short distance from the point where the lattice arms go through resonance. An attenuation peak is produced where the two curves intersect, since the lattice type of structure is essentially a Wheatstone bridge and at the crossover point the bridge is balanced. A fixed attenuation band is produced between the resonant and anti-resonant points of the crystal and this is so located that it is at the high frequency limit of the band for which the filter is designed.

A loss band. is produced from zero frequency to the resonant point of the lattice arms. The location of the cut-off point of this band is varied by changing the capacities of the

variable condensers

2 and 3. Since the portion of the reactance curve of the series arms below the axis is nearly horizontal over the range where the lattice arm curve crosses it, the frequency separation between the cut-off point and the attenuation peak will remain nearly constant as the band width is varied on the low frequency side.

In Fig. 3 are the reactance curves and the loss vs. frequency characteristics of section B. It will be seen that the resultant attenuation curves are the reverse of those for section A.

The reactance curve for the series arms of the section is that of a resonant circuit shunted by a capacity. This is produced by the quartz Qrystals l6 and H. The condensers 8 and 9 shunting the crystals are for the purpose of making the curve nearly horizontal on the high frequency side of the anti-resonant frequency. This is done so that the reactance curve of the lattice arms will cross it at only a short distance from the anti-resonant frequency of the lattice arms. An attenuation peak is produced where the two curves cross. A fixed attenuation band is produced between the resonant and anti-resonant points of the crystal and this is so located that it is at the low frequency limit of the band for which the filter is designed.

A loss band is produced from the anti-resonant frequency of the lattice arms to infinite frequency. The location of the cut-01f point of this band is varied by changing the capacities of the

Variable condensers

4 and 5. Since the portion of the reactance curve of the series arms below the axis is nearly horizontal over the range where the lattice arm curve crosses it, the frequency separation between the cut-off point and the attenuation peak will remain nearly constant as the band width is varied on the high frequency side.

In operation, the four

variable condensers

2, 3, 4, and 5 may, if desired, be ganged together and controlled from a single dial to produce the variation in band width which is sought.

It is readily Within the abilities of any one skilled in the art to follow the teachings hereinabove given by way of modifying the design of the filter sections, particularly so as to provide impedance matching transformers at each end of the filter in addition to the ones between the filter sections. In each case loss vs. frequency characteristics of these transformers may be so designed as to improve the loss characteristics of the filter in the regions outside the pass band.

Other modifications of my invention will doubtless suggest themselves to those skilled in the art. The invention itself is, therefore, to be given the broadest interpretation that is permitted by the scope of the claims to follow.

I claim:

1. An electric wave filter of variable band pass characteristics having two intercoupled impedance network sections including series-arms and lattice-arms, a piezo electric device in each of the series arms of said sections, means including a capacitor in shunt with each of said piezo electric devices for minimizing the frequency separation between the cut-off point and the variable attenuation peak of each section, and for keeping this frequency separation substantially constant as the location of the cut-off point is varied, each lattice arm of the first section including an inductance in series with a variable capacitor, each lattice arm of the second section including an inductance in parallel with a variable capacitor, means associated with the first section for varying the cut-off point on the low-frequency side of the pass band, and means associated with the second section for varying the cut-off point on the high-frequency side of the pass band.

2. A filter in accordance with claim 1 and further characterized in that the lattice arms of the first section are substantially resonant at the low frequency edge of the pass band, while the lattice arms of the second section are substantially anti-resonant at the high frequency edge of the pass band.

3. A variable band electric wave filter comprising two intercoupled sections, each section having two series branches and two lattice branches, each series branch of the two sections havin therein a piezo electric device and means including a capacitor in shunt therewith for minimizing.

the frequency separation between the cut-off point and the variable attenuation peak and for keeping this frequency separation substantially constant as the location of the cut-off point is varied, said series branches having substantially similar resonant characteristics, means including an inductance in series with a variable capacitor in each of the lattice branches of the first section for controlling the cut-off point on the low frequency side of the band to be passed, and means including an inductance in shunt with a 7 variable capacitor and constituting an antiresonant circuit for controlling the cut-off point on the high frequency side of said band.

4. A filter in accordance with claim 3 and having a variability of its band width in excess of H 13.5% of the mid-band frequency.

5. A variable band pass filter comprising two intercoupled sections, each section having two lattice branches and two series branches, the lattice branches of the first section having means including an inductance in series with a variable capacitor for independently determining the position in the frequency spectrum of the cut-off point on the low frequency side of the pass band, the lattice branches of the second section having means including an inductance in shunt with a variable capacitor for independently determining the position in the frequency spectrum of the cut-off point on the high frequency side of the pass band, each of the series branches of the two sections having therein a piezo electric device and means including a capacitor in shunt therewith for providing a minimum frequency separation between the cut-off point and the attenuation peak on the one side of the pass band, and on the other side respectively, and means for independently varying the location of said cut-01f points, thereby to provide a filter of variable width and of variable mid-frequency response.

JOHN BOYD ATWOOD.