Note: Descriptions are shown in the official language in which they were submitted.
~ :~ 6~33 2 ~
The present invention relates to electrical band-
pass filters.
Electrical band-pass filters which exhibit a
narrow passband centered on a preselected center frequency
and sharp skirts have been known in the art for several
decades. Good technical discussions of prior art electrical
band-pass filter design theory and apparatus can be found
in many standard texts, including: Reference Data for _adio
Engineers, 6th edition, Howard W. Sams & Co., Inc., Indianapolis,
Indiana, 1975, Chapters 7-10; Single Sideband for the Radio
~mateur, 5th edition, American Radio Relay League, Inc.,
Newington, Connecticut, 1970; The Radio Amateur's Handbook,
57th edition, American Radio Relay League, Inc., Newington,
Connecticut, 1978.
Various approaches have been used to attempt
to achieve the various passband characteristics needed
in present-day sophisticated communications systems and
the like: narrow passband response centered on a preselected
center frequency; sharp skirts down to 50 dB or more; very
low ripple in the passband; low inter-modulation distortion;
high frequency stability with respect to temeprature; low
passband insertion loss; high isolation between the input
and the output; etc.
One approach has been the high-frequency crystal
band-pass filter. Piezoelectric quartz crystals are used
fi8321
as the filter elements because of their very high
Q values. Examples o such filters can be found in the
references listed above.
Such high-frequency crystal band-pass filters, while
producing some of the desired passband characteristics,
nevertheless, exhibit many major deficiencies. For
example, fabricatlon of such filters is both complicated
and costly bec~a~se each of the crystals must be checked
and often modified before being put in-to the filter
assembly in order to obtain satisfactory performance.
While a narrow passband response can typically be achieved
at a 3 dB down point using only a few crystals, many crys-
tals ~re required to produce sharp skirts down to 50 dB or
more. It is also very dificult to remove ripple in the
passband because each crystal has its own electrical pole.
Low inter-modulation distortion is very difficult to
achieve because frequency-sensitive elements, such as
inductors and capacitors, must be used in the filter
assembly. High-frequency stability with respect to
'temperature is difficult to achieve because many of the
required components are temperature sensitive. The
reguired usP of several crystals and associated components
to produce a filter having a sharp passband and fairly
sharp skirts necessarily results in a ,h,igher passband
insertion loss. High isolation between the input and the
output of ~he ~ilter is difficult to achieve because of
the hlgh degree of coupling between various filter compo-
nents. These and other deficiencies limit the usefulness
of prior axt high frequency crystal band-pass filters.
3 2 ~
The ever-expanding use and concomitan-t crowding
of the electromatic spectrum has generated an enormous need
for technically better and less expensive high-frequency
band-pass filters. Another approach other than the high-
frequency band-pass filter has been the active filter. Technical
discussions of such active filters can be found in the references
listed above. Active filters, however, also exhibit many
deficiencies.
In accordance with the present invention, there
is provided a band-pass filter and gain stage comprising:
(a) a monolithic filter having a first filter element and
a second filter element, the first and second filter elements
being connected to have a common ground; (b) a first resistor
having a first end and asecond end and having a substantially
pure resistance value selected from the range of 200-2200
ohms, the first end being connected to the first filter
element; (c) a second resistor having a first end and a
second end and having a substantially pure resistance value
selected from the range of 200-2200 ohms, the first end
being connected to the second filter element; and (d) gain
stage means having an input connected to the second end
of the second resistor for amplifying an input signal from
the second end an amount greater than 20 dB.
The band-pass filter provided in accordance with
the present invention is both easy and inexpensive to fabricate
and maintain and overcomes the prior art problems referred
to above.
The invention is described further, by way of
illustration, with reference to the accompanying drawings,
wherein:
~16~32JI.
Figure 1 is a block diagram of the bandpass filter
and gain stage of the present invention;
Figure 2 is a circuit block diagram of the preferred
embodiment of the present irvention, whj.ch employs a piezoelec-
tric quartz crystal monolithic filter; and
Figure 3 is a plot of the passhand response produced
by the filter of Figure 2, where the horizontal c~xis plots
~'~
1 16832:~
fre~uency in KHz on either side of the center frequency
and the vertical axis plots band-pass rejection in d~.
.
DETAILED DESCRIPTIOI~ OF l~-IE PREFERRED EMBODIMENTS
Referring now to Figure 1, the functional blocks of
the band-pass filter and gain st~ge of the present inven-
tion are sho~n and are characteri~ed as being an input
terminal 10 connected to an input impedance stage 12,
which is connected to a monolithic filter stage 14, which
is connected to a second impedance stage 16, which is
connected to a gain stage 18, which, in turn, is connected
to an output terminal 20.
As will.become more apparent by the technical expla-
nation presented below, the present invention can be
viewed as being a very passive frequency door whose output
is lightly coupled to a broad band frequency amplifier
having a high gain --i.e., more ~han 20 dB at the center
frequency of the passband. The very passive frequency
.
door oIlly passPs those input signals whose ~reguencies
fall within the "door" or -passband, and substantially
attenuates all signals outside the "door" or passband,
including tho e that are very near in frequency to the
"door" or passband. The frequency door includes the input
impedance 12, the monolithic filter 14 and the second
- impedance 16. Because ~he input signals which pass
through the "door" or passband are substantially attenu-
ated, e.g., 15 dB is typical, the gain stage 18 brings
these signals back up to a desired output level.
2~
~ he heart of the very passive frequency door is the
monolithic filter 14. The monolithic filter 14 can be
characterized as a single mechanical package which
contains two electrical filter elements. One of the elec-
trical filter elements has a pole which is displaced in a
positive increment from the center frequency of the pass-
band, and the other electrical filter element has a pole
which is displaced a negative increment from the center
frequency of the passband. The absolute values of the
positive and negative increments are substantially egual,
and their absolute value sum approximately equals the
width of the passband at the 3 dB down point~ One side of
each of the two electrical ilter elements is connected to
a common ground.
As is stated in greater detail belo~, there are many
commercial available units which are suitable for the
monolithic filter 14. One group of commercially available
units employ peizoelectric quartz crystal electrical
filter elements. Such pieæoelectric ~uartz monolithic
filter units are particularly useful when a passband
having a width in the range of 10-40 KHz at 3 dB down and
a center frequency of 10.7 MHz is desired. Such piezo-
electri~ ~uartz monolithic filter units exhibit a very
high Q, a high resistance outside the passband and a rela-
tivel~ low resistance (e.g., 600 ohms) within the
passband.
A second group of commercially available units employ
ceramic piezoelectric electrical filter elements. Such
piezoelectric ceramic monolithic filt r units are particu-
larly useful when a passband having a width in the rangP
~ 1~832:1
of greater than 40 KHz at 3 dB down and a center freguency
of 10.7 MHz is desired. Such piezoelectric ceramic mono-
lithic filter units exhibit ,a very ~igh Q, a high resis-
tance outside the passband and a relatively low resistance
(e.~., 200 ohms) within the passband.
As shown in Figure 1, l:he input impedance stage 12provides the input electrical signal (not shown3 applied
to input terminal 10 to one of the piezoelectric electric
filter elements of the monolithic filter ~tage 14. It has
been found that it does not matter whether the input
impedance stage 12 is connected to the piezoelectric elec-
tric filter element of monolithic filter stage 14 having
the pole in a positive increment from the center fxeguency
or to the filter element having the pole in a negative
increment from the center frequency.
The input impedance stage 12 electrically couples the
electrical signal at terminal 10 to the monoli-~hic flter
stage 14 using a substantially pure electrical resistance
which doe~ not have any appreciable capacitive or induc-
tive components. Thus, the coupling provided by the input
~mpedance stage 12 between ~he input signal at input
terminal 10 and ~he monoli-thic filter stage 14 is substan-
t aily frequency insensitive.
It has been found that the level of coupling provided
by input impedance stage 12 is very important in order to
achieve the clesired passband response. For example, it
has been found that if too little of the input electrical
signal at input terminal 10 is provided by the input
impedance stage 12 to ~le monolithic filter stage 14, then
the passband response produced is one that is not optimum
1 1683~
because it is "bell-shaped": the skirts are not sharp in
the region from 0-3 dB down, and the passband is not flat
between the 0 dB pointsO
At the other extreme, it has been found that if too
much of the input electrica:L signal at input terminal 10
is provided by the input impedance stage 12 to the mono-
lithic filter stage, then the resultant passband response
is also not optimum because t.he skirts are not steep, etc.
Instead, it has been found that the proper level of
coupling is when input impedance stage 12 is a substan-
tially pure resistance element in the range between
200-2200 ohms. The explanation for this range is that
monolithic filter element 14 exhibits a very high resis-
tance to signals whose frequency is outside the passband,
but exhibits a low resistance to signals whose frequency
is within the passband. Thus, because the input impedance
stage 12 is in series electrical connection with the mono
lithic filter element 14, those signals whose frequency is
outside the passband are substantially attenuated in the
input impedance stage 12, while those signals whose
frequency is inside the passband are coupled to the mono-
lithic filter element 14. ~owever, the amount of coupling
by the input impedance stage 12 of those signals whose
frequency is within the passband is limited because of the
resistance value of input impedance stage 1~.
As shown in Figure 1, the other one of the two piezo-
electric filter elements of ~he monolithic filter stage 14
is in series connection with the second impedance
stage 16. ~he second impedance stage is a substantially
pure electrical resistance ~hat does not have any appre-
:1 ~6~3321
ciable capacitive or inductive components. Thus, thecoupling between the monolithic filter element 14 and the
second impedance stage 16 is substantially frequency
insensitive.
It also has been found that the level of coupling
between the monolithic filter stage 14 and the second
impedance stage 16 is very important in ordex to achieve
the desired passband response. It has been found that too
low or too high a level of coupling produces a passband
response that is not the optimum one.
It has been found that the proper level of coupling
between the monolithic filter stage 14 and the second
impedance stage 16 is when the second impedance stage 16
is a substantially pure resistance element in the range
between 200-2200 ohms. It is believed that this range is
due to the fact that the monolithic filter stage 14
exhibits a very high resistance to signals whose fre~uency
is outside the p~ssband but also exhibits a low resistance
tosi~nals whose frequency is within the passband. Thus,
since the monolithic filter element i4 is in series
connection with the second impedance stage 16, those
signals from input terminal 10 whose frequency is outside
the passband are very substantially attenuated before they
are provided by the monolithic filter stage 14 to the
second impedance stage 16. On the other hand, those
signals Lrom the input terminal 10 whose frequency is
inside the passband are passed by the monolithic filter
stage 14 to the second impedance stage 16 with only a low
level of attenuation: the attenuation introduced by the
series-connected resistances of input impedance stage 12,
lV
32~
second impedance stage 16 and the rnonolithic filter
stage 14.
It is now apparent why the combination of input
terminal 10 input impedance stage 12, monolithic filter
stage 14 and second impedance stage 16 can be viewed as
being a very passive freguency door. Because of the
level of coupling used between input impedance stage 12
and monolithic filter stage 14 and between monolithic
filter stage 14 and second im]pedance stage 16, those input
signals whose frequency is outside the passband are effec-
ti~ely blocked, while those input signals whose frequency
is within the passband are passed with moderate
attenuation.
As shown in Figure 1, the second impedance stage 16
is in series electrical connection with the gain stage 18.
Gain stage 18 has a wide-band frequency characterstic
especially in the range around the center frequency of the
passband. It has an input impedance which is substan~
tially a pure resistance without.any appreciable inductive
or capaci~ive components and which has an ohmic value less
than or egual to the resistance value of the second imped~
ance stage 16. In addi~ion, gain stage 18 has an output
impedance which is substantially a pure resistance without
any appreciable inductive or capacitive components and
which has an ohmic value equal to or greater than the
resistance value of the input impedance stage 12.
Because the resistance value of the input of gain
stage 18 is e~ual to or less than the resistance value of
the second impedance stage 16, the coupling between the
second impedance stage 16 and the gain stage 18 becomes
I 16~3321
less as the input impedance of the gain stage 18 is
lowered. It has been found that optimum passband response
is achieved when the input impedance of gain stage 18 is
less than one-half of the ~econd impedance stage 16.
Gain stage 18 must produce a gain in excess of 20 dB
in the frequency range of the passband of the filter.
Because the input signals whose frequency fall within
passband are attenuated approximately 15 dB due to the
light coupling between input impedance stage 12, monoN
lithic filter stage 14, second impedance stage 16 and gain
stage 18, these signals appear at output terminal 20 at a
signal level which is slightly greater than their level at
input terminal 10.
As is discussed in the examples which follow, the
passband filter and gain stage of the present invention
produces a passband response that is outstanding. For
example, the passband produced has no measurable ripple
and has skirts which are practically'vertical down to
120 dB or more. Inter-modulation dlstoxtion is practi-
cally non-existent because all couplings use purely resis-
tive elements which are freguency insensitive due to the
absence of capacitive and inductive components. Very high
adjacent channel rejection is observed. Moreover, very
high isolation is possible ~ithout having to resort to
c~reful shielding or layout. The passband response is not
affected by changes- in temperature. The filter is very
easy to fabricate and maintain, and does not require that
the monolithic filter stage 14 be modified to achieve the
desired filter response.
~ 32l
It is believed that one of the major reasons for the
outstanding performance of the present invention is due to
the light resistive coupling between stages. By not
coupling the input signal too heavily to the monolithic
filter stage 14 and by not coupling ~he monolithic filter
stage 14 too heavily to the gain stage 18, it is believed
that this permits the ~onolithic filter stage to act as a
very passive frequency door which passes only those
signals having a frequency within the passband.
It is believed that the use of light couplings
between the filter elements has not been recognized in the
past because it results in appreciable attenuation to
those signals whose fre~uency are within ~he passband,
e.g., 15 dB is typical. In the past, signal gain-was the
standard goal in communication design, and attenuation of
desired signal was not considered to be good design prac-
tice. This has changed recently, however, with the
appearance of inexpensive and commercially a~ailable
- solid-state de~ices that exhibit high amplification
factors at high frequencies.
It should be noted that several of the passband
filter and gain stages o~ the present invention can be
connected in series to produce an ever-more optimum pass-
band response.
It should also be understood that the passband filter
and gain stages of the present invention can be fabricated
on a chip or the like usins conventional semiconductor
integrated circuit fabrication techniques.
13
:1 16$~2:l
EXAMPLE 1
-
Referring to Figure 2, an embocliment of the present
inventlon which employs a piezoelectric c~uartz crystal
electrical filter for the monolithic filter stagé 14 is
shown. Like reference numerals refer to like stages in
Figures 1 and 2.
The input impedance stagle 12 includes a resistor 21.
Resistor 20 should exhibit a substantially pure resistance
in the range of 200-2200 ohms. The first end of
resistor ~1 is colmected to the input terminal 10, and the
second end is connected to one of the two piezoelectric
guartz crystal electrical filter elements of the mono-
lithic filter stage 14. As is stated above, it has been
found that it -does not matter whether the second end of
resistor 21 is connected to the piezoelectric quartz
crystal electrical filter element having the pole in the
positive increment from ~he center freguency or ~he elec-
trical filter element ha~ing ~he polé in the negative
increment from the center frequency. For purposes of
explanation, the second end of resistor 21 is said to be
connected to electrical filter element 24.
Suitable piezoelectric quart~ crystal monolithic
filter units for the monolithic filter stage 14 are
commercially available from- many different sources,
including Tama, Inc. of Kawaski, Japan. As is stated
above, s-~ch pie~oelectric guartz crystal monolithic filter
units are particularly useful when a passband having a
width in the range of 10-40 KHz at 3 dB down and a center
frequency of 10.7 MHz is desired.
~ 32
The second impedance stage 16 includes a resistor 22~
Resistor 22 should exhibit a substantially pure resistance
in the range of 200-2200 ohms. The firs~ end of
resistor 22 is connected to the other one of the two
piezoelectric quartz crystal electrical filter elemen~,
designated as electrical filter element 26.
~ ode 28 constitutes the input of the gain stage 18.
The second end of resistor 2~2 is connected to node 28.
Gain stage 18 exhibi~s a broad band frequency
response characteristic, espec:ially in the frequency range
around the center freguency of the desired passband, and
also produces a high gain of 20 dB or greater. Xt is
desirable that gain stage 18 ha~e an input impedance that
îs substantially a pure resistance that is of an ohmic
value equal to or less than the value of resistor 22, and
that gain stage 18 have an output impedance that is
substantially a pure resistance that i~ of an ohmic value
egual to or greater than the value of 'resistor ~1. Gain
stage 18 sh~uld also be designed to be insensitive to
temperature changes.- It is apparent that many differ~nt
amplifier desi~ns will produce the desired characteristics
As shown in Figure 2, a resistor 30 is connected
between node 28 and ground, and the base 34 OI a tran~
sistor 32 i5 also connected to node 28. A resistor 36 and
a capacitor 38 in series connection are connected between
the emitter 40 of transistor 32 ~nd ground~ The voltage
supply Vcc (not shown) for gain stage 18 is connected to
one end of a resistor 42, whose second end is connected to
node 50. One end of a capacitor 48 is connected to
node 50, and its second end is connected to ground. One
:l ~6~32 ~
end of a resistor 44 is also connected to node 50, and the
second end is connected to node 28. One end of a
r.esistor 46 is also connected to node 50, and the second
end is connected to the collector 50 of transistor 32.
The collector 52 is also connected to the output
terminal 20.
A pas~band filter and gain stage of the present
invention was designed and constructed to achieve a pass-
band of 14 KHz and a center freguency of 10.7 MHz
according to the circuit of Figure 2 using the component
values of Table 1 shown below.
. TABLE 1
Resistor 21 1,800 n 1/8 watt
Monolithic Filter 4 2 piezoelectric ~uartz crystal
electrical elements in one
mechanical casing with poles
~ 7 K~2 at 10.7 M~z
Resistor 22 1,800 n l/B watt
Resistor 30 10,000 Q 1j8 watt
Transistor 32 2N5134 NPN silicon
Resistor 36 2~000 Q 1/8 watt
Capacitor 38 o001 ~fd ceramic
Resistor 42 100 Q 1/8 watt
Resistor 44 47,000 n 1/~ watt
Resistor 46 10,00D Q 1/8 watt
Capacito~ 48 . 001 ~ fd ceramic
A filter and galn stage of the present invention
employing the component values of Table 1 was constructed
using a straightforward circuit board layout without any
attempt to shield or ele~trically isolate any of the indi-
vidual components. Using a voltage supply Vcc in the
16
I ~fi~32 ~.
range of 10-13 . 5 volts, the passbancl response as shown in
Figure 3 was measured using a H.P. 140 T spectrum analyzer
with a 100-cycle bandwidth and a 1 KHz division sweep.
The horizontal axis of Figure 3 plots freguency in KHz on
either side of the center fre~uency, and the vertical axis
plots passband rejection in d]3.
As Figure 3 illustrates, the measured passband
response was outstandin~. Specifically, it should be
noted that the skirts are al~most vertical down to 120 dB
and more, and there is no measurable ripple in the pass-
band. Inter-modulation distortion was negligible.
Extreme isolation between the input 10 and OUtpllt 20 was
observed without special shielding, and very high rejec-
tion of adjacent channel signals as close as 25 KHz was
also observed. Moreover, the embodiment was unaffected by
changes of the voltage Vcc in the range given above, and
was also unaffected by temperature changes. In addition,
the output signal was 10 dB higher than the level of the
input signal. Finally, it should be noted that all of the
possible components were 10% tolerance units, and no
adjustment of the monolithic filter 14 was made.
A better band-pass response has been obtained by
connecting in series several of t~e band-pass filter and
gain stages of the present invention. Figure 3 shows the
response characteristics of 2, 3 and 4 units.
-
EXAMPLE 2
A second example of the preferred embodiment of theinvention using the same circuit of Figure 2 will llOW be
discussed. The actual component values and passband
performance will be presented.
~ 321
Suitable piezoelectric ceramic monolithic filter
units for the monolithic filter stage 14 are commercially
available from many dif~erent sources, including
Murata, Inc. of Japan. As is staked above, such piezo-
electric ce.ramic monolithic filter units are particularly
useful when a passband having a width of 40 KHz or more at
3 ~B down and a center frequlency of 10.7 ~Iz is desired.
A ~assband and gain stage of the present invention
was designed and constructed to achieve a passband of
200 KHz and a center frequency of 10.7 MHz according to
the circuit of Figure 2 using the component values of
Table 2 shown below.
TABLE 2
Resistor 21 500 Q 1/8 watt
Monolithic filter 14 2 piezoelectric ceramic electrical
elements in one mechanical casing
with poles ~ 100 KHz at 10.7 MHz
Resistor 22 500 n l/8 w.att
Resistor 3Q 10,000 Q 1/8 watt
Transistor 32 2N5134 NPN silicon
Resistor 36 2,000 n 1/8 watt
Capacitor 38 .001 ~fd ceramic
Resistor 42 lO0 Q 1/8 watt
Resistor 44 ~7,000 Q 1/8 watt
Resistor 46 10,000 n l/8 watt
Capacitor 48 .001 ~fd ceramic
~ he filter and gain stage of the present invention
employing the component values of Table 2 was constructed
using a straightforward circuit band layout ~ithout any
attempt to sh:ield or electrically isolate any of the indi-
18
~ ~6g32~L
vidual components. Using a voltage ~upply Vcc in therange of 10~13.5 volts, the passband response as shown in
Figure 3 using the lower X-axis scale was measured using a
H.P. 140 T spectrum analyzer w~th a 100-cycle bandwidth
and a 1 KHz division sweep. The horizontal axis lower
scale of Figure 3 plots fre~lency in KHz on either side of
the center frequency, and the vertical axis plots passband
rejection in dB.
As Figure 3 illustrates, the measured passband
response was outstanding. Specifically, it should be
noted that the skirts are almost vertical down to 120 dB
and more, and there is no measurable ripple in the pass-
band. Inter-modulation distortion was negligible.
Extreme isolation between the input 10 and the output 20
was observed without special shielding, and very high
rejection of adjacent channel signals as close as 25 KHz
was also observed. Moreover, the embodiment was
unaffected by changes of the voltage Vc~ in the range
given above, and was also unaffected by temperature
changes. In addition, the output signal was 10 dB higher
than the level of the input signal. Finally, it should be
noted that all of the passive components were 10% toler-
ance units, and no a~justment of the monolithic filter 14
was made.
An even better passband response has been obtained by
connecting in series several of the band~pass filter and
gain stages of the present invention.
Although the invention has been described as
constructed using integrated circuit technology, other
techniques may be employed. For example, hybrid inte-
19
~ 32~.
grated circuits using active and passive components on achip can be employed. In such a case, the monolithic
elements would remain discrete components. Alternatively,
using total hybrid technology, ~he entire device,
including monolithic elementsv can be placed on a chip.
It is apparent that various modifications and varia-
tions can be made without departing from the scope of this
invention.
.
. 20
