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WIDE BANDWIDTH LOW COST SAWNOTCH FILTERS

P.A. Lorenz and D.F. Thompson

R F M onolithics, Inc., 4441 Sigma Road, Dallas, TX 75244

Abstract - SurfaceAcoust icWave SAW)NotchFilters

prevailingproblemhasbeen topbandnulldepthand

have been fabricated in the pas t with limited success. A

bandwidth versus a reasonably broad passband response.

Typically,otchiltersevelopednheastave

achieved either one or the other. Notch Filters using the

SinglePhaseUnidirectionalTransducer SPUDT) SAW

notchelementhaveachievedgood null depthand null

bandwidth,buthavehad ittle ucce ss in achievinga

broad passband response [ I ] . Other ilters, used in theCATV industry, have achieved v e p good broad passband

too narrow to reliably operate over a arge emperature

responses,but have produced notch bandw idths that are

range. SAW notch filters of he past have been physically

large, t times ver 7 mmnength nd therefore

requiredexpensivepackaging.Also, hese filters have

requiredargendxpensiveuningomponents to

achieveheesirederformance.hehysical size

requireme nts esultedn ircuitshatwereimited in

applications.

In an effort to solve these problems, a new approach has

been considered where a SAW element was optimized fo rhigh input impedance at the notch center frequent) Fo A

Coupling of Modes CO M) model was used to predict the

performance of the notch element and the notch network

data. he result was a SAW notch element under 7 mm in121 The results are prese nted along with experime ntal

length that could easily fit in a TO-39-5 pa ckage. Tuning

for this notchnetworkrequired on )’ two smallsurface

mount inductors. T)’pical passband pe rformance w as less

than -7.0 dB rom 350Mhz to 1.5Ghz. Lower passband

loss can be achieved when higher quality nductors are

bandwidth of over 95KHz at design notch center

used.ypical notch performance was -40dB overa

frequencies ranging from 375 MHz to 450MHz.Maximum ull epth the esign enterrequency

exceeded -5SdB.

P E R F O R M A N C E P A R A M E T E R S

Several electrical perform ance parame ters arc critical for a

notch filter. he notch depth at the the notch c c n k r

0-7803-4095-7iYXiS10.00 998 IEEE

frequency, Fo, should he many decibels (dB) below the

passband level. Along with this specification, the stopband

should e wide and at a level manyB below the

passband of the filter. This wide notch bandwidth allows

for aging, temperature drift, and set-on for manufacturing

attribute of notch filters. The -3dB bandwidth should also

variation and has typically been the most critical electrical

be narrownough to avoidistortion to adjacent

frequencies that the ilter hould pass [ l ] . Finally, the

passband should have a broad frequency pectrum and

have low loss. For the filter described in this presentation,

thedesired pecification goals for theaboveparameters

were a total notch de pth of m ore than -60 dB, a stopband

width of approximately 100 kHz at -40 dB, a -3dB

bandwidth of 0.500 MHz and a passband loss of less than

5dB from 350 MHz to 1500MHz.

LC N O T C H F I L T E R S

Discreet component notch or bandstop filters are common

inmanyapplications [ l ] . One popular LC configuration

shown in Figure 1 is called a T-type hand elimination filter

[3]. Th is circuit is designed as a high pass filter and a low

].;cl1

rrl1

~~ .~-<

h ” ” .

Ri: y

PC

W-c L 5 c1

a c = R c / X cQ1= RI / XI

R T

’Figure 1: T-Type Notch Filter

pass ilterconnected i n parallel. Thestopbandsof each

eliminated. Tu obtain thc notch pcrfclrrnancc compar;lhlc to

filter overlap in thc range of the frcqucncics t o he

thc goals statcd ahovc. :I mlution Icw thc nccccury

capacitor (C1 and C ? ancl inductor (L1 m d 1.2) v;~lucs A S

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found at a hosen F0 of 420MHz.Using a circuit

simulation program, the values for C1 and C 2 were found

to be 4.575 nF and 4.575 fF espectively and the values for

L1 and L2 were found o be 31.385pHand 31.385 uH

respectively.Also,he required values of Q for the

com ponen ts as defined in Figure 1 were found to be 18.000

for L1 and L2 and 36 000 for C l and C 2 [4,5]. Figure

exhibits the theoretical performance of this T-Type notch

filterapplyingdiscreet nductorsandcapacitorsusingacircuit imulationprogram.Clearly,discreet components

with such inductance, capacitance, and Q values cannot be

found to build the T-type configuration at 420 M Hz. T hus,

to meet the required specifications, alternative notch filter

components areneeded.

Figure 2: Simulation of T-Type Notch Filter

PREVIOUS SAW BASED NOTCH FILTERS

One alternative is a notch filter design incorporating SA W

transducers.reviously, SA W notch filters have

incorporated he Single PhaseUnidirectionalTransducer

(SPUDT ) SAW Notch Element or SNE The SPUDT SNE

can be designed to provide a constant admittance over a

specified requency ange [ l ] . Also, the susceptance isconstant verhe same frequency pan as that of the

configuration uses the SNE in parallel with a conventional

“valley” of the conductance response. On e possible circuit

reversingransformer. At the SNE frequency,he

reference impedance with both connected to a phase

resistance of the SNE equals the resistance of the reference

impedanceand he signal is cancelled by means of the

transformer. While this notch circuitrovidesood

stopband erforman ce, the passbandmay not be wide

enough for some applications. Also, the cost of the circuit

and he SA W device tself can he prohibitive in many

applications. Depending on Fa, the SAW device die can be

expensivepackaging. Th e tuningcomponents hemselves

as large as 17 mm. The se large die also equire large,

can be exp ensi ve, particularly he transformer. Also, he

packaged die, transformer,andother tuning com ponents

take up a sizeable amoun t of physical space, which is one

of the key limiting parameters in many applications.

These circuits are physicallyimilar to theiscreetOther circuits using the SNE are Bridged -T filters [ l ] .

component T-type configuration i n Figurc I hut use he

SNE’s as tuned adm ittance elemen ts. Also, a notch circuit

using aQuadratureHybridCoupler has been developed

[ l ] . Thesecircuits requiremultiple SN E’s and extensiv e

tuning, again raising the total cost and increasing the size

of the com plete circuit.

“2-PER” SAW IMPEDANCE ELEMENT

Keeping in mind the size and cost rcstrictions imposed by

communicationspplicationsoday, less cxpcnsive,

smallerlternative notch circuit was developed. SA Wdevices have been previouslydesigned for adesignated

admittance response at the resonant frequency , such as the

above mentioned SN E. However, a SAW device can also

he designed to have a maximum impedance response at a

designated anti-resonant frequency. This brings to mind the

two LC tank circuits in theseriessections of the T-type

notch filter n Figure I Ideally, each of these parallel

resonantankswould ppear as an open circuit at the

resonantrequencyeingpplied to them, assuming

infinite Q values.

CS1 ~

Rrn Lm Cm

Figure 3: I-Port Resonator Eqivalent Circuit

These tank circ uits ppe ar electrically similiar to the

equivalent circuit of a Two Electrodes per Wavelength, or

‘ Per”, SA W One -Port Resonator as shown Figure 3 [7].

Therefore, sing SA W design CA D program and a

Coupling of Mo des CO M) analysis rogram, -Per

SAW resonator was designed. The AW device was

designed for maxim um real impeda nce at the desired notch

centerrequency, Fo. of 420MHz. Theotional

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inductance,Lm, the motional apacitance, Cm , and the

static capacitance, CS, were fou nd to have values of 48.56

uH .95 f F and 3 66 pF respectively, with Q values for

the motional componen ts found to be 500 000 nd the Q

value for CS foundo be 100 OOO

=. i

Figure 4: S21 Simulation of 2-Per S A W Impedance Element

Figure 4 shows the simulated transmission response of the

formed by Lm, Cm , and he mallmotional esistance,

SAW device. The peak is due to the series resonant circuit

Rrn, and appears at theeriesesonance as a high

conductance. The null is due to the parallel resonance of

CS with the quivalent nductance of the series branch

composed of themotional components at the arallel

resonant frequency. Th e Q of the resonance produced by

this tank circuit is 11 494 [4]. At the parallel resonant

frequency, the real part of the mpedance of the SA Wtransducer is maximum. This null frequency corresponds to

the desired notch frequency.

TWO ELEMENT SAW N O T C H FILTER

A circuit similiar to that of the T- type circuit of Figure 1

was developedusing wo of the SAW devicesdescribed

circuits. However, the real impedance of the SAW is at a

above with these devic es essentially eplacing he anks

maximum of 950 ohms when the device is capactive, due

to the CS of the SAW . Th us, the T-type circuit had to he

modified. The series resonant circuit in shunt between the

two tank circuits is replaced with a single inductor,as

seenin Figure 5. This induc tance resonates with the CS of the

SAW devices so that the circuit appears to have a nominal

75 ohm real impedance t o the source over a wide passband,

with the exception of the notch response.

(Patent Pending)

Figure 5: Two Element SAWNotch Filter

. . .... . . .. . ..._

..

Figure 6 :Theoretical S21 Response of Two-Element S A W

Notch Filter

Figure 6 show s the transmission performance of the Tw oElementSA W notch filter, as generated by the ircuit

simulation program using a CO M analysis for the SA W.

The notch i lter has a total notch depth of over -35dB,a

-20dB stopband width of over 100 IcHz a -3dB bandwidth

of less than 280 kHz nd a passband loss of less than

-3dB.Figure shows he input impedance on a Smith

Chart . The circuitstarts by appearing o the source as a

nominal 75 ohm impedance on the low side of the

passband. Th e circuit then quickly becom es inductive and

the mpedance ncreases apidlyuntil i t crosses the real

attenuation for the notch filter. The null depth at this

axisagain at 950 ohm s, corresponding to themaximum

frequency, which is the Fo of the iltercircuit, is at amax imum . From this point, hecircuitquicklybecomes

capacitivend the impedanceapidly decrea ses until

returni ng to the nominal 75 ohms on the high side of the

passband. It should be noted here that the response can be

rotatedabout hecenter of the Smith Chart by adding

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electricaldelay. In fact, the esponse can be rotated 180

degrees so that the maximum mpedance poin t descrihed

above becomesa maximum admittance point.

Figure 7 : Theoretical S1 Response ofTwo-Elem ent

SAW Notch Filter

Figure 8: Theoretical S21 Response of Four Element

SAW Notch Filter

FOUR ELEMENT SAW NOTCH FILTER

As was stated before , more attenuation ov er a bandwidth

wide enough to compensate for temperature drift is desired.

Th e performance of the above mentioned circuit may not

he adequate for many applications. Simply cascading wo

Figure 6 provided improved performance. When cascading

of the TwoSAW mped ance Element filters shown in

two of the Tw o Element sectio ns together, concern may be

expressed hat he uning nductance between each SA W

device pair may have to change. However, this was found

to have negligible effect. In addition, no signiticant change

was found in the inductance value needed for each section.

Figure 8 shows the simulatedperform ance of theFour

Element SA W Notch Filter. Tota l notch depth is greater

than -6 0 dB below the passband with a -3dB bandwidth of

less than 370 kHz. This circuit alsn has a -40dB stopband

width of approximately 86 lcHz . The loss in the passband

is less than S dB from 350 M H z to 1.SGHz.

EXPERIMENTAL RESULTS

TheSAW devicesdeveloped for this notch circuit wcrc

designed to be small, easy to produce in large quantities,

andnexpensive to manufacture. Tw oAW 2-pcr

resonators were fabricated on a single die. Eachelementwas designed for a frequency of 420 M hz. The metal used

in the abrication of these dcvices was a two percent

AluminumCop per alloy. Th e substrate was 39 degree

rotated Y-cutquartz. Thedie dimensions were 6.4 0 by

1.40 mm . T he ackage type used was the TO-39-5.

Fjgure 9: S21 Response of Prototype Four Element

Notch Filter

Figure 9 shows the transmissionerforman ce of the

prototype Four Element SA W Notch Filter circu it. For this

prototype, commercially available surface mount inductors

with Q values of approximately S5 at FOwere used for the

shunt uning nductors. Th e total null response is greater

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than -60dB with a -3dB bandwidth of less than 41 0 kHz.

Figure 9 also displays what was stated before as the major

adv anta ge of this notch circuit, the wide stopband width.

The width of the -40dB stopband is exactly I O kHz.Along with this, the total notch frequency variation over a

temperature range of -40 to +60 degrees Celsius was 46

W z . Co mparing this variation to the -4MB notch

bandwidth of 1 kHz, a reasonable set-on for production

variation is obtained . The passband loss is less han 5dB ,with the exception of bulk modes at 1.6 and 1.8 times Foand the 2-Per response occuringapproximately 520 W z

dB and the 2-Per response is approximately -9.0dB .higher than Fa. Th e bulk modes are no greater han -6.0

Figure 10:S21 Response of Prolotype Four-ElementSAW Notch Filter over a Wide Frequency Span

LIMITATIONS

One limitation of this circuit is that i t is also a high pass

filter. As shown in Figure 10 the Four Element circuit has

often desireable to havea notch filterperform as an alla cutoff frequency of approximately 332 MH z. While it is

pass circuit from DC to above 1 GHz , the performance of

this filter is acceptable for many applications. If inductors

of approp riate values are connected in shunt a cross each

pair of S A W elements, the lower frequencies will be

passed downo 50 MHz. However, doinghis will

adverselyffecthe null depthubstantially. Higher

performance can be achieved with higher Q value uning

inductors.

CONCLUSIONS

To develop these SAW NotchFilters,different options

were explored. Designinga notch element or maxim um

admittance at the desired notch frequency, Fo was

considered and explored , but found to be too expensive due

element to “block” O proved to be more successful. For

to increased dieize. Developing a SAW impedance

this entireevelopment, size of the SAW elements

themselves and the circuit were of great importance. It was

found that using a simple 2-Per SAW resonator as an

impedanceelementprovided an excellent olution or a

low co st , wide stopban d width notch filter circuit design.

The Four SAW Impedance ElementNotch Filter proved to

be the best developmentorheost,ize, and

performance. The die size and therefore the package size

optimum filter performance was simply impedance

was kept small and inexpensive. The tuning approach for

matching the equivalent RLC circuit of each SAWelement

in theilter at Fo. There fore, theuningomponents

necessary for o ptimum filter performance were two surface

mount inductors, keeping the cost of the comp lete circuit

low. This development chieved a notch filter with a

broad, low loss passbandand a deep null with a -40dB

bandwidth of approximately 100 kHz.

REFERENCES

[ ] C.S.artmann,.C. Andle and M.B . King,

pp. 131-138.

“SAW Notch Filters,” IEEE Ultrason. Symp. Proc., 1987

SAW transducersand gratings,” Proc.43th Ann. Symp.

121 P.V. Wright, “A new generalizedodeling of

Frequency Control, 1989, pp.596-605.

Electrical Engineering, New York John Wiley Sons

[31 I.E.Pyros, Handbook of ModernElectronics nd

1986, pp. 668-669.

Circuit Analysis, 3rd Ed. New Y ork McGraw-Hill , 1978,

[41 W.H . Hayt,r.nd J.E. Kemmerly, Enginnering

p. 450.

Ed. New York: McGraw-Hill, 1980, pp. 130-131.[51 R.L. Schrader, Electronicommunication, 4th

[61 C.S. Hartmann, US Patent No 4,599,168, Oct.,

1986.

[71 See,orxample,.A. Ash,Fundamentals of

Signal Processing”, Topics in Applied Physics, vol. 24, pp.112-114, 1978.

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