RS Microwave Technical Corner

July/August 1998

Quasi-Elliptic Notch Filters for AMPS/GSM Separation

For the bandpass case, quasi-elliptic filters achieve steeper attenuation slopes with at least one region of non-monotonic stopband.  The swept response has some of the characteristics of the well-known “elliptic” response, in which the stopband levels oscillate but after achieving a minimum level of attenuation (such as 40 dB) do not go above that level (attenuation is always greater than the 40 dB).  The quasi-elliptic filter has at least one of these oscillations, but then usually then continues monotonically towards an ultimate attenuation floor of perhaps 85 or 90 dB.  In notch filters, a similar characteristic pertains, but the result is a bit different.  In this case, the stopband levels are achieved more quickly than with Chebychev designs, but the rippling or oscillation is not apparent.  However, with designs of this type, wider passbands and steeper stopbands are possible.  This is achieved by proper selection of the main and shunt line impedances.  Synthesis, combined with modeling and optimization, is the key.

Fig. 1 illustrates the response of a cascade of 4 5th order bandstop filters, each of which is in itself a quasi-elliptic bandstop network.  The cascade of these sections provides equivalent attenuation slope to a 26th order Chebychev filter so the economy of design is apparent.
 

 
 
FIGURE 1
Cascade of Four 5th order Quasi-Elliptic Notch Filters
 
 

The specifications for the previous bandstop filter are as follows:

P/N 71321A-5
Minimum passband: 891 to 960 MHz
Max. insertion loss over above passband: 2.75 dB (less than 1 dB 895-960 MHz)
Average insertion loss over above passband: less than 1.5 dB, 1 dB typical
Min. -40 dBc rejection: 870 to 888.75 MHz
Min. -35 dBc rejection: 888.75 to 889 MHz
Min. -30 dBc rejection: 889 to 889.9 MHz
 

Figure 2 depicts the interior configuration for one of the four identical blocks within the filter.  Although this bandstop example does not use cross-coupling as in a bandpass filter, the center section varies in impedance along its length, and a short length of non-50 ohm air slabline connects each 5th order element, with the combination such as to provide  bandstop quasi-elliptic response analogous to the bandpass case. Figure 3 is an outline drawing for the filter described above.
 

 
FIGURE 2
One of four 5th order sections
 
 
 
 

 

FIGURE 3
Outline-Cascade of four 5th order Quasi-elliptic bandstop sections
 
RS Microwave has produced many such filters, with implementation in coax, dielectric resonator, waveguide, stripline and lumped element.  We will be happy to apply this exciting technique to your particular problem.
 
 
© RS Microwave Company, Inc. 1998
 

NOTE:  More details on this topic will be presented at the European Microwave Conference from October 5-12 in Amsterdam, The Netherlands.