SF Filters

(Replace Round Tubular Filters)


SF 101: 9 to 30 GHz

SF 102: 1 to 10 GHz

SF 103: 200 MHz to 5.5 GHz


  • Bandpass
  • 200 MHz - 30 GHz
  • Low Loss (Much less than tubular filter)
  • 3 db Bandwidths 1% to 80%
  • Small Size (Less volume than tubular filter)
  • 50, 70 or 93 Ohm
  • Mounting Provisions Built-In
  • 1.5 VSWR in passband
  • Pins or Connectors
  • High Power
  • Evanescent Design (Wide-Band spurious free)
  • 3 to 22 Pole
  • Low Ripple Response
  • Reduced height configurations    

Figure 1 Outline

SF Series Filters offer a high performance alternative to conventional tubular bandpass filters. Using the evanescent mode technique, insertion loss is reduced to half that of the tubular filter, while power handling capability is more than doubled. Volume is reduced as well.

Use of standard parts permits rapid delivery with economical cost. SF Filters are available with connectors or pins. Placement of connectors may be specified on the ends or on a side. Mounting provisions are “built-in”. See Figure 1 for standard configurations.

SF Filters are easy to specify (see RFQ form) and are suitable for channelized receivers, multiplers, amplifier output filtering, etc. They may be readily multiplexed. SF Filters are high performance alternatives to tubular bandpass filters.

For SF Filters, low side attenuation will approximate 15% less (in db) than .01 db Chebychev theoretical values down to the 80 db level. High side values will follow 10% less (in db) than an N pole .01 db response, where N is the number of poles in the specific filter, down to the 80 db level. The low-side selectivity characteristic can be improved on special order, if a non-monotonic skirt shape is allowable.

Two Pictures SF Filters                     Picture SF Filter

Center Frequency insertion loss can be calculated using the following formula:

EQ(1)              Loss=K1  (0.66N - 0.83)      1                K1=0.9 SF 101 Series
                        BW                   (fo 1/2)            K1=1.3 SF 102 Series
                                                                K1=1.9 SF Series

Where              N = number of Bandpass poles

                         BW = VSWR Bandwidth in percent (approximately 70% of 3 dB bandwidth for 5 pole, 90% of 3 dB for 9 pole).

Note:             fo1/2 = Square Root of fo

Physical length (exclusive of connectors) can be calculated from EQ.2:

"A"= Length = (10[LOG10 BW - 2.4/tan (93.5 + 1.9 (N-3))])  x K2

 K2=1.0  (SF 101)

K2=1.8 (SF 102)

K2=3.1 (SF 103)

Channelized microstrip test fixtures are available to alleviate compatibility difficulties which have been common in the use of drop-in components. Installation of a typical filter is shown in Fig. 2. The channelized structure results in maximum input to output isolation. Alternatively, many SF Filters can be supplied with field replaceable connectors to eliminate the need for test fixtures entirely.


Figure 2

  • fo = 1030 MHz
  • “Flat” Bandwidth = 80 MHz Minimum
  • 3 dB Bandwidth = 104 MHz Minimum
  • Required 60 dB Bandwidth = 392 Maximum
  • Number of Poles = 5 (66 dB on a .01 dB Chebychev chart)
  • Percentage Flat

                BW                         = ( 80/1030) x 100 = 7.76%

  • Insertion Loss = (1.0/7.76 ) (.66 x 5-.83) 1/(1.03)1/2 = .32 dB
  • Length = 2.65 inches (EQ.2.)
  • Delivery = Start 30 Days
  • Part Number = SF 102-1030-80-5

RS Microwave can provide diplexed or multiplexed configurations as well as high power evanescent structures. Packages with a meandered or folded signal path or unusual connector locations are available.

Last Updated on Tuesday, 27 December 2011 14:48