Adjustable filter provides lowpass response

By Richard Kurzrok, Queens Village, NY -- EDN, 3/1/2001

You can configure simple lowpass filters as pi sections with nominal three-pole, 0.1-dB Chebyshev response to provide a moderate amount of stopband selectivity. You can put four of these filters into one enclosure and then select discrete-filtering steps by using toggle switches. Manufacturers of commercially available stepped attenuators and adjustable baseband equalizers commonly use this technique (Reference 1). In an adjustable lowpass filter, each filter section uses commonly available components (Figure 1). This example uses filter-section cutoff frequencies for standard inductors and capacitors without the need for any extra components in series or parallel. Fixed inductors are Coilcraft 90 series axial-lead chokes with ±10% tolerance. Fixed capacitors are polyropylene units, available from any distributor, with ±2% tolerance for the 1200-pF devices and ±5% tolerance for the other values.

The adjustable lowpass filter is in a 3.625-in.-long×1.5-in.-wide×1.0625-in.-high Bud CU-123 die-cast aluminum box with input and output BNCs. Miniature toggle switches for the individual filter sections are accessible at the enclosure's exterior. Internal ground returns use solder lugs. The four filter sections have 50Ohm characteristic impedance and nominal 3-dB cutoff, from left to right in Figure 1, of 3.083, 6.586, 14.491, and 21.310 MHz. Table 1 shows the measured amplitude response for the box alone and for the four individual filter sections. The low-cost, adjustable lowpass filter delivers reasonable performance. As the frequency approaches 100 MHz, the transmission performance of the enclosure deteriorates with all filter sections switched to the off position. The interconnections between switched sections use available bus wire without any precautions to minimize parasitic circuit elements. Stray series inductance, estimated at approximately 55 nH, arises from the 3.5-in. physical length of the enclosure, plus 2 in. for the four switches.

References
Kurzrok, Richard, 'Adjustable-amplitude equalizer provides small discrete steps,' Electronic Design, May 31, 1999, pg 76.