Lowpass filter uses only two values
Edited by Bill Travis and Anne Watson Swager
Richard M Kurzrok, Queens Village, NY -- EDN, July 5, 2001
In recent years, image-parameter design of LC filters has received new consideration (references 1 and 2). The composite lowpass filter uses interior constant-k full sections terminated by m-derived half-sections. For best passband response, you usually select m to equal 0.6. However, m=0.5 can still give useful filter performance while reducing the number of component values. A low number of component values is advantageous for low-cost manufacturing. The design technique is also applicable to highpass and wideband filters (references 3 and 4). Figure 1 shows a schematic of the composite lowpass filter. The filter uses four inductors of two different values and five capacitors of two different values. Figure 2 shows the schematic of an equivalent composite lowpass filter. This filter uses judicious combinations of components in series, parallel, and series-parallel. The filter uses eight inductors and 14 capacitors of only one value each.
Table 1 gives test results for the lowpass filter of Figure 2. We constructed the filter on Vector board in a die-cast aluminum enclosure with BNC input and output connectors. The 3-dB cutoff design frequency is 3.88 MHz with source and load impedances of 50V . All capacitors are polypropylene units of 820 pF±5%. All inductors are 2.05 µH and made of 20 turns of number 26 AWG magnet wire on Micro Metals T37-2 toroids. The toroidal inductors, with unloaded Q exceeding 100, provide low passband-insertion loss. Surface-mount inductors with unloaded Q of 10 to 20 yield higher passband losses, which are acceptable in many applications.
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