A previous Idea for Design described an R-C twin-tee circuit designed to reduce power-supply hum at 50 to 60 Hz ("R-C Twin-Tee Reduces Power-Supply Hum,"). This Idea for Design presents a new L-C high-pass filter that reduces both power-supply hum at 50 to 60 Hz and ripple at 100 to 120 Hz. The circuit is a relatively simple composite high-pass filter employing the classic design technique of image parameters.¹

The high-pass filter uses input and output half sections with m = 0.5 and one interior constant-k T-section (see the figure). The filter, designed for source and load impedances of 50 , was assembled on vector board (type 169P44C1). It was mounted exterior to a proprietary test piece that can switch between test and bypass modes.

All capacitors were 50-V electrolytic 10- F units with radial leads, 20% tolerances, and an unspecified equivalent series resistance. Note that all capacitors are polarized. The inductors were Fastron radial and pluggable epoxy coils with 5% tolerances, obtainable from Mouser Electronics. Each has a minimum unloaded Q of 50 at a test frequency of 79.6 kHz. Typical small-quantity component costs are $0.25 for the capacitors and $1.05 for the inductors.

The filter was tested over a range that included both audio and AM RF bands. The measured insertion loss is shown in the table. Greater circuit selectivity can probably be achieved using more than one interior constant-k T-section between the m-derived input and output half sections. The composite filter is a symmetrical circuit with any number of interior constant-k sections possible. Circuit symmetry is not ordinarily realizable for higher-order elliptic function filters.

The L-C high-pass filter can be used in test accessories, with the unit's pc board installed in a die-cast aluminum enclosure. The filter can also be integrated into manufactured equipment.

Reference
1. R.M. Kurzrok, "Update the Design of Image-Parameter Filters," Microwaves & RF, May 2000, p. 119-126

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