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 (). 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
Frequency
(Hz) |
Insertion loss
(dB) |
Frequency (Hz) |
Insertion loss
(dB) |
50 |
34.9 |
500 |
1.9 |
60 |
31.9 |
700 |
1.1 |
80 |
27.5 |
1000 |
0.7 |
100 |
24.0 |
2000 |
0.4 |
120 |
21.1 |
5000 |
0.3 |
150 |
16.9 |
10,000 |
0.3 |
200 |
11.0 |
15,000 |
0.2 |
250 |
6.7 |
20,000 |
0.2 |
300 |
4.7 |
100,000 |
0.2 |
400 |
2.7 |
200 kHz-2 MHz |
0.2 |