[Lowfer] Filters?

[Ralph Hartwell]

> Where can I find info on filter design?  I wish to block 60 Hz and
harmonics
> if possible, and have a cut-off from 50 KHz and above.  Any ideas?

From my experience, most of the 60 Hz and harmonically related garbage
is caused by ground loops between the receiver and the antenna system.
I find almost all the trash goes away when I use a shielded torroid
transformer between the antenna and the RX.  you have to break the
shield side of the coax - no DC path allowed or you get hummmm.

As for the low pass filter, running TuneKit (available at
http://members.aol.com/maxfro/index.html )  for a 5-pole low pass filter
with a cut off frequency of 50 KHz  and a 50 Ohm design impedance gives
the following values:

CHEBYSHEV LOW PASS FILTER
FREQ= 50000 HZ
POLES= 5
RIPPLE=.1dB

CAP 1  C = 7.30 E-08F
IND 1   L = 2.18 E-04H
CAP 2  C = 1.26 E-07F
IND 2   L = 2.18 E-04H
CAP 3  C = 7.30 E-08F

Where F = Farads and L = Henries.

The filter is configured with CAP 1 connected between the input and
ground, IND 1 and IND 2 in series with each other and connected between
the input and output, CAP 3 connected between the output and ground and
CAP 2 connected between the junction of IND 1 and IND 2 and ground.
Place IND 1 and IND 2 so that they are at right angles to each other and
do not couple magnetically to each other.

73,

Ralph   W5JGV / WC2XSR / 13

http://home.att.net/~shmrg

http://home.att.net/~ralph.hartwell