September/October 2018 13
name, L-C. By stepping through a sample design, you’ll
get a feel for how to turn specifications like we discussed
in the last column into a real filter design.
Begin by downloading the current version of
ELSIE ( 2. 82 as of mid-June 2018) from Tonne Software
www.tonnesoftware.com). Follow the install wizard’s
instructions, then run ELSIE. Begin by clicking New
Now it’s time to tell ELSIE what kind of circuit you
want. This is the filter’s topology describing the general
arrangement of the filter components. (Filter basics were
covered in the previous column.)
Since we’re designing a broadcast-reject filter for
160 meter reception, we need a high-pass response.
ELSIE gives us two choices for high-pass filters: capacitive
input and inductive input.
A capacitor in series with the filter at the input
(capacitive input) blocks any DC and low frequency
signals, so select that topology.
Next, we must select from the many types of LC filter
circuits called families — each with a slightly different
type of response. For example, the Butterworth family
has a very flat response but a gradual roll-off between the
passband and the stop band. The Chebyshev family allows
some variation in the passband and stopband in trade for a
Bessel filters have a constant time delay through the
filter in the passband. If you click the ? button next to
Butterworth in the Family section, a pop-up window will
show the general behavior for each family.
For our filter, we need a very sharp rolloff that passes
signals with little attenuation at 1.8 MHz, but with lots of
attenuation at 1.6 MHz — the highest frequency of the AM
broadcast band at which full-power stations are permitted.
(Above 1,600 kHz in the US, AM stations are limited to
10 k W during the day and 1 k W at night. These smaller
stations are less likely to cause overload problems than the
full-power 50 k W transmitters.)
Chebyshev would be a good choice, but the Cauer
family is even better at creating the necessary steep rolloff.
The tradeoff is that attenuation of the Cauer filters varies
quite a bit in the stopband. That’s okay, as long as we
maintain the minimum required attenuation. So, select
Cauer as the filter family.
Now, the program needs some performance
specifications entered at the right-hand side of the screen.
How much attenuation (Stop Band Depth, AS, in dB) is
enough for our filter? In my experience, 40 dB is enough
to keep even nearby AM stations from clobbering a
modern receiver. For Ripple Bandwidth (FC), enter “1.8M”
(1.8 MHz) as the lowest frequency of the filter passband.
The highest frequency at which we want our 40 dB of
attenuation, “1.6M” (1.6 MHz), is the Stopband width (FS).
Filter Order (N) can be thought of as the number of
resonances created by pairs of L and C components. The
higher the order, the more components are required to
create the circuit. Start by entering a filter order of 3 to see
if we can meet our design goals.
Viewing the Filter Response
To set up the program’s calculations and display
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n FIGURE 1. The third-order AM broadcast-reject filter
schematic and the design parameters used or calculated
by the ELSIE program.
What is Q?
Q is the symbol for “quality factor” and for inductors
(L) and capacitors (C), it represents the losses in the
component. Q is the ratio of the component’s reactance (X)
to its resistance (R). The resistance accounts for all losses in
the component, such as for skin effect, dielectric losses, and
other parasitic effects.
Q = X / R
A perfect L or C has zero resistance, so its Q is infinite.
A perfect resistor, R, has no reactance, so its Q is zero!
Typical values of Q for capacitors used in RF circuits are
around 1,000. Inductors have Q values of 250 and below.
Try different values of Q for the components in your
design and watch for the effect in the simulated filter