14 September/October 2018
configuration, click the Analysis tab. This is where the
inductor and capacitor Q are specified (250 and 1,000,
respectively). Lower values of Q result in less stopband
attenuation and less sharp rolloff, among other effects.
Leave them at their default settings for this exercise.
Specify an Analysis start frequency of 0.5 MHz and an
Analysis stop frequency of 10 MHz.
Leave all other selections and values
at their original default settings.
Before looking at the filter
response plot, click the Schematic
tab at the top of the screen to
show the information in Figure 1.
Passband ripple ( 21. 3 dB) and the
ratios are calculated by the program.
Now, click the Plot tab for the
frequency response graph of Figure
2. Place the cursor on the blue
response line and hold down the
left mouse button. At the bottom
of the screen, you’ll see the filter
performance at that frequency. The
figure shows performance at the
response peak of 1.93 MHz.
Move the mouse to the stop
band notch near 1.5 MHz to find
attenuation there (- 69 dB at 1.49
MHz). Attenuation varies by more
than 20 dB over the 160 meter band
— that’s too much! A third-order filter
can’t meet our specifications, so create
a higher-order filter. (While you’re at it, use the Design
window to select different filter families and compare their
This is where the value of easy-to-use design software becomes
apparent. Instead of re-starting a
laborious design process, simply
re-enter new specifications and try
again. Return to the Design tab and
increase the filter order from 3 to
4, then click Plot. Performance is
improved but attenuation still varies
by more than 10 dB across 160
Back at the Design tab, increase
the filter order to 5, resetting FS to
1.6 MHz. (The program changes
some values when order is changed.
PRACTICAL TECHNOLOGY FROM THE HAM WORLD
n FIGURE 2. Frequency response of the third-order filter shows too much
variation in attenuation across the 1. 8-2.0 MHz 160 meter band and does not
meet the 40 dB attenuation requirement at 1.6 MHz.
n FIGURE 3. Frequency response
of the fifth-order filter meets
the broadcast band rejection
requirement with only 3 dB of
variation in the 160 meter band.