September/October 2018 15
Check your settings whenever you
change filter order.) This response
(Figure 3) is much more useful.
Attenuation varies by about 3 dB (1/2
S-unit) across the 160 meter band,
and we just meet our design goal with
- 40 dB of attenuation at 1.59 MHz.
Click the Save tab to hold on to this
design version before proceeding.
Imagine doing this in the “good
old days” before the desktop PC
became a commodity! For each
design, detailed calculations would
have to be worked out with a calculator or
slide rule at numerous frequencies, then
plotted on graph paper if reviewing the full
response was necessary. Today’s process
takes seconds and allows a lot of “what if” experimentation
that was impractical before.
Standard Value Components
The schematic shows all the component values are
in a reasonable range. Nevertheless, I don’t think your
local component vendor will have, say, 538.436 pF
capacitors in stock, nor do you want to have to adjust
variable capacitors. Now is the time to redesign the filter
using standard fixed-value parts. This will degrade filter
performance a bit, but remember that we can continue to
work with the design.
Return to the Design window and click the Nearest
5% tab. You’ll be presented with several options, including
changing all the components to the nearest standard value
in the 5% series.
Other options include just changing the capacitors or
inductors, assuming you’ll wind the Ls
or tune the Cs. You can also change
just the capacitors or inductors, and
the program will re-calculate the
remaining values exactly so that you
can tune up the filter yourself.
Let’s take the easy way out
and select the first option to use all
standard values. Return to the Design
tab, then check the schematic shown
in Figure 4. How about performance?
Viewing the frequency response, not
much has changed.
We have a little more variation
across the band (now 4 dB), but
attenuation at 1.6 MHz is still the
same, only failing to reach 40 dB
below 840 kHz by less than a dB. This
design can be built with off-the-shelf components requiring
no tuning to provide useful performance.
Testing a Real Filter
After publishing this design in QST (January 2016
issue), I received an email from Scott Roleson KC7CJ who
built a very nice version of the filter, shown here in Figure
5. He used a sturdy die-cast aluminum box, even making
the PCB (printed circuit board) from scratch. The capacitors
are silvered-mica with a 5% tolerance. For the inductors, he
used miniature encapsulated components.
Roleson created an interesting way of bypassing the
filter. Using individual SPDT subminiature toggle switches
at the input and output avoided a multi-pole switch that
might have allowed unwanted signals to “leak” from the
filter’s input to the output through internal capacitance.
A piece of aluminum rod has a hole in each end that slips
over the switch handles so that both are moved together
n FIGURE 4. The schematic of the fifth-order filter after standard 5%
series component values are substituted for exact calculated values. Filter
performance is substantially unchanged from the exact value version.
n FIGURE 5. KC7CJ’s homemade version of the BCI filter.