Nuts and Volts - September/October 2018

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.

Using ELSIE

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 Design.

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 steeper rolloff.

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

PRACTICAL TECHNOLOGY FROM THE HAM WORLD

Post comments on this article and find any associated files and/or downloads at www.nutsvolts.com/magazine/issue/2018/09.

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.