Electronic Design

Inductance-Substitution Circuit Uses Variable Coils

When prototyping RF circuits, computer simulation or de-sign programs can potentially yield the wrong component values. These errors are caused by the omission of capacitances or inductances originating from the board layout or other considerations. This can be quite problematic when it comes to surface-mount or through-hole components where, after a few changes, the pc-board pads can be lifted or ruined.

One alternative is the use of old-fashioned component substitution, where different components are quickly switched in or out to achieve a range of values. The process is greatly simplified if individual values are grouped into a convenient sequence. Since BCD code is most easily added mentally, I chose the following sequence: 1, 2, 4, 8, 10, 20, 40, 80, 100, 200, 400, 800 (see the figure). Unfortunately, standard capacitors and inductors aren't available in these values. For an inductance-substitution circuit, the easy solution is to use adjustable coils. Even though 1, 10, and 100 µH are standard inductor values, variable coils have the added advantage of allowing the user to set these and all of the other inductances to within 1% of the desired values. To achieve a value of 2.00 µH, adjust a 2.2-µH coil downward; for a value of 4.00 µH, adjust a 3.9-µH coil upward; and for a value of 8.00 µH, adjust an 8.2-µH coil downward. The same adjustment process is repeated for the other two decades.

In this circuit, each coil is shorted out of the sequence through default by its corresponding switch. To add the coil's value to the total substitution inductance, just open the switch. This scheme yields a range of values from 1 to 1665 µH (1.665 mH) in 1-µH steps.

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