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Switch between different capacitor values

I have posted before about using a Maxim 281 Low pass Bessel filter. I need to be able to switch cutoff frequencies between three different values. I'm using an external clock oscillator to set my cutoff frequencies. It's cascaded with a couple of flip flops used as divide by twos to halve the frequency twice. I know how to wire a manual switch to choose between these three values to send to the Maxim chip.

However.

The Maxim requires an external resistor and capacitor to operate as part of a feedback loop and as one pole of the filter. They have to be computed as 1/(2*pi*R*C)=fc * 1.1579, or performance degrades. I can use the same resistor value and have 3 different capacitor values.

So in order to change the cutoff, I need to change not only the clock frequency, but also a capacitor value

I want to use another manual switch (my clock is on a different board, so it can't be the same switch) to switch between 3 capacitor values. Is this possible? I've just never seen it done.

The cutoffs will probably be around 600, 1200, and 2400 Hz and capacitor values around 4, 8,and 16 nF.

Thanks for any tips, concerns, suggestions for types of switches, caveats, etc.

Amanda

Reply to
Amanda Robin
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` The values aren't so small that you can't use switches. For the values you show, you could use two 8 nF caps:

in ------ | | | | \\ o | \\ ---

16\\ --- 8 nF | | | | | o | \\ | \\ | 4 o \\o 8 or 16 ------| | | | --- | 8 nF --- | | | |---|

out

If you don't have to switch them very often, you could use dip switches.

-- John

Reply to
John O'Flaherty

Thank you, thank you, thank you. I probably would have done something clunky with three caps.....

Amanda

Reply to
Amanda Robin

Pretty neat. The same thing can be done with a single on-off-on switch, and capacitors of 8, 8, and 12 nF. If I have time later, I'll post a schematic (have to run to an appointment in 5 minutes).

Mark

Reply to
redbelly

... and here it is, all three capacitor values available with a single on-off-on switch. I AM a little leery in general of connecting switches to caps, but others on here know better than I about when it's suitable. John did say it should be okay here.

-- Mark

. | . | . o------- . | | . | | . o --- . / --- 8.2nF . / | . (16)o x o---o . | (4) (8) | . | | . --- --- . --- 12nF --- 8.2nF . | | . | | . -----o----- . | . |

Reply to
redbelly

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