Switch between different capacitor values

[Amanda Robin]

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

 

[John O'Flaherty]

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.

`
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.

 

[Amanda Robin]

`
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.

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

Amanda

 

[redbelly]

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.

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

 

[redbelly]

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

.... 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-----
.. |
.. |