MOTM

Thought this might be helpful to a few people when reading schematics.

Cheers!
Shemp

----- Original Message ----- 

From: Brousseau, Paul E (Paul) <noise@...>
A question:  are variable capacitors actually in use? 
They seem not nearly so common as variable resistors.

I have seen only two common applications in my very limited experience.
1.  Radio reciever tuning units.
2.  Foot volume controls in older Hammond Spinet organs.

I am sure there are others.

>I have seen only two common applications in my very limited experience.
>1.  Radio reciever tuning units.
>2.  Foot volume controls in older Hammond Spinet organs.
>
>I am sure there are others.

The tuning trimmers in old tube theremins used variable capacitors -- hey,
it's a synth application!!

>Variable capacitors tend to be very "physical," therefore very expensive
>>and a general nightmare to work with. So in an RC type circuit where you
>>can vary either one, the resistor ends up being a MUCH simpler
>proposition.

This is true.

>Variable capacitors (at least all the ones I've seen) tend to be rows of
>semi-circular plates mounted on a spindle, and that hangs over another set
>of fixed plates in such a way that the two dovetail alternately. Turning
>>the spindle moves the variable plates in or out of the others, sort of
>like
>moving nuclear control rods up or down, if you'll buy that analogy.

Yes, it changes the surface area of the capacitor.

>The problem is that to get any real capacitance, you need a giant set of
>plates. I had one of these in an ooooold radio and the thing weighed about
>15 pounds for that one part! It was huge! Yet I think it was a relatively
>small capacitance.

These are called gang capacitors -- one knob tunes several circuits, AM,
FM, etc.  I had one in an old Heathkit tuner.  It was about the size of a
beer can, not that anyone with enough taste to appreciate motm would ever
drink canned beer :)

Otoh, I built a crystal radio when I was a kid -- it had variable capacitor
about the size of postage stamp.

Also, some circuits use varactors -- which in my limted understanding is a
semi-conductor variable capacitor.  I believe that's what used in a Manley
de-esser.  Otoh, acceleration limiters use switched caps.

Has anyone ever built a varactor synth filter??

Oh dear, another part I've not heard of.  What's a "switched cap"?  Is it
simply a cap with two possible capacities?

--PBr

-----Original Message-----
From: mark@... [mailto:mark@...]
Sent: Wednesday, June 20, 2001 7:02 AM
To: motm@yahoogroups.com
Subject: [motm] variable capacitors (was: Schematic Icon Reference)

Also, some circuits use varactors -- which in my limted understanding is a
semi-conductor variable capacitor.  I believe that's what used in a Manley
de-esser.  Otoh, acceleration limiters use switched caps.

Has anyone ever built a varactor synth filter??

At 10:34 AM -0700 06/20/01, Brousseau, Paul E (Paul) wrote:
>
>Oh dear, another part I've not heard of.  What's a "switched cap"?  Is it
>simply a cap with two possible capacities?

Don't worry, it isn't another part.  It's a combination of parts, or rather
an adjective describing a noun.  For example, each input of a 110 ring mod
has a switched cap.  In the case of limiters using switched caps for time
settings, it's a rotary selector switch with a different capacitor for each
setting -- regular, BBQ, or extra-crispy :)

>Also, some circuits use varactors -- which in my limted understanding is a
>semi-conductor variable capacitor.  I believe that's what used in a Manley
>de-esser.  Otoh, acceleration limiters use switched caps.
>
>Has anyone ever built a varactor synth filter??

> Don't worry, it isn't another part.  It's a combination of parts, or
rather
> an adjective describing a noun.  For example, each input of a 110 ring mod
> has a switched cap.  In the case of limiters using switched caps for time
> settings, it's a rotary selector switch with a different capacitor for
each
> setting -- regular, BBQ, or extra-crispy :)

Bzzzzzzzzzzzzzzzzzzzzzz! -10 points.

A 'switched cap' is a semiconductor structure that implements a variable
capacitance
by modulating an analog switch at high frequencies. National, Linear Tech,
and Maxim
all make various filter ICs that you feed in a digital clock that is usually
100X or 50X the
cutoff frequency. They do not have resonance.

Other similar structures are 'flying capacitors', which are used in some
analog-to-digital converters.

Entire books and 1000s of papers have been written on these structures. They
are not in widespread use,
because of cost (not all that cheap) and they tend to be noisy.

Paul S.

At 2:05 PM -0500 06/20/01, Paul Schreiber wrote:
>
>> Don't worry, it isn't another part.  It's a combination of parts, or
>> rather an adjective describing a noun.  For example, each input of a 110
>>>> ring mod has a switched cap.  In the case of limiters using
>> switched caps for time settings, it's a rotary selector switch with a
>> different capacitor for each setting -- regular, BBQ, or extra-crispy :)
>
>Bzzzzzzzzzzzzzzzzzzzzzz! -10 points.
>
>A 'switched cap' is a semiconductor structure that implements a variable
>capacitance by modulating an analog switch at high frequencies. National,
>>Linear Tech, and Maxim all make various filter ICs that you feed in a
>>digital clock that is usually 100X or 50X the cutoff frequency.
>They do not have resonance.

So it's also a compound noun referring to another part, and in that case I
was wrong.  However, that is not to what I was referring.  Like I said, in
the example below, it's an adjective describing a noun.  So any cap that is
switched is a switched cap.  Just like one could say, "mastering
compressors use switched resistors instead of pots."  Now, "switched
resistor" might also be a compound noun referring to something else, but
the speaker wasn't using that word.

While I more than appreciate your vast knowledge of electronics, you'll
notice I don't go around grading people's posts :)

>>>Also, some circuits use varactors -- which in my limted understanding
>>>is a semi-conductor variable capacitor.  I believe that's what
>>>used in a Manley de-esser.  Otoh, acceleration limiters use
>>>switched caps.