Tuning LC with potentiometer only

The pot would alter the bandwidth. This would be a nice demo if you have a spectrum analyzer (or FFT program + soundcard if the frequencies are low). If you want to change the tuning frequency the variable plate caps are the way to go. IIRC they are not that expensive. If you want to filter multiple bands, you'll need to connect them in 'parallel'. I.e. a distribution amp with multiple independent outputs and a summing amp.

Anyway, this might be easier if the frequencies are low enough to be processed by a soundcard. Most decent soundcards go up to 90kHz these days.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
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"Larry Towers"

** Long as your noise source operates in the audio band - all you need is a basic "graphic equaliser". The simplest have 5 or 6 bands and come in a little box run from a 9V battery - check out eBay.

Set all the sliders at minimum and then bring one up to maximum - that will give you a narrow band of noise that can be viewed on a CRO or heard through a speaker.

.... Phil

1: Use an inexpensive variable cap: 2: Use a potentiometer to set a voltage across a varactor.

3: Use a superhet receiver as a cheap tunable volt meter.

4: Raid an antique electronics store for a real tunable volt meter.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

** Of all the whacky drivel you have posted - this s**te takes the cake.

Does not help the OP with a single one of his requests.

.... Phil

Look for "Notch Filters"

OR Look in the FFT "Fast Fourier Transforms"/ DFT, IDFT's etc.

You didn't specify the freqs you are working with so we don't know..

Jamie

..

cake.

But that's often what OPs need the most--new ways of approaching / thinking about about their problems.

-- Cheers, James Arthur

If you need a specific bandwidth (say 100 Hz) regardless of frequency, use a VCO (controlled by the pot) to a higher frequency (say 100 kHz for audio),then using a fixed 100 Hz wide band pass filter at 100 kHz (Q=1000, may require a few filter stages), then mix it down to the original audio frequency, using the same VCO. Filter the image frequency above 100 kHz.

The output will a 100 Hz wide noise that can be easily moved across the input frequency range with the pot.

Use a varactor diode, sometimes called a varicap.

Good Luck! Rich

Even easier than that download Daqarta - a lovely little soundcard based scope for the PC that will do realtime FFT display as a function of time. I used it a couple of years back for a lecture on sound and music. Good up to around 20kHz on a decent soundcard maybe a bit higher.

Regards, Martin Brown

On a sunny day (Sat, 21 May 2011 00:00:35 GMT) it happened snipped-for-privacy@portal.com (Larry Towers) wrote in :

You could use a ferrite core and tune by sliding the core in and out, or use a magnet to change L by driving it into saturation a bit.

I am top posting my OP.

The idea folks was to use a pot. What kind of adjustment can I demonstrate with fixed RC values?

For example:

Someone mentioned bandwidth. How can that be set up to produce a successive narrowing effect, say by a factor of ten over a few stages?

Larry Towers

"Tim Wescott"

• Did you READ the OP's question at all ??

Fuckhead.

..... Phil

Phil thinks that "wacky drivel" and "not the Phil way" are one and the same. Just ignore him.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

You can use permeability tuning, with a pot controlling drive to a motor and the motor pushing a slug into a coil. It may not be practical for you, I don't know. But it worked great for measuring the height of the tide in a canal. No motor - the tide raised and lowered the "slug" to change the coil inductance. It was a BIG coil.

You could also modify your idea and use a bunch of fixed C's in series across an L, where varying the pot selects more C's for lower total capacitance, or fewer C's for higher.

I think the simplest idea for RF was already proposed - a varactor diode LC circuit, and for audio, an equalizer. Is there a reason they won't work for you?

Ed

One other possibility no-one so far seems to have pointed out is to use a passive RC twin T tuned circuit and a pair of identical twin gang pots with a rubber band round the stems so they will turn in sync. Pots in series on the R,R,C/2 T and in parallel on the C,C,R/2 T. It will obviously present a very variable load on the signal source...

Normally used as a notch reject filter but the signal difference between the input and terminal is a bandpass provided you don't load it.

Regards, Martin Brown

I have a wideband noise signal and want to tune to specific frequency ranges (not precise) of same but not use expensive variable plate caps.

Is there any way to do this, even on a rudimentary level, perhaps by fitting a coil and cap somehow across the lugs of a pot?

The idea is to string several of these in series, each with a slightly higher resonance, and demonstrate on a CRO that the frequency content of the original noise has been altered.

Thanks for any pointers in this direction.

Larry Towers

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