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'[EE]: varactors vs. air variable caps and LED win'
2001\08\09@152715 by Mike Kendall

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I origionally  accidently didn't  include the brackets around the "EE" so am
re-sending this message with the correct subject.

Have a project I've been working on for months  now.
1st question-  What is a good  source for the plastic used for LED windows
for the red 7 segment LED's?
2nd question-   Polling for input on Varactors vs air variable capacitors.
I recently purchased 16 RF tuner capacitors at $1.25ea from Electronix
Express only to find that there is no such thing as a mating knob for them
in the USA (they have two flat sides on the tuning shaft and a threaded hole
in the center of that.  Also, the shaft is extremely short).  I've
considered using a varactor tuned oscillator circuit with the 120pF variety.
Saw a web page that uses two varactors chained together with the tuning
voltage at the  tie point.  The circuit web page mentioned the low Q of
varactors and  I'm  concerned about this affecting my circuit negatively.
I'm aware of homebrew amateur 2meter transcievers using varactors and their
usage in radio receiver tuners.
Regards,
Mike

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2001\08\10@044528 by Alan B. Pearce

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>2nd question-   Polling for input on Varactors vs air variable capacitors.
>I recently purchased 16 RF tuner capacitors at $1.25ea from Electronix
>Express only to find that there is no such thing as a mating knob for them
>in the USA (they have two flat sides on the tuning shaft and a threaded
hole
>in the center of that.  Also, the shaft is extremely short).  I've
>considered using a varactor tuned oscillator circuit with the 120pF
variety.
>Saw a web page that uses two varactors chained together with the tuning
>voltage at the  tie point.  The circuit web page mentioned the low Q of
>varactors and  I'm  concerned about this affecting my circuit negatively.
>I'm aware of homebrew amateur 2meter transcievers using varactors and their
>usage in radio receiver tuners.

I think you should check out the Hyper-Abrupt junction Varactors that are
available. I think you will find that these give a higher Q than older
varactors. You do not state what frequency range you want, but if you are
talking in terms of 120pF then I suspect it will be HF rather than broadcast
or VHF. To get this you may need several diodes in parallel to get the
capacitance you need, but it is a while since I looked at varactor data
sheets so am not sure of the capacitance ranges available these days.
Certainly every TV and Video tuner has them in.

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2001\08\10@145546 by dbengtson

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On Thu, 9 Aug 2001 14:31:49 -0000, you wrote:

>I origionally  accidently didn't  include the brackets around the "EE" so am
>re-sending this message with the correct subject.
>
>Have a project I've been working on for months  now.
>1st question-  What is a good  source for the plastic used for LED windows
>for the red 7 segment LED's?
>2nd question-   Polling for input on Varactors vs air variable capacitors.
>I recently purchased 16 RF tuner capacitors at $1.25ea from Electronix
>Express only to find that there is no such thing as a mating knob for them

What are the specifics of the Circuit that you are tuning? It may be
limited by stuff other than the Varactors. Although you might need a
DAC to drive a Varactor, they might be a lot easier to deal with.
Dave

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