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'[PICLIST] EE: varactors vs. air variable caps and'
2001\08\09@144145 by Mike Kendall

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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@051533 by Peter L. Peres

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You don't say what frequency but 120 pF suggests low HF or lower.
Varactors have problems if the RF amplitude is comparable inmagnitude to
the control voltage. This is almost always the case in equipment not
designed to work with varactors. The Q is at least one order of magnitude
lower than that of an air variables. Plus they are prone to
intermodulation and other effects if used near a front end that can be
overloaded.

Apart from that, every TV and VCR tuner manufactured in the last at least
25 years used and uses varactors and there seem to be no serious problems
so it depends on what you really do.

Peter

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2001\08\10@111958 by Mike Kendall

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Thanks for the input Peter and Alan,  I was considering using them in a grid
dip oscillator.  If a varactor is used, I wonder if it would need to be
buffered?  I saw that the Motorola varactors are discontinued and even some
of the varactors on digi-key are discontinued and only available in a
surface mount package.  NTE makes some varactors in a TO-92 type case.  I'm
starting to wonder if a varactor is not the correct part for a GDO circuit?
There was a NTE618 AM tuning varactor.  If I tied them together to isolate
the tuning voltage in the oscillator circuit, the capacitance would be
halved and in the range of 250pF but with low Q and possibly the need for
switched-in fixed capacitance values?
Thanks/regards,
Mike
{Original Message removed}

2001\08\11@043645 by Peter L. Peres

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Mike, I think that you can use varactors in a gdo if it has some
provisions for amplitude limiting the oscillation. A Lambda diode
oscillator comes to mind as a appropriate. Almost everything else will
require that you limit the minimum control voltage seriously even if you
use two antiseries varactors.

I built my gdo using a polyethylene variable meant for small radios. It
has the same axle you described but I was able to find buttons for it in
this country. ;-) (actually a tuning scale wire wheel).

Peter

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2001\08\11@125743 by Mike Kendall

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Hi Peter,
   I'm guessing that the tuning wire wheel that you mentioned must be the
older type of tuning assembly in radios that has the string that provides
the reduction ratio when turning the air variable cap.  Really good idea.
I've got  a "made in Taiwan" GDO that  has a large dial with teeth on it.
The cover on the "wheel" must cover up the fact that it may just be one of
those tuning wheels.
   I did a search for Lambda diodes and only came up with microwave diodes.
Are the Lambda diode oscillators Gunn diode oscillators, and if so, can they
be used in low frequencies?
   The specification sheets on the varactors have specific voltage values
necessary to obtain the desired capacitance. Do you mean that  I need to
carefully choose  the correct N-channel JFET in the oscillator when you
mentioned " provisions for amplitude limiting the oscillation"?  I was
looking at using the 2N7000 as it seems superior to some of the older  MPF
type JFETs?
Regards,
Mike
{Original Message removed}

2001\08\12@151001 by Peter L. Peres

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A Lambda diode osc. is actually realized with transistors inside a chip
normally. It is a negative resistance device so it only needs one port for
the LC or crystal. It also inherently limits the amplitude of the
oscillation to 1.2V pk-pk (more commonly 0.3-1.0 Vpkpk), and is suitable
for GDOs because you can easily turn it into a passive receiver/detector
and also adjust the gain. Many PLL chips and color/TV/video chips that
have the crystal or tank connected to only one pin and GND have lambda
diode o.'s. The low L.O. amplitude is good for low radiation/parasitics.
The associated tuned circuit is usually operated in series resonance
(this is favored by Rneg and the current drive output of the device).

You can make your own using an emitter coupled pair of NPNs. Improved
versions use 4 NPN (one extra for the current source and one for the
output buffer/amp). There are also FET versions used in CMOS chips. Some
are very high performance (using FETs). Since none of the devices can
saturate in normal operation they go up to respectable frequencies.

If you can't find a lambda diode circuit on the web I'll find something
and send it to you.

Peter

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