xpyttl wrote:
. . .
Varactors do have some temperature coefficient, and they are often coupled
with toroid inductors, which also have some considerable temperature
coefficient. Most of the designs you see out there are for CW rigs in the
ham bands, where temperature stability is extremely important. The maze of
capacitors around the varactor are there to balance the temperature
coefficients. Usually there is a polystyrene capacitor which has a
temperature coefficient opposite to the toroid and varactor, but you can
never get exactly the right value for that, so it is a question of getting
the right combination of positive and negative temperature coefficients AND
the right value of capacitance. For AM in the broadcast band, you can
probably come close enough with one or two capacitors. For a rig with a 200
Hz CW filter at 15 meters, it can be a real bear keeping the frequency to
within the 0.0001% that you need for comfortable operation.
. . .

All toroid inductors aren't equal, and neither are capacitors.

I routinely build VFOs with no temperature compensation which have about
200 Hz total warmup drift on 40 meters. The trick is to use components
which have inherently low temperature coefficients rather than try to
make ones with high coefficients compensate each other. Polystyrene
capacitors have a fairly high temperature coefficient, but it's in the
opposite direction than a typical poor inductor. Sometimes people get
lucky and the combination works ok, but often they don't and it doesn't.

The other thing you have to do is design your oscillator so that its
frequency depends almost solely on the tank components and not the
active device.

I found that good quality NPO ceramic capacitors have the lowest
temperature coefficient of any commonly available parts, and inductors
wound on type 6 powdered iron cores were the best. It's the inductor
which dominates the drift in my VFOs, and that small amount can easily
be compensated if desired by replacing part of the tank C with a
capacitor with controlled temperature coefficient.

I described these techniques (except for compensation) in more detail in
"An Optimized QRP Transceiver", in August 1980 QST.

Roy Lewallen, W7EL

laura halliday wrote:
. . .
I'd try R5D3 first, then see if I could find something worth
cannibalizing at Cascade (note new address) or Wacky
Willy's (lots of junk, but you never know...). Then I'd hit the
various thrift stores and see if I could find an old FM tuner.

At Sea-Pac this year I found out that Wacky Willy's has also moved from
its long time location across from the Reedville Cafe.

Are there any ham radio swap meets coming up?

There's the Salem club swap meet at Rickreall in February, and Sea-Pac
at Seaside in June, but no other major local ones I know of. I'm sure
that a query on a local repeater would bring full details of any others.

. . .

Roy Lewallen, W7EL

Here's a radio tuner based on a variometer and fixed capacitor. The
variometer housing is a CDROM container; the capacitor is built from
blank CDROMS.

http://www.hpfriedrichs.com/rr-cdrom.htm

73
Pete
AC7ZL






Hamateur wrote:

SparkySKO wrote:

[Basically this says, are there any practical alternatives to an air
variable capacitor that a beginner can use in building his first
receiver radio?]


This homebrew 10-600pF book-shaped air variable has knob tuning:
http://www.leradiodisophie.it/CV-libro.html

Wind a simple coil, add a diode and audio amplification -
see what you can hear.

Check out the Tin Ear receiver:

http://www.amqrp.org/kits/tin_ear/TinEar%20Manual.pdf

It uses a plastic drinking straw wrapped with wire and a brass screw
to make a variable inductor that is part of a permeability tuned VFO.

3 FETs 4 NPNs and 1 PNP
1 toroid

It is no longer available as a kit but all the info is there to build
one yourself.

BTW, real air trimmers seem to be far better for temperature stability
than normal plastic dielectric trimmers! In an AM broadcast
transmitter using a VXO, a plastic trimmer in series with one of two
crystals(mixer setup) gave instability and a deep hetrodyne drone that
got worse over several days(unattended remote pirate rig) even though
the outside temperature was the same.

Replacing it with an air variable trimmer whose plates were apparently
cut from two solid pieces of brass mude for utter stability, with no
sign of drift over at least a 20 degree temperature change without
having to reset it.

For noncritical applications, those little plastic variable caps from
car stereos are pretty good. Have not used one in an ultra-critical
application in a role giving a lot of control over frequency, but when
I used one in the 2004 rig to shift a crystal maybe 2KHZ out of 16MHZ,
it didn't seem to drift.

Of course, a VXO(pulled crystal) is a hell of a lot more stable than
any VFO! A VXO and a similar but not the same frequency fixed crystal
can give a suprisng tuning range with stability. Put in a crystal oven
or even a heated/air conditioned room it would leave little to be
desired in stability.

For a VFO for any application, the better your parts, the better
your results. Wind coil on "air" or unity permeability cores such as
wood or ceramic, and epoxy the windings in place. Blow on an
oscillator's coil while listening to the beat note with and without the
epoxy, and hear the difference for yourself. Wood cores seem to work
fine with the epoxy covering.

With an air-core coil,a good tuning capacitor, and a circut that
minimizes active device contribution to drift, you end up with a lot
less chasing drift to do. Best active device for any VFO and probably
any VXO as well is a JFET. Almost no heat(unlike a tube), and no
junctions in the current path to change characteristics with
temperature(unlike a bipolar). If you must use a powdered iron core,
keep DC out of the windings as changes in the DC current change the
permeability opf any ferrite or powered-iron core. Ferrite cores of any
type have been named as an especially bad source of drift in
oscillators, so don't use modified IF transfomers as tuners in
oscillators expected to be stable. They are fine in tuned small-signal
amps, just not in oscillators.

In that VXO with the bad trimmer, I got lucky and found the bad part
first try, but this is unusual. If you have to track down drift expect
hours of work. That's why it takes less tiem to use the good stuff from
the start, unless it takes hours of driving or biking to obtain it, of
course.