Re: VFO Stability

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From: Geoff Schecht (geoffs@onr.com)
Date: Tue Aug 08 1995 - 01:02:45 EDT


>Clark Fishman <cfishman@fsac3.pica.army.mil> recommended -7 core material
>to help stability in VFOs. I have heard of this, but the last Amidon
>flyer I got with an order (about 3 months ago) had nothing to say about
>them, though it was listed on the enclosed price list. Something to try.

Amidon gets their powdered iron (carbonyl iron) cores from
Micrometals. Micrometals should have info on the latest/greatest
high-stability materials that they make. I doubt if Micrometals will
ship catalogs to just anyone who asks but if you're an engineer you
can get a copy sent to your company with a phone call.

>
>Meanwhile, the basic consensus is that ANY core material is a
>compromise for VFO use. An air core coil is by far the most stable
>inductor to use. The traditional ceramic form is most recommended, with
>glass tubing and acrylic rod also getting some nods. Toroids have become
>popular for compactness, of course, but unless that -7 material is really
>good, air core is still the ultimate if you make room for it.

I built a Vackar circuit a number of years ago that used a 1uH
aircored inductor at around 5MHz. This is a fairly small value for
such a low freq but the high C/L ratio kept the Q high and the Vackar
loads the tank less than some of the simpler oscillators.

Anyway, remember that toroids are a closed magnetic structure and are
generally self-shielding whereas air-core solenoids couple beautifully
to each other unless placed on perpendicular axes (*and* putting a
solenoid-type inductor next to a chassis wall or shield amounts to
coupling it into a shorted turn!).

Core materials that are well characterized/stabilized may indeed have
higher tempcos than air coils but it's possible to track this out over
a wide variety of conditions by a judicious choice of temperature
compensating capacitors. Look inside a Collins PTO and you'll see what
I mean. An isothermal circuit environment and keeping tank circuit
circulating currents at reasonable levels helps minimize temp drifts,
too.

Anyone try injecting isocyanate foam into their VFO's cavity to keep
the temp constant? I did this years ago in a VFO for a PLL and it did
reduce microphonic susceptibility somewhat (I never checked the
thermal drift in a before/after test, sorry!).

Geoff NQ7A Austin


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