Re: VFO Stability


From: Geoff Schecht (
Date: Tue Aug 08 1995 - 01:02:45 EDT

>Clark Fishman <> 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,

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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