[time-nuts] Coil sensitivity to external magnetic fields...

[Burt I. Weiner]

Pardon me for inter-loping here - I don't know if this has any 
bearing on the subject but the following was certainly a learning 
experience for me.  About a thousand years ago I had an ICOM IC-21A 
two-meter radio.  The speaker coil opened and the only convenient 
replacement I could find had a larger magnet.  The speaker for this 
radio is mounted in the lid.  I replaced the speaker and checked it 
out before re-assembling the radio.  Everything was fine.  Once I 
re-assembled the radio it was deaf.  Figuring I had bumped something 
I opened it up to check.  With the lid off it worked fine.  After a 
few rounds of this same exercise I decided I was going to put the lid 
on with the receiver operating.  What I discovered was that the 
magnet on the speaker came within about 1/8" of one of the I.F. 
cans.  The field from the magnet detuned the I.F. coil.  I finally 
had to order a replacement speaker from ICOM, which solved the 
de-tuning problem because of the smaller magnetic.

What I learned is that, depending on the resolution of your test 
equipment,  yes, very small fields, A.C. or D.C. can impose a tuning 
effect on an inductor with a Ferris type core.  A D.C. field can 
(shift) de-tune a circuit and an A.C. field will modulate it by 
changing the reactance of a Ferris type core inductor.

Burt, K6OQK



>Subject: Re: [time-nuts] OCXO sensitive to gravity
>To: Discussion of precise time and frequency measurement
>-1; format=flowed
>
>Bruce Griffiths wrote:
> > Some caution is in order as some ferrites used in RF transformer coils
> > may be permanently altered by application of a strong magnetic filed.
> > Testing at lower fields first would be safer.
> > Setupo a pair of Helmholtz coils and excite them with low frequency AC
> > and look for associated sidebands in the oscillator output.
> > NB a spectrum analyser is unlikely to be sensitive enough for this.
>
>I was just about to say the same. Also, it has much higher repeatability
>and makes quality measures on measures and counter-measures much easier
>to achieve.
>
>Cheers,
>Magnus

Burt I. Weiner Associates
Broadcast Technical Services
Glendale, California  U.S.A.
biwa at att.net
K6OQK