[R-390] Theory Question - RF Transformer Alignment

[Dan Merz]

 Mike,  it seems you answered your own question. But not in detail. Your
adjustments chase after the right amounts of inductance and capacitance
until you find that magic combo where both are right at the two frequencies.
The reason you don't miss the combination that is right for both is because
you make adjustments that correct any initial deviancy of inductance and
capacitance toward the right values and never away from the right values,
and hence towards the right values in seqential adjustments.  This is how it
must be if the final point is found.  And the core/coil has to be designed
so that such a point actually exists.

Others have suggested why L is adjusted at low freq and C at high freq but
haven't pointed out one difference that moveable core tuning causes  so I'll
offer my limited insight.  

I've convinced myself that the cam does not enter into why the adjustment
works.  Instead the change of inductance when the core is moved relative to
the bracket does affect the success in finding the magic position.  Once the
cam is set,  it simply brings the bracket back and forth to the same two
positions during the adjustment when you go from low to high frequency (i.e.
the bracket is tied to the digital dial readout via the cam.  But how the
inductance varies with core position at these two points is very important
in understanding why the magic point ends up being found.   In fact, you can
easily show by calculation that if adjusting the inductance at the lower
freq. simply shifted the inductance at the higher freq by the same amount,
the sequence of your adjustments would lead you away from having the two
frequencies adjusted at the same time.   In fact,  you would have to adjust
inductance at the higher freq and capacitance at the lower freq (just the
opposite of the recommended method)  to have the tuning converge on
simultaneous tuning for both points if the inductance span stayed fixed.  By
working with a spreadsheet experiment,  I convinced myself that sequences of
adjustments only converge to the magic point if the change in inductance at
the higher freq. bracket position is much less than the inductance change at
the low freq. brack position.  That is if you tweak the inductance at the
low freq by moving the core in the bracket and then move the bracket to the
high freq position,  the inductance change there has to be less than the
inductance change at the low position frequency.  How much less does it have
to be.  My conclusion was that it has to be less by a factor of the square
of the ratio of the two frequencies.  Otherwise sequential adjustments
result in divergence away from the magic point.  In other terms,  this means
if you plotted the inductance versus core position,  the slope would be much
steeper at the low freq end than it is at the high freq end.    Perhaps
someone else has more specific info on this.  I was surprised when a simple
spreadsheet calculation lead me to my conclusion that if a change in core
position changed inductance by the same amount at low and high positions,
the magic point would not be found and sequential adjustments would lead one
away from that point. This is different from the usual case in radios that
tune across the tuning range with a variable capacitor rather than a
moveable core inductance.  I'm guessing the inductance versus core position
was designed to be flatter at high frequency than at low frequency.
Intuitively it would seem that if you shoved a core through a coil the
inductance would increase more and more as the coil was entered (the curve
would become steeper) and then would flatten out as the core filled the coil
and finally decrease again as the core was shoved thru.  I'm guessing an
appropriate segment of this overall curve could be picked with an
appropriate shape that meets the above condition for successful adjustment.
I notice the manual adjustment procedure advises alternate positions be
selected within the the recommended pair if the recommended pair are not
used.  I don't know if this relates to inductance/core position curves or to
some other consideration.
Perhaps someone has data on inductance versus core position for a typical
tuning unit in the 390a or 390. 

 Dan.

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of Mike Hardie
Sent: Sunday, September 14, 2008 12:37 PM
To: R-390 at mailman.qth.net
Subject: [R-390] Theory Question - RF Transformer Alignment

I'm not having any difficulty performing the alignment, just don't
understand what's happening.

When peaking the RF transformers the technique is to tune to a "lower" 
frequency, adjust the slug, then tune to a "higher" frequency, adjust the
trim cap, and repeat as necessary.  I can understand why the two adjustments
are inter-related, can anyone explain how it is that eventually both the
lower and the higher frequencies are peaked?

In other words the slug adjustment seems to affect the lower frequency more
than the higher frequency, and the trim cap the other way around.  If this
wasn't the case the transformer couldn't be peaked for the lower and higher
frequencies, every time one was adjusted the other would "move" too far.

Mike VE7MMH 

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