I didn't have the time to complete my last post since towards the end of it I had a shout for food (Must get one's priorities right! 
).
So, to resume . . . .
A closer look at your design requirement.
You have a 225 pF variable with an appropriate law (You may have an SLF or an SLW: the difference in practice is relatively small: the the SLF gives a curve with a slightly greater curvature, since it follows a cubic law; the SLW follows a square law. This is illustrated in Terman's article, to which I referred earlier.) You require 150 pF: a quick bit of arithmetic shows that a series padder of 450 pF is required.
I used a 520 pF SLW for my measurements, which is different to your available 225 pF, but my measurements & conclusions still have a validity for your design need, nevertheless: I'll explain . . .
My second graph shows how the the curves deviate from a straight line as Cp is varied; there is also a table shown of the ratio [total C / Cmax] for various values of Cp. {Cmax = the maximum value of the variable capacitor; total C = the total effective capacitance.} The closer the ratio [total C / Cmax] is to unity, the closer the law is to a straight line. [It is that ratio which is important here, so the absolute value of the variable capacitor is not that relevant - hence my earlier comment]. For your Cmax = 225 pF and total C = 150 pF, that ratio = 0.67, and looking at graph #2 and interpolating for the corresponding curve, we can see that yes, there is a deviation from the required straight line, but that it is not a dramatic deviation - so I would expect that the resultant dial / freq. characteristic will produce a degree of linearity that should be quite adequate for your need. If nothing else, it indicates that building a prototype using those component values should not be a complete waste of time - and, moreover, I would expect that it would approach quite closely to your identified need.
Having said all that, if, on test, the degree of linearity is indeed inadequate, there is (of course) always the last (and debatably dramatic) recourse of removing vanes from the 225 pF cap. to make it into a 150 pF. (I myself have had to do such a 'modification' on more than one occasion). The necessary calculation (how many vanes to remove) should be relatively straight-forward, but the 'prune-and-test' approach with the aid of a reliable and calibrated capacitance meter still has a lot to be said for it. In either case, it should not be necessary to change the value of the corresponding inductance - which, quite understandably, you are loath so to do.
All of this has reminded me of an HRO which I did a complete electrical re-design job on as a project many years ago (with apologies to HRO aficionados
). Since I chose my own mixer and local oscillator circuits, it was necessary to re-calculate the inductances, trimming and padding capacitors for all the mixer & L.O. ccts. in all the coil packs. All in all, quite a task - but it was eventually successful - and also very enlightening as a design problem: the Radio Designer's Handbook (Langford-Smith) was a useful aid.
Al. / April 10, 2013 //
).So, to resume . . . .
A closer look at your design requirement.
You have a 225 pF variable with an appropriate law (You may have an SLF or an SLW: the difference in practice is relatively small: the the SLF gives a curve with a slightly greater curvature, since it follows a cubic law; the SLW follows a square law. This is illustrated in Terman's article, to which I referred earlier.) You require 150 pF: a quick bit of arithmetic shows that a series padder of 450 pF is required.
I used a 520 pF SLW for my measurements, which is different to your available 225 pF, but my measurements & conclusions still have a validity for your design need, nevertheless: I'll explain . . .
My second graph shows how the the curves deviate from a straight line as Cp is varied; there is also a table shown of the ratio [total C / Cmax] for various values of Cp. {Cmax = the maximum value of the variable capacitor; total C = the total effective capacitance.} The closer the ratio [total C / Cmax] is to unity, the closer the law is to a straight line. [It is that ratio which is important here, so the absolute value of the variable capacitor is not that relevant - hence my earlier comment]. For your Cmax = 225 pF and total C = 150 pF, that ratio = 0.67, and looking at graph #2 and interpolating for the corresponding curve, we can see that yes, there is a deviation from the required straight line, but that it is not a dramatic deviation - so I would expect that the resultant dial / freq. characteristic will produce a degree of linearity that should be quite adequate for your need. If nothing else, it indicates that building a prototype using those component values should not be a complete waste of time - and, moreover, I would expect that it would approach quite closely to your identified need.
Having said all that, if, on test, the degree of linearity is indeed inadequate, there is (of course) always the last (and debatably dramatic) recourse of removing vanes from the 225 pF cap. to make it into a 150 pF. (I myself have had to do such a 'modification' on more than one occasion). The necessary calculation (how many vanes to remove) should be relatively straight-forward, but the 'prune-and-test' approach with the aid of a reliable and calibrated capacitance meter still has a lot to be said for it. In either case, it should not be necessary to change the value of the corresponding inductance - which, quite understandably, you are loath so to do.
All of this has reminded me of an HRO which I did a complete electrical re-design job on as a project many years ago (with apologies to HRO aficionados
). Since I chose my own mixer and local oscillator circuits, it was necessary to re-calculate the inductances, trimming and padding capacitors for all the mixer & L.O. ccts. in all the coil packs. All in all, quite a task - but it was eventually successful - and also very enlightening as a design problem: the Radio Designer's Handbook (Langford-Smith) was a useful aid.Al. / April 10, 2013 //
