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Old 19th Mar 2018, 8:47 pm   #1
VHF_BUILDER
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Default Tapped Capacitor Impedance Matching....

Hi guys, does anyone know a simplified formula for calculating the capacitors of a 50 ohm tapped tuned circuit?
I spent most of yesterday with a scientific calculator and reading several papers on the subject and yet I am no further forward.
See my sketch below:

Cheers

Rob.
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Old 20th Mar 2018, 10:42 am   #2
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Default Re: Tapped Capacitor Impedance Matching....

That seems very complex - what is the objective?
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Old 20th Mar 2018, 11:56 am   #3
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Default Re: Tapped Capacitor Impedance Matching....

1) decide what Q you want in the tuned circuit to be loaded to, allow for unloaded Q
2) calculate the inductor reactance at the centre frequency
3) multiply by Q

This is the resistive component you want to transform 50 Ohms up to

4) You can use the Star-Delta transform to calculate values and you get a net shunt capacitance, so you need to subtract this from your resonator capacitor.

5) But it's a lot easier to just use a Smith chart calculator like 'LL Smith' to do the calculations.

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Old 20th Mar 2018, 12:50 pm   #4
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Default Re: Tapped Capacitor Impedance Matching....

As a first approximation you can assume that the capacitances affect impedance in the same way as transformer turns. So equal values means about a quarter impedance, and the inductor is tuned by the capacitors in series.

The long way is to do series-parallel impedance transformations and tot these up in series or parallel as appropriate. Alternatively, use the formulas for this and do some algebra to arrive at a design equation for the particular circuit you are considering. I managed to design an FM tuner using nothing more complex than a calculator and lots of paper.
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Old 20th Mar 2018, 3:52 pm   #5
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Default Re: Tapped Capacitor Impedance Matching....

Quote:
Originally Posted by VHF_BUILDER View Post
Hi guys, does anyone know a simplified formula for calculating the capacitors of a 50 ohm tapped tuned circuit?
Hey Rob,

What's the big picture here? The 1pF capacitor alone has a reactance of 1600R at 100MHz, for example.

What is the resonant frequency of the two tank circuits? Are the capacitors voltage dividers or is the capacitor in each pair the same value?
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Old 20th Mar 2018, 6:51 pm   #6
VHF_BUILDER
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Default Re: Tapped Capacitor Impedance Matching....

Thanks for the reply guys, basically I have re-desinged the front end for my 70MHz SSB transceiver and went off down the rabbit hole of tangents and over thinking.
So the attached circuit is a representation of a 70MHz bandpass filter, the 220nH are Toko MC120 types and those will form the adjustable element of the front end tuned circuits.
I have simulated this in LTSpice with some known values from tinkering but this is guess work and I would like to understand the how and why.
So, once I get home this evening I will get the calculator, pen and paper out and try working through the above suggestions and report back.

Cheers

Rob.

Last edited by VHF_BUILDER; 20th Mar 2018 at 6:52 pm. Reason: Typo
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