EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Veramacor on July 27, 2015, 01:36:14 am
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Hello,
Attached I have a 40 meter band pass filter for ham radio - its placed right after the antenna.
1) Does anyone recognize this filter?
2) Can C8 and C13 be combined into one equivalent Capacitor?
I know Capacitors in parallel are Cs = C1 + C2
Capacitors in series are 1/Cs = 1/C1 + 1/C2
C8 and C13 are configured much like a Voltage Divider. I'm just not sure if combining these two caps is valid?
Thank you all!
Circuit Author's Note:
"As you can see on the schematic above it is made of two resonant circuits: first one being C13, C8, L2 and the second one L3 C12. First one is also an impedance transformer that rises the impedance from antenna’s 50 ohm to about 7k. This is done to achieve better frequency response (narrower filter) by the second resonant circuit."
My ps question is how does C13 C8 and L2 act like an impedance transformer? By the way this circuit avoids using toroids - this is why I'm chosing to work with this particular design.
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2) Can C8 and C13 be combined into one equivalent Capacitor?
Nope.
My ps question is how does C13 C8 and L2 act like an impedance transformer? By the way this circuit avoids using toroids - this is why I'm chosing to work with this particular design.
Fancy answer: Complex Conjugate matching.
See the wiki article here:
https://en.wikipedia.org/wiki/Impedance_matching
And if you can get your hands on it, read 'RF Circuit Design' by Chris Bowick.
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There's no source or load resistor, so it cannot be said to be a filter as such. It's just a bunch of reactances sticking around. There's no source or load to define a filter coefficient on.
Whenever you're doing something with RF (and in fact, this generalizes to everything, not just RF), you must have loss resistance, somewhere. Loss means work performed. Usually as heat. But it also means work generated, which is why sources always have resistance. It also means the potential for more work being done, as an amplifier has input and output impedances: the amplifier's input needs to be matched, in order to maximize gain, and therefore available work at its output!
Also, since this appears to be LTspice, beware of the default loss values given (which are not shown on the schematic by default either :( ), which may be completely unsuitable to your circuit.
Tim
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Attached I have a 40 meter band pass filter for ham radio - its placed right after the antenna.
I recognise the circuit and the component values as this filter was discussed here about a month ago.
https://www.eevblog.com/forum/beginners/ham-radio-input-filter-question/msg700699/#msg700699 (https://www.eevblog.com/forum/beginners/ham-radio-input-filter-question/msg700699/#msg700699)
It looks to me like it is designed for a 50R input impedance and a 1500R output impedance to suit an NE602 or SA602 mixer. It's also a 20m filter and not 40m so I think you made a typo in your first post :)