# Resonant Circuit Output Point?

#### SamR

Joined Mar 19, 2019
3,614
I've been studying resonance and as a general question as to applying them, where is the output point? I've shown here as antenna input but it could be any input signal. Where would be the proper take-off point to say an amplifier? Also what are the merits/disadvantages between series and parallel?

#### SamR

Joined Mar 19, 2019
3,614
Actually it is commonly used to "match" an Antenna to a xcvr with a variable cap and roller or multi-tapped inductor. But where is the signal output point on the tuner? I assume it is somewhere on inductor?

#### MrAl

Joined Jun 17, 2014
8,556
I've been studying resonance and as a general question as to applying them, where is the output point? I've shown here as antenna input but it could be any input signal. Where would be the proper take-off point to say an amplifier? Also what are the merits/disadvantages between series and parallel?

View attachment 186820
Hi,

It really depends on what you are using the filter for.
A few examples for the series string would be:
1. across the cap
2. across the resistor
3. actuall the current is the output
4. across all three
You might ask how can it be across all three if that is the input.
The input could be from a network with some impedance itself so the series string will cause some voltage change at the top.

So really you have to have the entire circuit and it's application to tell where the output is.

#### SamR

Joined Mar 19, 2019
3,614
Thanks Al. I looked at several "Antenna Tuner" circuits and for the parallel tank circuit, it seems that the output point can be at any node point for the circuit. I also see many RLC variations of PI, L, etc. used for frequency input schemes. Some with a single variable cap and some with multiple and variable cap far more prevalent than variable inductors. So it does seem to not be a simple answer that I was looking for.

Sam

#### crutschow

Joined Mar 14, 2008
28,204
A parallel tank develops a high voltage across the inductor at resonance, and the series tank develops a low voltage, so it rather depends upon what you want the tank signal to do in the rest of the circuit.

Edit: Correction. In a series circuit, the low voltage appears across the inductor-capacitor, not the inductor.
The inductor will still have a high voltage.

Last edited:

#### ci139

Joined Jul 11, 2016
1,696
( /!\ i can't be too much trusted in this particular matter /!\)

for your first Fg. it's likely tapped near C ( FM Receiver with one Transistor )

for the 2-nd there's likely coupling TF used "below" LC-tank . . . (can't find such example)
. . . ?? to enable "free" ~AC path to antenna and "blocked" to GND . . . but i don't remember the exact schematic for nor the frequency range involved ...
___________

?? ↓ J1 ↓ . . .

#### MrAl

Joined Jun 17, 2014
8,556
Thanks Al. I looked at several "Antenna Tuner" circuits and for the parallel tank circuit, it seems that the output point can be at any node point for the circuit. I also see many RLC variations of PI, L, etc. used for frequency input schemes. Some with a single variable cap and some with multiple and variable cap far more prevalent than variable inductors. So it does seem to not be a simple answer that I was looking for.

Sam
Hi,

Well you will gain experience as you go and be able to spot these things right off most of the time as seeing the whole scheme helps a great deal. Keep asking here and you'll gain a lot of experience

#### Janis59

Joined Aug 21, 2017
1,357
A parallel tank develops a high voltage across the inductor at resonance]

---> Just contrary, sorry, the serial tank makes kiloVolts. Im using it everyday and know for 1000% sure. Parallel generates kiloAmperes instead.

#### MrAl

Joined Jun 17, 2014
8,556
Hi,

Yes i am sure he meant series.

A series circuit of RLC with R=0.1, L=1, C=1, excited with a 1vac source at frequency w=1, the inductor voltage shows a peak of 10v at that frequency. So in this case the voltage across the inductor went up by 10 times.

A parallel RLC shows an inductor voltage of 1vac when excited by 1vac.

#### SLK001

Joined Nov 29, 2011
1,548
With a resonant circuit, almost everything you do to it detunes it. To couple to an amplifier, you need to lightly couple energy out of the resonant structure to your amplifier. This can be done with a small loop of wire near the inductor (or a tap near the low voltage end of the inductor) , or a small value capacitor.

#### Picbuster

Joined Dec 2, 2013
1,027
I've been studying resonance and as a general question as to applying them, where is the output point? I've shown here as antenna input but it could be any input signal. Where would be the proper take-off point to say an amplifier? Also what are the merits/disadvantages between series and parallel?

View attachment 186820
The answer is simple the ' output' is a point that has a minimum effect on the resonance.

Picbuster

#### MrAl

Joined Jun 17, 2014
8,556
The answer is simple the ' output' is a point that has a minimum effect on the resonance.

Picbuster
Hi,,

Sorry to disagree but it depends on the design.
That is, circuits are designed based on known configurations which dont pay much attention to frequency change, but do take into account loading factors.

#### Picbuster

Joined Dec 2, 2013
1,027
Hi,,

Sorry to disagree but it depends on the design.
That is, circuits are designed based on known configurations which dont pay much attention to frequency change, but do take into account loading factors.
Yes however; SamR is talking about antennae the first step is to 'amplify' the signal. (achieve a maximal Q for that frequency)
A normal antennae stage is using a coil coupling with a capacitor to create the second resonance fed into the mixer.
But yes it's all open to the designer and lots of variations are used.

Picbuster