# Tuned circuits

Discussion in 'General Electronics Chat' started by Ric Pompey, May 27, 2014.

1. ### Ric Pompey Thread Starter New Member

Mar 8, 2012
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Within my profession, we use impedance bonds. One of the purposes of an impedance bond is to couple coded track circuit and CAB energy to the train rails. The coded audio frequency uses frequency multiplexing to carry transmit the audio signal down the rail to a reciever. Eight different frequencies are used and the boundaries of each track circuit is represented by these impedance bonds.
Within each impedance bond are three tuned circuits; CAB, transmit and recieve. The CAB and the transmit tuned circuits are just a capacitor and inductor in parallel, however, the recieve tuned circuit has the capacitor, inductor and a resistor added.
My question is the addition of the resistor there for antiresonance to prevent distructive current build up or to compensate for phase shifting as the signal moves down the rail?

2. ### timwhite Member

Apr 10, 2014
50
7
In an LC circuit such as both the CAB and Transmit circuits you're referring to, a charge oscillates back and forth between the capacitor's electric field (E) and the inductor's magnetic field with very little current decay. When you add in the resistor, you limit the current and begin to see a strong decay over time. The practical use of this in the transmission circuits you describe, I am not 100% sure about. Maybe someone else can offer some insight.

Wiki Source to RLC circuits

I think that with the edition of the Resistor, you can begin to filter different frequencies, and thus act as a receiver. That's probably why, not 100% though.

3. ### Ric Pompey Thread Starter New Member

Mar 8, 2012
7
0
It is true. The addition of too large a resistor will cause the current amplitude to decay, hwever, if the resistance equals two times the square root of inductance divide by the capacitance, the resistance will decrease the maximum size of the currents amplitude but it does notchange the resonant frequency.
All inductors contain amounts of stray capacitance. This can cause resonance problems ( unwanted oscillations). I thought added resistance that fits the above criterion would prevent that.

4. ### #12 Expert

Nov 30, 2010
16,657
7,303
My thoughts on this are that adding a resistor lowers the Q of the circuit so that it is relatively equal in sensitivity to a wider range of frequencies. You wouldn't want that in the transmit circuit. I might be saying, "anti-resonance" in the way you use that word.

5. ### AnalogKid Distinguished Member

Aug 1, 2013
4,679
1,295
#12 is correct. The resistor decreases sensitivity to small signals, but also increases sensitivity to signals that are slightly off-frequency due to component ageing or temperature effects or calibration drift. It is similar to the way some long-range radio links are designed; the transmit antenna is as narrow-beam as possible, but the receive antenna lobes have a bit more spread to account for mis-alignment.

ak

6. ### alfacliff Well-Known Member

Dec 13, 2013
2,449
428
also could be their method of diplexing, putting transmit and recieve signals on the same track without killing the reciever.

7. ### Ric Pompey Thread Starter New Member

Mar 8, 2012
7
0
#12 you are absolutely correct. The circuit will be less selective ( lower "Q" ) but it will have the same natural frequency. This is the one notable exception to the rule of circuit resistance. As far as the comment about diplexing being used to put transmit and recieve signals on the same track, interesting thought. Would that need be required because as signals are transmitted through a conductor they may shift out of phase? Thanks everyone. I believe my main question was answered.