4. 2 Decoupling devices

The decoupling device is an eight-pole circuit with different attenuation between the poles. The transmission direction has small attenuation, and a direction of a decoupling (inhibition = protection) - high attenuation. The next conditions are required for the normal signal transmission and absence of generation (the Ideal TS):

1. Absence of attenuation in a transmission direction (А₁₂=А₄₁=0 dB).

2. Infinite attenuation in a decoupling direction (А₄₂=А₁₃ =∞ dB).

3. The input resistance balance with the load resistance.

The TS is called as a reversible if the next conditions are satisfied: А₁₂=А₂₁; А₄₁=А₁₄, in other way – they are irreversible.

Terminating set (Hybrid system) is used when parallel operating of filers, at the input and output circuits of amplifiers, VBC construction, pilot frequency inputting, using of duplex amplifier, for transition from 2w circuit to 4w and back when using the same frequency band for transmission and reception.

Figure 4.13

The transformer type of TS is widely used. Its circuit.

Figure 4.14

So the TS represents the differential transformer, to the 1-1 jacks of which the 2w line circuit is connected, to the 3-3 jacks – the circuit with the resistance Z_bc that is balancing it. The 2-2 jacks are connected to the 4w transmission path of the channel, and the 4-4 jacks – to the receiving path. The input resistances should be chosen according to the next conditions in order to provide the decoupling:

Z₃ = Z_bc = m Z₁

Z₄ = m Z₁/(m+1)

Z₂ = (m+1) Z₁/ n²

The transformation coefficient:

The equal-arm coefficient:

The TS is unequal-arm if m ≠ 1, and the TS is equal-arm if m=1.

I f the half-windings have the same quantity of transformer coils (w1’’ = w1’) and the resistances (Z₁=Z_bc) are equal a balanced bridge circuit is formed. When there is a complete balance circuit i₁ = i_bc. As the equal but oppositely directed currents flow through half-windings, the equal but oppositely directed magnetic fluxes appear. The total magnetic flux equals to zero, therefore electromotive force and current i₂ are absent at the output of the circuit. In this case the decoupling device is balanced and the А₄₂ =∞ and the feedback currents are absent.

Figure 4.15

In other case signals from the 4-4 pole will be reflected and sent to the pole 2-2 (the input resistance ).

Attenuation in a 4-2 direction is equal to:

.

The reflected attenuation is equal to:

.

At TS is balanced and . The balance attenuation is equal to

So . TS is balanced if А₄₂=А₁₃ =∞ dB and TS is unbalanced if А₄₂ ≠ dB.

The transmission direction attenuations:

T he unbalance coefficient equals to:

The circuit of the resistor type decoupling device.

Figure 4.16

5 Transmission channels.

5.1 The main parameters of vbc

Channel – is the hookup of hardware, software and the propagation medium that provides the signals transmission from source to receiver.

Transmission channels in MTS SDC can be subdivided into:

1. VBCs (0.3-3.4 kHz)

2. broadband channels

• SubG 12…24 kHz

• PG 60…108 kHz

• SG 312…552 kHz

• MG 812…2044 kHz

3. Sound broadcasting channels

• 1^st rank 0.05…6.5 kHz

• 2^nd rank 0.05…10 kHz

VBC is widely used, so let’s observe it.

The simplified diagram of VBC is shown below.

Figure 5.1

TY - TE – trough-line extender (adds 3.5 dB attenuation)

(СЛ) – trunk

(АЛ) – SL – subscriber line

(ATC) – AE – automatic exchange

DC - decoupler

The main parameters of VBC are:

• The residual attenuation deviation from requirements (AFCh)

• Group delay time characteristic (Phase-frequency characteristic)

• Amplitude characteristic

We can determine that residual attenuation of VBC equals to (from the previous picture):

A_res = p_in­ – p_out = –13 – 4 = – 17 dB

In practice the frequency dependence is normalized by the residual attenuation deviation from the requirements but not by the residual attenuation.

ΔA_res (f) = A_res (f) – A_res (800Hz)

P_out (f) = p_in – A_res (f)

P_out (f) = p_in – (A_res (800Hz) + ΔA_res (f))

Norm: residual attenuation at 800 Hz frequency is – 17 dB.

Residual attenuation at different frequencies is given by the following graph (template):

f, kHz	0.3…0.4	0.4…0.6	0.6…2.4	2.4…3.0	3.0…3.4
ΔAres (f), dB	1.4	0.72	0.6	0.72	1.4

f, kHz

dB

Figure 5.2

Speech intelligibility – upper limitation

Possibility of selfgeneration in the channel – lower limitation

Bandwidth of boundary frequencies of which (0.2 and 3.4 kHz) the residual attenuation is 8.7 dB higher than the residual attenuation at 800 Hz frequency is called efficiently transmitted frequency bandwidth. Phase frequency characteristic is taken as the group time delay for convenience:

τ_gr = – dφ(ω) / dω

There is a template for its normalization. Amplitude characteristic is normalized by the following way: when the measuring level is changed from -17.5 dB to +3.5 dB in the point of zero measuring level the residual attenuation deviation shouldn’t exceed 0.3 dB.

When measuring level is increased to 8.7 dB and 20 dB, ΔA_res shouldn’t exceed 1.75 and 7.8 dB respectively.