TEA2028 Ver la hoja de datos (PDF) - STMicroelectronics
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TEA2028 Datasheet PDF : 47 Pages
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TEA2028 - TEA2029 APPLICATION NOTE
V.3.2 - Functional duty of individual blocks
V.3.2.1 - Phase comparator
The duty of this comparator is to issue an output
current proportional to the phase difference be-
tween φIN and φOUT.
V.3.2.2 - Low-pass filter
This filter suppresses the parasitic component con-
taining the sum of phases, smoothens the phase
difference component and determines the timing
characteristics of the loop.
V.3.2.3 - VCO centered on 500kHz
This is a voltage-controlled oscillator which gener-
ates an output frequency proportional to the volt-
age applied to its input.
This voltage is delivered by low-pass filter.
V.3.2.4 - Divider stage
It is used to divide the VCO frequency (500kHz) by
32 so that it can be compared with the line sync
signal frequency of 15625Hz.
V.3.3 - Functional description of building blocks
V.3.3.1 - Phase comparator ”φ1”
The comparator is functionally equivalent to a sig-
nal multiplier (see Figure 12).
Let’s assume that :
- iLS = I sin (ωHt + φIN) and Vφ1 = k cos (ωHt + φOUT)
then :
-
i
=
iLS ⋅
2
k
[
sin
(ΦIN
-
ΦOUT)
+
sin (2ωHt
+
ΦIN +
ΦOUT)]
(see Figure 13)
- the low-pass filter will suppress the 2fH frequency
Figure 12
component
- φIN - φOUT difference being low :
sin (φIN - φOUT) ≈ φIN - φOUT
- the output current will be therefore proportionalto
the phase difference between the signals com-
pared.
In other words, the average current over one period
is :
IAV
⋅
TH
=
I
tS
2
+
∆t
-
I
tS
2
−
∆t
=
2I
∆t
IAV
=
2I
∆t
TH
and
∆t
=
∆ΦT2πH
The comparator conversion gain is thus :
A=
i
∆Φ
=
I
π
(in A/rd)
Later in our discussion we shall consider the two
possible values of the current I.
For the time being, let’s define these values as
follows :
- I = 500µA for ”long time constant” or normal
operation
- I = 1.5mA for ”short time constant” VCR mode or
synchronization search (Mute).
The values of A are therefore :
- ALONG = 0.16mA/rd
- ASHORT = 0.47mA/rd
Use of comparator inhibition signal is quite useful
under noisy transmission conditions. It eliminates
risk of incorrect comparison during the line scan-
ning phase which would be due to the noise present
on LS signal. Horizontal phase and image stability
are thus highly enhanced.
Characteristics of this inhibition signal will be dis-
cussed at the end of this chapter.
φOUT
vϕ1 Signal
1.26V
+
VCC
LS = 1
0
tS
TH
VφI
∆t
i
1.26V
I+
iO
I-
2I
Video Recognition
1mA
Mute
≤1
VCR
VCR Mode
Sw i tc hing
i LS
I
500µA
φIN
LS
&
Long ϕ1 inhibition
10/46
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