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Motore di ricerca datesheet componenti elettronici |
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Motore di ricerca datesheet componenti elettronici |
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TEA2028 Scheda tecnica(PDF) 10 Page - STMicroelectronics |
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TEA2028 Scheda tecnica(HTML) 10 Page - STMicroelectronics |
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10 / 47 page  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 ⋅ k 2 [ sin ( ΦIN - ΦOUT) + sin (2ωHt+ ΦIN + ΦOUT)] (see Figure 13) - the low-pass filter will suppress the 2fH frequency component - φIN - φOUT difference being low : sin ( φIN - φOUT) ≈ φIN - φOUT - the output current will be therefore proportional to 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= ∆Φ TH 2 π 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 = 500 µA 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. i V + CC 1.26V i LS I 2I 1mA 500 µA & LS Long ϕ1 inhibition ≤ 1 Mute VCR Video Recognition VCR Mode Switching φ OUT 1.26V v ϕ1 Signal φIN VφI i O LS = 1 T H t S 0 ∆t I+ I- Figure 12 TEA2028 - TEA2029 APPLICATION NOTE 10/46 |
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