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APPLICATION NOTE

STMicroelectronics 

TEA2028 - TEA2029 APPLICATION NOTE

It is therefore deduced that the system can follow

all input phase variations without producing any

static error.

In practice, there will be a slight error due to the

input bias current ”IB” of VCO, which is 0.55µA at

fO = 500kHz. This DC current is delivered by a

phase comparator which will generate a phase

error of :

- long time constant :

∆ΦLONG

=

ALIOBNG=

0.55

⋅

10−3

0.16

= 3.4 ⋅ 10-3 rd or 35ns in ∆t

-

short

time

constant

:

∆ΦSHORT

=

IB

ASHORT

=

12ns

These two errors cause a horizontal picture dis-

placement. On a large screen of 54cm wide, this

will be : 64 - 12 = 52µs, which for both modes

corresponds to a shift of :

∆LINE

=

∆ΦLONG−∆ΦSHORT

52

⋅

520

=

0.24mm

It is obvious that such displacement can be fully

neglected.

Response to a Frequency Step

- The input phase is : ΦIN(t) = ∆ωt

which

as

a

function of

(p)

is

:

ΦIN(p)

=

∆ω

p2

- The accuracy is :

lim (ΦIN −

p−>0

ΦOUT)

= lim

p−>0

p

∆ω

+ ABf(o)

=

∆ω

ABR

where R = 500kΩ at f(o)

In this case, the phase error depends on both, the

magnitude of the frequency step and the static gain

ABR.

In

general,

∆f

∆f

which

is

the

open-loop

static

gain,

is

taken into consideration.

∆ω

∆Φ

=

ABR

=

2π∆f

∆t × 2π

=

A

⋅

2

π

⋅

B’

⋅

R

⇒

∆f

∆t

=

AB’R

⋅

2π

TH

(B’ in

kHz/V)

- In normal mode : ALONG = 0.16 mA/rd

⇒

∆f

∆t

=

5.5kHz/µs,

R

= 500kΩ

- In VCR mode : ASHORT = 0.47 mA/rd

⇒

∆f

∆t

=

16.5kHz/µs

Note : The capture range is specified within

± 500Hz with respect to 15625Hz.

Numerical Example

Let’s suppose that in VCR mode there is a fre-

quency variation of ± 100Hz, this will yield a phase

variation of 0.1/16.5, i.e. ± 6ns which, on a 54cm

wide screen, will produce a horizontal shift of

∆LINE = ± 0.06mm !

It is obvious that an excellent image stability is thus

obtained.

V.3.6.2 - Dynamic study

The loop response in transient mode is quite im-

portant. It determines the overall system stability

and the phase recovery time, which are imposed

by the external filter ”f(p)”.

The close-loop transfer function is equivalent to a

second order system. These time constants are in

practice displayed on screen by a bar delivered by

a special pattern generator representing the phase

errors.

The following optimizedresults were obtained from

filter f(p) connected to Pin 22.

Filter component values are :

R1 = 4.7kΩ, C1 = 2.2µF, C = 10nF

A. LONG TIME CONSTANT

- At ∆t of 4µs ⇒ N=18 lines, i.e. τLONG = 1.15ms.

System oscillations are perfectly damped. Image

stability with a noisy video signal is very satisfac-

tory.

B. SHORT TIME CONSTANT

- At ∆t = 4µs ⇒ N = 5 lines, i.e. τSHORT = 0.32ms

- n = 5 lines

One should notice fast phase recovery, naturally

followed by bounced oscillations due to the char-

acteristics of a second order device.

As given in application diagram section 6, an other

alternative would be to use the following compo-

nent values : R1 = 3.9kΩ, C1 = 4.7µF, C = 15nF.

16/46

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