5

SL1466

The SL1466 is normally used as a type 2 second order loop

and can be represented by the above diagram. for such a

system the following loop parameters apply.

T

1

= C1 R1

T

2

= C1 R2

and

T

1

= K

o

K

D

/

ω

n

2

T

2

=2

ξ/ω

n

where:

K

O

is the VCO gain in radians seconds per volt

K

D

is the phase detector gain in volts per radian

ω

n

is the natural loop bandwidth

ξ is the loop damping factor

From these factors the loop 3dB bandwidth can be

determined from the following expression;

which approximates to

ω

3dB

= 2

ξω

n

, when

ξ >>1

N.B. VCO gain within the PLL is three times higher than at the

video drive pin due to gain in the output stage.

NOTE: R1 is the loop amplifier input resistor. R2 and C2 are

the generic designators for the loop components R7-R9 and

C9, C14 on the circuit diagram.

AGC FACILITY

A sophisticated two stage level detect circuit has been

provided which will control both internal IF AGC and external

tuner AGC amplifiers in order to maintain a fixed level to the

input of the phase detector of around –20dBm for optimum

threshold performance. The internal AGC amplifier provides

50dB of gain adjust and the external AGC control provides for

15dB of gain adjust, thus covering 65dB of dynamic range at

the tuner input.

The RF output current RF AGC CONTROL can be

converted to a positive gradient control voltage by an external

resistor.

AFC FACILITY

The SL1466 contains a digital frequency error detect circuit,

which generates an output consisting of two logic flags,

DIGFHI and DIGFLO, dependant on whether the LO

frequency is above or below the input frequency. These flags

have an overlap region where both are high; this is equivalent

to the deadband window.

The function of the AFC outputs is shown in Fig. 7 and the

accompanying Table.

3dB

n

n

Fig. 4 RF AGC control current and AGC time cinstant voltage vs Input power

AGC TIME

CONSTANT (V)

-10

400

350

300

250

200

150

100

50

0

-90

-70

-50

-30

RFCONT

TC

Pin dBm

RFC AGC CONTROL CURRENT (

µA)

4V

3V

2V

1V