TDA7326 datasheet(6/16 Pages) STMICROELECTRONICS

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3.3 DIVIDER FROM VCO FREQUENCY TO

REFERENCE FREQUENCY

This divider provides a low frequency fSYN which

is phase compared with the reference frequency

fREF.

3.4 OPERATING MODE

Four operating modes are available:

- FM mode,

- AM swallow mode,

- AM direct mode,

- Standby mode

They are user programmable with the SWR/DIR

and AM/FM bits in the byte 2.

Standby mode: all functions are stopped. This al-

lows low current consumption without lost of infor-

mation in all register, it is activated by forcing bit 0

(AM/FM) and bit 1 (SWM/DIR) both at zero value.

3.4.1 FM and AM (SW) Operation (Swallow

Mode)

The FM or AM signal is applied to a four

modulus: 32/33/64/65 high speed prescaler,

which is controlled by a 6 bit divider ’A’.This

divider is controlled by the 6 bit SC register.

In parallel the output of the prescaler is con-

nected to a 8 bit divider ’B’. This divider is

controlled by the 8 bit PC register. For FM

mode with 25kHz reference frequency opera-

tion, the divider A is a 5 bit divider. The high

speed prescaler is working in : 32/33 dividing

mode. Bit 6 of the SC register has to be kept

to ”0”.

Dividing range calculation :

For FM mode with 12.5kHz reference fre-

quency and SW swallow mode operation :

fVCO =[ 65

⋅ A1 +(B1 +1 - A1) ⋅ 64 ] . fREF or

Important : For correct operation B

≥ 64 and B

≥ A.

At FM mode with 25kHz reference frequency :

fVCO =[ 33

⋅ A2 +(B2 +1 - A2) ⋅ 32 ] ⋅ fREF

Important: For correct operation B

≥ 32 and B

≥ A.

A and B are variable values of the dividers.

To keep the actual tuning frequency after a

modification of the reference frequency, the

values of the dividers have to be modified in

the following way.

Switching from 25kHz to 12.5kHz reference

frequency : B1 =B2,A1 =A2

⋅ 2

Switching from 12.5kHz to 25kHz reference

frequency:

B2 =B1,A2 =

and A2 =

(A1 + 1)

for odd values A1.

The AM signal is directly applied to the 14 bit

static divider ’C’. This divider is controlled by

both SC and PC registers.

Dividing range:

PREDIVIDER

OSC IN

SC5 .. SC0

COUNTER

PREDIVIDER

M/M+1

AM IN

FM IN

PC7 .. PC0

COUNTER

fref

fsyn

D94AU101

Figure 2: FM and AM (SW) operation (swallow mode)

MODE SECTION

SWM/DIR

AM/FM

STAND-BY

AM SWALLOW

AM DIRECT

fVCO = (64

⋅ B1 +A1 + 64) ⋅ fREF

fVCO = (32

⋅ B2 +A2 + 32) ⋅ fREF

fVCO =(C + 1)

⋅ fREF

TDA7326

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