TDA7326 Datasheet, PDF (6/16 Pages) STMicroelectronics – AM-FM RADIO FREQUENCY SYNTHESIZER

TDA7326
3.3 DIVIDER FROM V_CO FREQUENCY TO
REFERENCE FREQUENCY
This divider provides a low frequency fSYN which
is phase compared with the reference frequency
f_REF.
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.
MODE SECTION
STAND-BY
FM
AM SWALLOW
AM DIRECT
SWM/DIR
0
1
0
1
AM/FM
0
0
1
1
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”.
Figure 2: FM and AM (SW) operation (swallow mode)
Dividing range calculation :
For FM mode with 12.5kHz reference fre-
quency and SW swallow mode operation :
f_VCO = [ 65 ⋅ A₁ + (B₁ + 1 - A₁) ⋅ 64 ] . f_REF or
f_VCO = (64 ⋅ B₁ + A₁ + 64) ⋅ f_REF
Important : For correct operation B ≥ 64 and B
≥ A.
At FM mode with 25kHz reference frequency :
fVCO = [ 33 ⋅ A2 + (B2 + 1 - A2) ⋅ 32 ] ⋅ fREF
f_VCO = (32 ⋅ B₂ + A₂ + 32) ⋅ f_REF
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
=
A₁
2
and
A₂
=
(A₁ +
2
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:
f_VCO = (C + 1) ⋅ f_REF
OSC IN
PREDIVIDER
R
fref
PD
fsyn
6/16
AM IN
FM IN
REGISTER
SC5 .. SC0
COUNTER
A
PREDIVIDER
M/M+1
REGISTER
PC7 .. PC0
COUNTER
B
D94AU101