ST7538
■ Carrier/Preamble Detection
The Carrier/Preamble Block is a digital Frequency detector Circuit.
It can be used to manage the MAINS access and to detect an incoming signal.
Two are the possible setting:
- Carrier Detection
- Preamble Detection
CARRIER DETECTION: The Carrier/Preamble detection Block notifies to the host controller the
presence of a Carrier when it detects on the RAI Input a signal with an harmonic component close to
the programmed Carrier Frequency. The CD_PD signal sensitivity is identical to the data reception
sensitivity (1mVrms Typ. in Normal Sensitivity Mode).
The CD_PD line is forced to a logic level low when a Carrier is detected.
PREAMBLE DETECTION: The Carrier/Preamble detection Block notifies to the host controller the
presence of a Carrier modulated at the Programmed Baud Rate for at least 4 Consecutive Symbols
(“1010” or “0101” are the symbols sequences detected).
CD_PD line is forced low till a Carrier signal is detected and PLL is in the lock-in range.
To reinforce the effectiveness of the information given by CD_PD Block, a digital filtering is applied on
Carrier or Preamble notification signal (See Control Register Paragraph). The Detection Time Bits in the
Control Register define the filter performance. Increasing the Detection Time reduced the false
notifications caused by noise on main line. The Digital filter adds a delay to CD_PD notification equal to
the programmed Detection Time. When the carrier frequency disappears, CD_PD line is held low for a
period equal to the detection time and then forced high.
Figure 10. CD_PD Timing during RX
T
T
CD
DCD
CD_PD
RAI
D03IN1418
Figure 11. Receiving Path Block Diagram
RXFO
31
Bit 3,4
Bit 3,4
Bit 0-2
Bit 23
MIXER
3
8
32
AGC
RAI
RxD
Bit 3,4 &14-21
PLL
Low Pass
Band Pass
IF FILTER
Band Pass
Band Pass
CHANNEL
FILTER
DIGITAL
FILTER
FSK
DEMODULAOR
PRE-FILTER
CLR/T
LOCAL
OSC
GAIN
CONTROL
Bit 9 & 10
Bit 12 & 13
Bit 0 -2
CARRIER/
PREAMBLE
DETECTION
1
9
Carrier Detection
CD_PD
BU
Low Pass
BAND
IN
USE
D03IN1419
16/30
STMICROELECTRONICS [ ST ]
