AMIS-49587_15 Datasheet, PDF (23/56 Pages) ON Semiconductor – Power Line Carrier Modem

AMIS−49587
6.2 TRANSMITTER PATH DESCRIPTION (S−FSK
MODULATOR)
For the generation of the space and mark frequencies, the
direct digital synthesis (DDS) of the sine wave signals is
performed under the control of the microprocessor. After a
signal conditioning step, a digital to analog conversion is
performed. As for the receive path, a sigma delta modulation
technique is used. In the analog domain, the signal is low
pass filtered, in order to remove the high frequency
quantization noise, and passed to the automatic level
controller (ACL) block, where the level of the transmitted
signal can be adjusted. The determination of the signal level
is done through the sense circuitry.
TX_EN
ALC_IN
TX_OUT
Transmitter (S−FSK Modulator)
ALC
control
LP
Filter
D/A
Transmit Data
& Sine Synthesizer
ARM
Interface
&
Control
fMI f MQ fSI fSQ
TO RECEIVER
Figure 17. Transmitter Block Diagram
6.2.1 ARM Interface and Control
The interface with the ARM consists in a 8−bit data
registers R_TX_DATA, 2 control registers R_TX_CTRL
and R_ALC_CTRL, a flag TX_RXB defining transmit and
receive and 2 16−bit wide frequency step registers R_FM
and R_FS defining f_M (mark frequency = data 1) and f_S
(space frequency = data 0). All these registers are memory
mapped. Some of them are for internal use only and cannot
be accessed by the user.
The processing of the physical frame (preamble, MAC
address, CRC) is done by the ARM.
6.2.2 Sine Wave Generator
A sine wave is generated with a direct digital synthesizer
DDS. The synthesizer generates in transmission mode a sine
wave either for the space frequency (f_S, data 0) or for the
mark frequency (f_M, data1). In reception the synthesizer
generates the sine and cosine waves for the mixing process,
f_SI, f_SQ, f_MI, f_MQ (space and mark signals in phase and
quadrature). The space and mark frequencies are defined in
an individual step 16 bit wide register.
Table 24. FS AND FM STEP REGISTERS
ARM
Register
Hard
Reset
Soft
Reset
Description
R_FS[15:0]
0000h
0000h
Step register for the
space frequency f_S
R_FM[15:0]
0000h
0000h
Step register for the
mark frequency f_M
The space and mark frequency can be calculated as:
• f_S = R_FS[15:0]_dec x f_DDS/2¹⁸
• f_M = R_FM[15:0]_dec x f_DDS/2¹⁸
Or the content of both R_FS[15:0] and R_FM[15:0] are
defined as:
• R_FS[15:0]_dec = Round(2¹⁸ x f_S/f_DDS)
• R_FM[15:0]_dec = Round(2¹⁸ x f_M/f_DDS)
Where f_DDS = 3 MHz is the direct digital synthesizer
clock frequency.
After a hard or soft reset or at the start of the transmission
(when TX_RXB goes from 0 to 1) the phase accumulator
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