There is one FREQUENCY-SYNTHESISER used to generate the

FSK-signal.

With the input signal TXD strobed with the high going edge of CKTX the control input of the

synthesiser gets modified which results in frequency shift corresponding to the data-input.

In receive-mode, the same synthesiser is used to generate the Mixer reference-clock.

The Mixer-Frequency Fmixer is set to a value to fold down the FSK-signal to one of two

possible IF-frequencies (2.7kHz / 5.4kHz).

The SCCLK-PLL is used to filter the phase jitter of the second frequency-synthesiser

generating the reference-clock for the SC-Filter. There is an external capacitor needed as

low-pass filtercomponent of the PLL-loop.

The BANDPASS Filter is used to limit the output-spectrum properly for power-line modem

applications.

The OUTPUT-STAGE is designed to be connected to an external buffer arrangement for

minimising the output-impedance and increasing the output-swing. The interface to the

external circuit is done with special I/O-pins allowed to operate with voltages up to +24V.

With two bias pins M1M and P1M the external buffer-stage gets biased (activated). When

these two pins are inactive, the buffer is in a high impedance-mode.

1.2.1 FREQ-GEN

Since the FSK-signal shall be programmable in steps of 150Hz, and the CKSYS

clock-frequency is 5.5296MHz the following structure is used for frequency generation:

The SC-CLK is defined to be 16

times the center-frequency of the

bandpass filters.

For generating the FSK or MIXER-

frequency, Fsynth gets divided by 16

for generation of proper DAC input

signals.

This means for both synthesisers the

frequency steps are 2400Hz.

To get a resolution of 2400Hz a

division by 2304 has to be done by subtraction of 2304 whenever the contents of the

SUM-REG exceeds 2303.

SUBSTR = 5529.6kHz / 2.4kHz = 2304 (=> res 2048+256)

To generate mark-frequencies in the range of 63.9 ... 140.55kHz, the adder factor Nmark has

to be:

Nmark = 16*Fmark / 2.4kHz

To cover the wanted frequency-range with a 9 bit word, a fixed number of 426 is added.

MRK-REG

Nmark = MRK-REG+426

Fout

Fmark

min

0

426

1022.4kHz

63.9kHz

max

511

937

2248.8kHz

140.55kHz

AS5501 / AS5502 Multimode Powerline Modem

Data Sheet

Rev A, May 2000

Page 9/25

ADDER

SUM-REG

&

s

q

r

1k

2k

256

res2304

CKSYS

5.5296M

12

10

N

Fout

FSYNTH