tza3015hw NXP Semiconductors, tza3015hw Datasheet - Page 19

• Manufacturer Part Number: tza3015hw
• Description: 30 Mbit/s To 3.2 Gbit/s A-rate 4-bit Fibre Optic Transceiver
• Manufacturer: NXP Semiconductors

Philips Semiconductors
Transmitter
C
The transmitter frequency can be set independently of the
receiver frequency. For this a clock synthesizer is provided
that drives the multiplexer. Just like the DCR the clock
synthesizer is built around a fractional N synthesizer
offering A-rate functionality for the transmit path.
The clock synthesizer consists of a VCO, several dividers,
a phase frequency detector, an integrated loop filter, a lock
detection circuit and a prescaler output buffer (see Fig.14).
The internal VCO is phase-locked to the reference clock
signal provided at pins CREF(Q). This frequency is
internally scaled down (if necessary) to a frequency in the
range of 18 to 21 MHz by divider R.
2003 Dec 16
handbook, full pagewidth
LOCK SYNTHESIZER
30 Mbit/s to 3.2 Gbit/s A-rate
4-bit fibre optic transceiver
TXPRSCL(Q)
CREF(Q)
divided
LOL
Fig.14 Schematic diagram of the clock synthesizer.
FREQUENCY
DETECTOR
PHASE
down
up
CHARGE PUMP
19
LOOP FILTER
DIVIDER N, K
Because of the 22 bits fractional N capability, any
combination of bit rate (30 Mbit/s to 3.2 Gbit/s) and
reference frequency between 18 and 672 MHz is possible.
The LSB (bit k0) of the fractional divider, should be set to
logic 1 to avoid limit cycles. These are cycles of less than
maximum length, which generate spurs in the frequency
spectrum. This leaves bits k[21:1] available for
programming the fraction, allowing approximately 10 Hz of
frequency resolution without altering the reference
frequency.
To meet most transmission standards, the reference
frequency should be very accurate. In order to be able to
synthesize a clean RF clock that is compliant with the most
stringent jitter generation requirements, it should also be
very clean in terms of phase noise.
MAIN
AND
÷
N
VCO
OCTAVE
DIVIDER
÷
M
Preliminary specification
to LM MUX
and
multiplexer
TZA3015HW
MGU682