GS9005B Re - clocking Receiver - Detailed Device Description
The GS9005B Reclocking Receiver is a bipolar integrated
circuit containing a built-in cable equalizer and circuitry
necessary to re-clock and regenerate the NRZI serial data
stream.
Packaged in a 28 pin PLCC, the receiver operates from a
single five volt supply at data rates in excess of 400 Mb/s.
Typical power consumption is 500 mW. Typical output jitter is
±100 ps at 270 Mb/s.
Serial Digital signals are applied to either a built-in analog
cable equalizer via the SDI and SDI inputs (pins 8,9) or via the
direct digital inputs DDI and DDI (pins 5,6).
Cable Equalizer
A logical HIGH applied to the Analog/Digital Select input (1)
routes the equalized signal while a logic LOW routes the
direct digital signal to the reclocker.
Phase Locked Loop
The phase comparator itself compares the position of
transitions in the incoming signal with the phase of the local
oscillator (VCO). The error-correcting output signals are fed
to the charge pump in the form of short pulses. The charge
pump converts these pulses into a “charge packet” which is
accurately proportional to the system phase error.
The charge packet is then integrated by the second-order
loop filter to produce a control voltage for the VCO.
The Serial Digital signal is connected to the input either
differentially or single ended with the unused input being
decoupled. The equalized signal is generated by passing the
cable signal through a voltage variable filter having a
characteristic which closely matches the inverse cable loss
characteristic. Additionally, the variation of the filter
characteristic with control voltage is designed to imitate the
variation of the inverse cable loss characteristic as the cable
length is varied.
The amplitude of the equalized signal is monitored by a peak
detector circuit which produces an output current with a
polarity corresponding to the difference between the desired
peak signal level and the actual peak signal level. This output
is integrated by an external AGC filter capacitor (AGC
CAP pin 2), providing a steady control voltage for the voltage
variable filter.
A separate signal strength indicator output, (SSI pin 28),
proportional to the amount of AGC is also provided. As the
filter characteristic is varied automatically by the application
of negative feedback, the amplitude of the equalized signal is
kept at a constant level which is representative of the original
amplitude at the transmitter.
The equalized signal is then DC restored, effectively restoring
the logic threshold of the equalized signal to its correct level
irrespective of shifts due to AC coupling.
As the final stage of signal conditioning, a comparator converts
the analog output of the DC restorer to a regenerated digital
output signal.
An OUTPUT 'EYE' MONITOR (pin 16), allows verification of
signal integrity after equalization but before reslicing.
Analog/Digital Select
D
E
D S
N N
E G
M SI
M E
O D
C
E W
R E
T N
O R
N O
F
During periods when there are no transitions in the signal, the
loop filter voltage is required to hold precisely at its last value
so that the VCO does not drift significantly between corrections.
Commutating diodes in the charge pump keep the output
leakage current extremely low, minimizing VCO frequency
drift.
The VCO is implemented using a current-controlled
multivibrator, designed to deliver good stability, low phase
noise and wide operating frequency capability. The frequency
range is design-limited to ±10% about the oscillator centre
frequency.
VCO Centre Frequency Selection
The centre frequency of theVCO is set by one of four external
current reference resistors (RVCO0-RVCO3) connected to
pins 13,14,15 or 17. These are selected by two logic inputs
SS0 and SS1 (pins 20, 21) through a 2:4 decoder according
to the following truth table.
SS1
0
0
1
1
SS0
0
1
0
1
Resistor Selected
RVCO0 (13)
RVCO1 (14)
RVCO2 (15)
RVCO3 (17)
As an alternative, the GS9010A Automatic Tuning Sub-
system and the GS9000B or GS9000S Decoder may be used
in conjunction with the GS9005B to obtain adjustment free
and automatic standard select operation (see Figure 20).
With the VCO operating at twice the clock frequency, a clock
phase which is centred on the eye of the locked signal is used
to latch the incoming data, thus maximising immunity to
jitter-induced errors. The alternate phase is used to latch the
output re-clocked data SDO and SDO (pins 25, 24). The true
and inverse clock signals themselves are available from the
SCO and SCO pins 23 and 22.
3 of 13
32466 - 1
A 2:1 multiplexer selects either the equalized (analog) signal
or a differential ECL data (digital) signal as input to the
reclocker PLL.
GENNUM [ GENNUM CORPORATION ]
