tza3015hw NXP Semiconductors, tza3015hw Datasheet - Page 10

• 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
Loss Of Signal (LOS) indicator
Besides the analog RSSI output, a digital LOS indication is
present on the TZA3015HW. The RSSI level is internally
compared with a LOS threshold, which can be set by
connecting an external resistor to pin LOSTH or by means
of an internal DAC which is accessible via the I
Bit I2CLOSTH of register LIMLOSCON (D1h) enables the
8-bit DAC, of which the value needs to be programmed
into register LIMLOSTH (D0h). The threshold level is
adjustable in 256 steps from 0 to 1.2 V.
If the received signal strength is below the threshold value,
pin LOS will be HIGH. A default hysteresis of 3 dB is
applied in the comparator. The hysteresis can be set with
bits HTLC[2:0] in register LIMLOSCON (D1h). The
programmable range is 0 to 7 dB.
The polarity of the LOS output can be inverted by bit
LOSPOL of register LIMLOSCON (D1h) to provide more
flexibility in the application.
LOSTH reference setting by external resistor
If the built-in DAC is not used, the reference voltage level
to pin LOSTH can be set by connecting an external resistor
(R2) between pin LOSTH and ground. V
determined by the resistor ratio between R2 and R1 (see
Fig.5). For resistor R1 a value of 10 to 20 k is
recommended, yielding a current of 120 to 60 A
through R1.
V
V
reference voltage of 1.2 V. The minimum threshold level
corresponds to 0 V and the maximum to 1.2 V. Hence, the
value of R2 may not be higher than R1. The accuracy of
V
external resistors.
Apart from using resistors (R1 and R2) to set the LOS
threshold, an accurate external voltage source may also
be used.
If no resistor is connected or an external voltage higher
than
circuit (including the RSSI reading) is automatically
switched off to reduce power dissipation. This ‘auto power
off’ function only works in the pre-programmed mode.
I
2003 Dec 16
2
LOSTH =
ref
LOSTH
C-bus mode allows flexible configuration.
30 Mbit/s to 3.2 Gbit/s A-rate
4-bit fibre optic transceiver
represents a temperature stabilized and accurate
2
/
3
depends mainly on the matching of the two
R2
------ -
R1
V
CC
is applied to pin LOSTH, the LOS detection
V
ref
LOSTH
is
2
C-bus.
10
Slice level adjustment
Due to asymmetrical noise in some optical transmission
systems, a pre-detection signal-to-noise ratio
improvement can be achieved by adding a DC offset to the
input signal. This is done by the slice level circuit in the
TZA3015HW. The required offset depends on the photo
detector characteristics in the optical front-end and the
amplitude of the received signal. The slice level is
adjustable between 50 and +50 mV in 512 steps of
0.2 mV.
Bit SLEN of register LIMLOSCON (D1h) enables the slice
function. The slice level is set by sign and magnitude
convention. The polarity sign is set by bit SLSGN in
register LIMLOSCON (D1h). The magnitude is set by an
8-bit DAC, accessible via register LIMSL (D2h), from
0 to 50 mV in 256 steps.
The introduced offset is not present on input pins
RXSD(Q), in order not to affect the logarithmic RSSI
detector, which would detect the offset as a valid input
signal.
Fig.5
V EE
1.2 V
V ref
Setting the LOSTH reference level by
external resistors.
I
RREF
R1
10 k
LOSTH
Preliminary specification
R2
TZA3015HW
RSSI
compare
LOS
MGU681
V CCA
RSSI
LOS
ground