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OQ2538U Schematic ( PDF Datasheet ) - NXP Semiconductors

Teilenummer OQ2538U
Beschreibung SDH/SONET STM16/OC48 main amplifiers
Hersteller NXP Semiconductors
Logo NXP Semiconductors Logo 




Gesamt 24 Seiten
OQ2538U Datasheet, Funktion
INTEGRATED CIRCUITS
DATA SHEET
OQ2538HP; OQ2538U
SDH/SONET STM16/OC48 main
amplifiers
Product specification
Supersedes data of 1997 Nov 26
File under Integrated Circuits, IC19
1998 Oct 14






OQ2538U Datasheet, Funktion
Philips Semiconductors
SDH/SONET STM16/OC48 main amplifiers
Product specification
OQ2538HP; OQ2538U
FUNCTIONAL DESCRIPTION
The OQ2538HP is comprised of four DC-coupled amplifier
stages along with additional circuitry for offset
compensation and level detection.
The first amplifier stage contains a modified
Cherry/Hooper amplifying cell with high gain
(approximately 20 dB) and a wide bandwidth. Special
attention is paid to minimizing the equivalent input noise at
this stage, thus reducing the overall noise level. Additional
feedback is applied at the second and third stages,
improving isolation and reducing the gain to 14 dB per
stage. The last stage is an output buffer, a unity gain
amplifier, with an output impedance of 100 .
The total gain of the OQ2538HP amounts to 48.5 dB, thus
providing a constant CML-compatible output signal over a
wide input signal range.
Two rectifier circuits are used to measure the input signal
level. Two separate RF preamplifiers are used to generate
the voltage gain needed to obtain a suitable rectifier output
voltage. For rectifier A the gain is approximately 18 dB, for
rectifier B it is about 14 dB. The output of rectifier A can be
used for AGC at the preamplifier stage in front of the
OQ2538HP. The output of rectifier B can be used for LOS
detection. There is a linear relationship between the
rectifier output voltage and the input signal level provided
the amplifiers are not saturated.
Because the four gain stages are DC-coupled and provide
a high overall gain, the effect of the input offset can be
considerable. The OQ2538HP features an internal offset
compensation circuit for eliminating the input offset.
The bandwidth of the offset control loop is determined by
an external capacitor.
REF and CAPA band gap output and decoupling
capacitance
To reduce band gap noise levels, a 1 nF decoupling
capacitor on CAPA is recommended. Since the band gap
is referenced to the negative power supply, the decoupling
capacitor should be connected between CAPA and VEE.
The band gap voltage is present on pin REF for test
purposes only. It is not intended to serve as an external
reference.
RF input and output connections
Striplines, or microstrips, with an odd mode characteristic
impedance of Zo(odd) = 50 must be used for the
differential RF connections on the PCB. This applies to
both the input signal pair IN and INQ and to the output
signal pair OUT and OUTQ. The two lines in each pair
should have the same length.
RF input matching circuit
The input circuit for pins IN and INQ contains internal
100 resistors decoupled to ground via an internal
common mode 6 pF capacitor. The topology is depicted in
Fig.3.
handbook, halfpage
100
6 pF
GND
100
COFF and COFFQ offset compensation
Automatic offset compensation eliminates the input offset
of the OQ2538HP. This offset cancellation influences the
low frequency gain of the amplifier stages. With a
capacitance of 100 nF between COFF and COFFQ the
loop bandwidth will be less than 1.5 kHz, small enough to
have no influence on amplifier gain over the frequencies of
interest. If the capacitor was omitted, the loop bandwidth
would be greater than 30 MHz, which would influence the
input signal gain. The loop bandwidth can be calculated
from the following formula:
floop = -2---π-----×-----1---2----5--1-0----------×-----C----e---x--t
(1)
where Cext is the capacitance connected between COFF
and COFFQ.
IN INQ
MGM114
Fig.3 RF input topology.
1998 Oct 14
6

6 Page









OQ2538U pdf, datenblatt
Philips Semiconductors
SDH/SONET STM16/OC48 main amplifiers
Product specification
OQ2538HP; OQ2538U
VAGC VAGCDC
(mV)
200
100
(2)
(1)
(3)
MGE746
VLOS VLOSDC
(mV)
200
100
MGE747
(2)
(1)
(3)
0
0 10 20 30 40 50 60 80
VIN (mV p-p)
0
0 1 2 3 4 5 6 7 8 9 10 11
VIN (mV p-p)
(1) Tamb = 20 °C.
(2) Tamb = +25 °C.
(3) Tamb = +85 °C.
Fig.11 AGC transfer characteristics.
(1) Tamb = 20 °C.
(2) Tamb = +25 °C.
(3) Tamb = +85 °C.
Fig.12 LOS detection characteristics.
AGC and AGCDC level detection
When using rectifier A as an input signal level detector, the
AGC and AGCDC pins must be decoupled to ground with
100 nF capacitors. The AGCDC output is intended as a
reference voltage against which the actual AGC output
voltage can be compared. This voltage difference,
VAGC VAGCDC, can be used as a control input in an AGC
loop. A graph depicting output voltage difference as a
function of the input signal level (typical) is shown in
Fig.11. Note that an input signal with the specified
peak-to-peak value is applied to both IN and INQ inputs,
but with complementary phase.
LOS and LOSDC level detection
The output of rectifier B can be used for LOS detection.
The LOSDC output provides a reference voltage against
which the voltage at the LOS output can be compared.
The voltage difference VLOS VLOSDC can be used as
input to a LOS detection circuit. Both outputs need to be
decoupled using 100 nF capacitors. A graph depicting
VLOS VLOSDC as a function of the input signal level
(typical) is shown in Fig.12. Note that an input signal with
the specified peak-to-peak value is applied to both IN and
INQ inputs, but with complementary phase.
Grounding and power supply decoupling
The ground connection on the PCB needs to be a large
copper area fill connected to a common ground plane with
as low inductance as possible, preferably positioned
directly underneath the LQFP48 package. The large area
fill will improve heat transfer to the PCB and thus aid IC
cooling.
All VEE pins (two at each corner) need to be connected to
a common supply plane with as low inductance as
possible. This plane should be decoupled to ground.
To avoid high frequency resonance, multiple bypass
capacitors should not be mounted at the same location.
To minimize low frequency switching noise in the vicinity of
the OQ2538HP, the power supply line should be filtered
once using an LC-circuit with a low cut-off frequency
(see Fig.14).
1998 Oct 14
12

12 Page





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