OQ2538HP_97 PHILIPS [NXP Semiconductors], OQ2538HP_97 Datasheet - Page 6

• Manufacturer Part Number: OQ2538HP_97
• Description: SDH/SONET main amplifier
• Manufacturer: PHILIPS [NXP Semiconductors]

Philips Semiconductors
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 pre-amplifiers 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
pre-amplifier 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.
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 were 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:
1997 Nov 26
f
loop
SDH/SONET main amplifier
=
----------------------------------------------- -
2
1250
1
C
ext
(1)
6
handbook, halfpage
where C
and COFFQ.
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 supply, V
capacitor should be connected between CAPA and V
The band gap voltage is present 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 Z
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 be the same length.
RF input matching circuit
The input circuit for pins IN and INQ contains internal
100
common mode 6 pF capacitor. The topology is depicted in
Fig.3. An external 200
recommended in order to match the inputs to a differential
transmission line, coupled microstrip or stripline with an
odd mode impedance Z
resistors decoupled to ground via an internal
ext
IN
is the capacitance connected between COFF
100
o,odd
Fig.3 RF input topology.
= 50
o,odd
resistor between IN and INQ is
must be used for the
of 50 .
6 pF
Preliminary specification
100
OQ2538HP
EE
GND
MGM114
, the decoupling
INQ
EE
.