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TZA3019CHT

2.5 Gbits/s dual postamplifier with level detectors and 2 × 2 switch

Philips Electronics 

Philips Semiconductors

2.5 Gbits/s dual postamplifier with level

detectors and 2 × 2 switch

Preliminary specification

TZA3019

FUNCTIONAL DESCRIPTION

The TZA3019 is a dual postamplifier with multiplexer and

loss of signal detection see Figs 1, 2 and 3. The RF path

starts with the multiplexer, which connects an amplifier to

one of the two inputs. It is possible to invert the output for

easy layout of the Printed-Circuit Board (PCB). The signal

is amplified to a certain level. To guarantee this level with

minimum distortion over the temperature range and level

range, an active control part is added. The offset

compensation circuit following the inverter minimizes the

offset.

The Received Signal Strength Indicator (RSSI) or the Loss

Of Signal (LOS) detection uses a 7-stage ‘successive

detection’ circuit. It provides a logarithmic output. The LOS

is followed by a comparator with a programmable

threshold. The input signal level-detection is implemented

to check if the input signal voltage is above the user

programmed level. This can insure that data will only be

transmitted when the input signal-to-noise ratio is sufficient

for low bit error rate system operation. A second

offset compensation circuit minimizes the offset of the

logarithmic amplifier.

RF input circuit

The input circuit contains internal 50 Ω resistors

decoupled to ground via an internal common mode 12 pF

capacitor (see Fig.6).

The input pins are DC-biased at approximately −0.33 V by

an internal reference generator. The TZA3019 can be

DC-coupled, but AC-coupling is preferred. In case of

DC-coupling, the driving source must operate within the

allowable input range (−1.0 to +0.3 V). A DC-offset voltage

of more than a few millivolts should be avoided, since the

internal DC-offset compensation circuit has a limited

correction range. When AC-coupling is used, if no

DC-compatibility is required, the values of the coupling

capacitors must be large enough to pass the lowest input

frequency of interest. Capacitor tolerance and resistor

variation must be included for an accurate calculation.

Do not use signal frequencies around the low cut-off

circuit frequencies (f−3dB(l) = 50 kHz for the postamplifiers

and f−3dB(l) = 1 MHz for the LOS circuits).

RF output circuit

Matching the main amplifier outputs (see Fig.7) is not

mandatory. In most applications, the transmission line

receiving end will be properly matched, while very little

reflections occur.

Matching the transmitting end to absorb reflections is only

recommended for very sensitive applications.

In such cases, pull-up resistors of 100 Ω should be

connected as close as possible to the IC from

pins OUT1 and OUT1Q, and pins OUT2 and OUT2Q to

VEE1B and VEE2B respectively. These matching resistors

are not needed in most applications.

handbook, halfpage

12 pF 420 Ω

IN1, IN2

IN1Q, IN2Q

50 Ω

50 Ω

GND1A,

GND2A

VEE1A,

MGS555 VEE2A

Fig.6 RF input circuit.

Postamplifier level adjustment

The postamplifier boosts the signal up to PECL levels. The

output can be either CML- or PECL-level compatible,

adjusted by means of the voltage on pins LEVEL1

and LEVEL2. The DC voltages of pins OUT1 and OUT1Q,

and pins OUT2 and OUT2Q match with the DC-levels

on pins LEVEL1 and LEVEL2, respectively. Due to the

receiving end 50 Ω load resistance, it means that at the

same level of Vo(p-p), VLEVEL1 and VLEVEL2 with

AC-coupling are not equal to VLEVEL1 and VLEVEL2 with

DC-coupling (see Figs 7 and 8).

The postamplifier is in power-down state when pin

LEVEL1 or LEVEL2 is connected to ground or not

connected (see Fig.8).

Postamplifier DC offset cancellation loop

Offset control loops connected between the inputs of the

buffers A1A and A2A and the outputs of the amplifiers A1B

and A2B (see Figs 1, 2 and 3) will keep the input of both

buffers at their toggle point during the absence of an input

signal. The active offset compensation circuit is integrated,

so no external capacitor is required. The loop time

constant determines the lower cut-off frequency of the

amplifier chain. The cut-off frequency of the offset

compensations is fixed internally at approximately 5 kHz.

2000 Apr 10

9

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