TZA3019AHT Philips Semiconductors, TZA3019AHT Datasheet - Page 13

• Manufacturer Part Number: TZA3019AHT
• Description: 2.5 Gbits/s dual postamplifier with level detectors and 2 x 2 switch
• Manufacturer: Philips Semiconductors

Philips Semiconductors
RSSI and LOS detection
The TZA3019 allows AC-signal level detection. This can
prevent the outputs from reacting to noise during the
absence of a valid input signal, and can insure that data
only will be transmitted when the signal-to-noise ratio of
the input signal is sufficient to insure low bit error rate
system operation.
The RSSI detection circuit uses seven limiting amplifiers in
a ‘successive detection’ topology to closely approximate
logarithmic response over a total range of 70 dB. The
detectors provide full-wave rectification of the AC signals
presented at each previous amplifier stage. Their outputs
are current drivers. Each cell incorporates a low-pass filter,
being the first step in recovering the average value of the
demodulated signal of the input frequency. The summed
detector output currents are converted to a voltage by an
internal load resistor. This voltage is buffered and
available in the A and B versions of the TZA3019. When
V
prevent the LOS comparator from switching to the standby
mode. The LOS comparator detects an input signal above
a fixed threshold, resulting in a LOW-level at the LOS
circuit output.The threshold level is determined by the
voltage on pins LOSTH1 or LOSTH2 (see Fig.10). A filter
with a time constant of 1 s nominal is included to prevent
noise spikes from triggering the level detector.
The comparator (with internal 3 dB hysteresis) drives an
open-drain circuit with an internal resistor (5 k ) for direct
interfacing to positive or negative logic (see Fig.11). Only
available in the B and C versions of the TZA3019.
The response is independent of the sign of the input signal
because of the particular way the circuit has been built.
This is part of the demodulating nature of the detector,
which results in an alternating input voltage being
transformed to a rectified and filtered quasi DC-output
signal. For the TZA3019 the logarithmic voltage slope is
independent through four feedback loops in the reference
circuit. The internal LOS detector output voltage is based
on V
waveform and the response depends roughly on the input
signal RMS value. This influences high frequencies, a
square wave input of 2.4 GHz (LOS circuit bandwidth
of 2.4 GHz) offsets the intercept voltage by 20%. V
can be calculated using the following formulae:
where S = sensitivity.
Example: a 200 mV (p-p) single-ended 1.2 GB/s PRBS
signal has an RSSI from 1003 mV.
2000 Apr 10
V
RSSI
= 1/13 dB/mV and is essentially temperature and supply
LOSTH
2.5 Gbits/s dual postamplifier with level
detectors and 2
ref
is used V
. The demodulator characteristic depends on the
=
V
RSSI
LOSTH
=
S 20log
must be connected to GND to
2 switch
Vi 18 V
LOSTH
(1)
13
A full understanding of the offset control loop is useful. The
primary purpose of the loop is to extend the lower end of
the dynamic range in any case where the offset voltage of
the first stage might be high enough to cause later stages
to prematurely enter limiting, caused by the high DC-gain
of the amplifier system. The offset is automatically and
continuously compensated via a feedback path from the
last stage. An offset at the output of the logarithmic
converter is equivalent to a change of amplitude at the
input. Consequently, with DC-coupling, signal absence,
either LOW-level or HIGH-level is detected as a full signal,
only signals with an average value equal to zero give zero
output.
Version B can be used for an auto function, which switches
the strongest input signal to output 1 and the weakest to
output 2. To achieve this output V
the reference voltage for input V
LOS1 can switch S1 and S2.
handbook, halfpage
(1) PRBS pattern input signal with a frequency <1 GHz.
(2) Linearity error typically 0.5 dB.
(3)
V i(se)(p-p)
(mV)
= 1/12.5 dB/mV.
10
10
10
10
Fig.10 Loss of signal assert level.
1
3
2
1
0.16
10
0.32
20
LOW-level
LOS1,
LOS2
V LOSTH1 , V LOSTH2 (% of V ref )
0.48
30
(3)
0.64
40
LOSTH
Preliminary specification
V RSSI1 , V RSSI2 (V)
RSSI2
HIGH-level
(1)
(2)
LOS1,
LOS2
0.8
50
. Then the output
must be used as
TZA3019
0.96
60
MGS564
1.12
70