U4275B
Functional description
Interference with FM reception can be triggered by
various causes, such as multipath reception, adjacent and
insufficient field strength. At the same time, multipath
reception is responsible for the most frequently occurring
types of interference. In order to suppress this
interference systematically, it is first necessary to analyze
the received signal so that it is possible to derive the type
of interference at any time. The circuit U4275B analyzes
the FM reception conditions and makes the data
available. This data can be requested by a system
controller and evaluated (see figure 1). It is thus possible
to introduce measures for optimizing reception – possibly
tailored to individual types of interference.
Acoustically relevant multipath interference, since it
requires a short reaction time, is suppressed directly by
the circuit via a Multipath Noise Cancellation (MNC)
function.
The U4275B can be used both in conventional receivers
and in receivers with digital signal processing.
Furthermore, it is designed so that it can activate
switchover between antennas on antenna-diversity
receivers. The integrated circuit can be connected in
cascade without much wiring for tuner-diversity
receivers.
TELEFUNKEN Semiconductors
The field strength voltage of the IF amplifier provides all
the information about the reception conditions. The
following demands are placed on the field strength output
of the IF amplifier:
(1) A linear characteristic, i.e. the output voltage must be
proportional to the logarithm of the IF voltage in the
range of RF levels encountered in practice. Deviations
from linearity lead to a different evaluation of
interference signals of the same intensity of the signal
levels are different.
(2) The “signal level” output has to react to extremely fast
dips in the field strength. Total field strength failures
lasting 500 ns and occuring a voltage dip to approx.
0 V at the field strength output.
Restrictions
The field strength voltage becomes increasingly noisy in
the range of low RF levels (< 30 dB
m
V). The noise
prevents reliable detection of interference. Therefore,
only the average field strength value can be evaluated
over a relatively large time constant in this range. The
pilot signal component in the receive signal is an
important basis for quality assessment. Multipath
detection and common channel detection therefore
function only in the case of stereo transmitters.
MPX in
FM – tuner
Fieldstrength
signal
Address
MPX out
AF
left
U4275B
Interrupt
output
ANA out
Digital or analogue
signal
processing
AF
right
System controller
94 8646
Figure 1 Block diagram
6
Preliminary Information
Rev. A1: 14.03.1995
TEMIC [ TEMIC SEMICONDUCTORS ]
