U4275B Datasheet, PDF (7/12 Pages) TEMIC Semiconductors – FM-Receiving Condition Analyser and Multipath Noise Cancellation

TELEFUNKEN Semiconductors
U4275B
Multipath detector
Typical multipath interference is characterized by fast,
modulation-dependent field strength dips, as shown in
figure 2. The multipath detector detects these field
strength dips and evaluates them in accordance with the
level of anticipated acoustic interference.
In the case of acoustically relevant interference, it
operates the interrupt output INT and the MNC function
as a reaction to the interference. If the acoustic effects of
interference can be reduced by monoblend, only pulses
will appear at output INT. In the case of stronger
interference, interference suppression takes place by
means of the Multipath Noise Cancellation (MNC)
function. The multipath interference also generates a
voltage proportional to the interference level at MPFIL.
The capacitor connected there is responsible for storing
the information to give the system controller sufficient
time to read out the value at ANAOUT. The capacitor at
MPFIL is discharged again slowly with a long time
constant. The corresponding addressing at ADDR0,
ADDR1 and OE to permit a rapid assessment when the
station is changed.
Multipath Noise Cancellation (MNC)
Extremely strong multipath interference remains audible
in spite of the monoblend function. The remaining
interference is reduced still further by the MNC circuit if
the output MPXOUT is connected to a reference voltage
for a few microseconds while the interference is still
occuring.
In order to eliminate the possibility of malfunctions,
activation of the MNC depends on the following
conditions:
– The average field strength must exceed a certain
value so that the noise in the field strength
signal does not cause the MNC to respond at low
RF voltages and
– the field strength dip must not be caused by
adjacent-channel interference. Blanking will
otherwise make the modulation of the adjacent
channel audibly. The information for this
purpose is derived from the common channel
detector.
The MNC function can also be activated externally via
the MUTE pin (low active). The circuit can therefore also
be used as a fast high performance mute stage.
Common channel detector
The common channel detector evaluates the spectral
composition of the field strength signal to distinguish
common channel interference from other types of
interference. This is a particularly difficult task, since
weak multipath interference results in a spectrum of the
field strength signal that is similar to that of common
channel interference. the pilot signal portion of the field
strength signal serves as a reference variable to permit a
clear distinction to be made. A voltage proportional to the
interference is generated at the filter input CCFIL in the
case of common channel interference. The external
capacitor is responsible for storing the information to give
the system controller sufficient time to read out the value
at ANAOUT. The capacitor is discharged again slowly at
CCFIL, with a long time constant. The capacitor can also
be discharged rapidly via corresponding addressment
when stations are changed.
The common channel detector is blocked for further
20 ms after the detectors are reset to prevent transitory
phenomena in the selective filters from causing an
indicating error.
Rev. A1: 14.03.1995
Preliminary Information
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