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參數資料
| 型號： | U4276B |
| 英文描述： | FM Multipath Noise Cancellation |
| 中文描述： | 調頻多徑噪 |
| 文件頁數： | 3/8頁 |
| 文件大小： | 88K |
| 代理商： | U4276B |
U4276B
Preliminary Information
TELEFUNKEN Semiconductors
Rev. A1, 05-Sep-96
3 (8)
Functional Description
Interference with FM reception can be triggered by vari-
ous causes, such as multipath reception, adjacent and
insufficient field strength. At the same time, multipath re-
ception is responsible for the most frequently occurring
types of interference.
Acoustically relevant multipath interference, since it re-
quires a short reaction time, is suppressed directly by the
circuit via a Multipath Noise Cancellation (MNC) func-
tion.
The U4276B can be used both in conventional receivers
and in receivers with digital signal processing.
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. Devi-
ations 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 fail-
ures 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 V). The noise pre-
vents reliable detection of interference. Therefore, only
the average field strength value can be evaluated over a
relatively large time constant in this range.
Digital or analogue
signal
processing
FM – tuner
U4276B
Fieldstrength
signal
MPX in
MPX out
System controller
95 10633
AF
left
AF
right
Mute
Figure 1. Block diagram
Multipath Noise Cancellation (MNC)
Extremely strong multipath interference remains audible
in spite of the monoblend function. The remaining inter-
ference is reduced still further by the MNC circuit if the
output MPXOUT is muted for a few microseconds while
the interference is still occuring (figure 2).
In order to eliminate the possibility of malfunctions, ac-
tivation of the MNC depends on the following condition:
–
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
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.
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