MC13022 Просмотр технического описания (PDF) - Motorola => Freescale
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MC13022 Datasheet PDF : 8 Pages
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MC13022
EXPLANATION OF FEATURES
Blend and Noise Reduction
Although AM stereo does not have the extreme difference
in S/N between mono and stereo that FM does (typically less
than 3.0 dB versus greater than 20 dB for FM), sudden
switching between mono and stereo is quite apparent. Some
forms of interference such as co–channel have a large L–R
component that makes them more annoying than would
ordinarily be expected for the measured level. The MC13022
measures the interference level and reduces L–R as
interference increases, blending smoothly to monaural. The
pilot indicator remains on as long as a pilot signal is detected,
even when interference is severe, to minimize annoying pilot
light flickering.
RF AGC/Meter Drive
A dc voltage proportional to the log of signal strength is
provided at Pin 6. This can be used for RF AGC, signal
strength indication, and/or control of the post detection filter.
Normal operation is above 2.2 V as shown is Figure 4.
Stop Sense
Multiplexed with the signal strength information is the stop
sense signal. The stop sense is activated when scanning by
externally pulling the blend capacitor on Pin 23 below 0.5 V.
This would typically be done from the mute line in a frequency
synthesizer.
If at any time Pin 23 is low and there is either no signal in
the IF or a noisy signal of a predetermined interference level,
Pin 6 will go low. This low can be used to tell the frequency
synthesizer to immediately scan to the next channel. The
interference detection prevents stopping on many
unlistenable stations, a feature particularly useful at night
when many frequencies may have strong signals from
multiple co–channel stations.
IF Bandwidth Control
IF AGC attenuates the signal by shunting the signal at the
IF input. This widens the IF bandwidth by decreasing the
loaded Q of the input coupling coil as signal strength
increases.
Post Detection Filtering
With weak, noisy signals, high frequency rolloff greatly
improves the sound. Conventional tone controls do not
attenuate the highs sufficiently to control noise without also
significantly affecting the mid–range. Also, notch filters are
necessary with any wide–band AM radio to eliminate the
10 kHz whistle from adjacent stations.
By using a twin–T filter with variable feedback to the
normally grounded center leg, a variable Q notch filter is
formed that provides both the 10 kHz notch and variable high
frequency rolloff functions. Typical range of response is
shown in Figure 3. Response is controlled by the dc voltage
on Pin 15.
Pin 15 could interface with a dc operated tone control such
as the TDA1524, or could be tied to Pin 6 for automatic audio
bandwidth control as a function of signal strength.
4
MOTOROLA ANALOG IC DEVICE DATA
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