MC13022A
EXPLANATION OF FEATURES
Blend and Noise Reduction
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. Pin 6 drives a comparator
which has a 1.7 V reference. Therefore the comparator
output, Pin 15, is low if Pin 6 is <1.7 V and high if Pin 6 is
>1.7 V.
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
MC13022A 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.
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.
Signal Strength
Post Detection Filtering
A dc voltage proportional to the log of signal strength is
provided at Pin 6. This can be used for signal strength
indication, and it directly controls the post detection filter.
Normal operation is above 2.2 V as shown is Figure NO TAG.
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 NO TAG. Response is controlled by Pin 6 for
automatic audio bandwidth control as a function of signal
strength.
Stop Sense
The signal strength information is multiplexed with the stop
sense signal. The stop sense is activated when scanning by
externally pulling the blend, Pin 23, below 0.3 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, Pins 6 and 15 will go low. This low can be used to tell
4
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