MC13022A

4

MOTOROLA ANALOG IC DEVICE DATA

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

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.

Signal Strength

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.

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

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.

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 NO TAG. Response is controlled by Pin 6 for

automatic audio bandwidth control as a function of signal

strength.