LM1865 datasheet(11/16 Pages) NSC

11 / 16 page

Application Notes (Continued)

voltage while VD is allowed to increase VD will increase

until it reaches the level of the wide band threshold at the

tuner output When this occurs VUD is no longer needed to

keep Vo l VWB as VD takes over the job Thus VUD will

drop as the amount of AGC increases while VD is held con-

stant by the AGC

When compared to the simple case of a single in-band sig-

nal we see that because of the presence of a strong out-of-

band signal AGC action has occurred earlier For the simple

case AGC started when VD t VWB For the two signal case

above AGC started when VD t VNB Thus the LM1865

achieves an early AGC when there are strong adjacent

channels that might cause IM3 and a later AGC when these

signals aren’t present

For the range of signal levels that the tuner was gain-re-

duced and VD k VWB there was loss in signal-to-noise in

the recovered audio as compared to the case where there

was no gain reduction in this interval

Note however that

the tuner is not desensitized by the AGC to weak desired

stations below the narrow band threshold

NARROW BAND AGC THRESHOLD ADJUSTMENT

Both the narrow band and wide band AGC thresholds are

user adjustable This allows the user to optimize the AGC

response to a given tuner Referring to

Figure 5 when the

meter output exceeds 1V a comparator closes SW1 A sim-

plified circuit diagram of this comparator is shown in

Figure

The 1K resistor in series with pin 8 allows for an upward

adjustment of the narrow band threshold This is accom-

plished by externally loading pin 8 with a resistor

Figure 11

illustrates how this adjustment takes place

From

Figure 11 it is apparent that loading the meter output

not only moves the narrow band threshold but also de-

creases the meter output for a given input

In general one chooses the narrow band threshold

based on what signal-to-noise compromise is considered

acceptable

TLH7509 – 12

FIGURE 10 Narrow Band Threshold Circuit

TLH7509 – 13

FIGURE 11 Affect of Meter Load on Narrow Band Threshold

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