LM1865 datasheet(10/16 Pages) NSC

10 / 16 page

Application Notes (Continued)

First examine what happens with a single in-band signal as

we vary the strength of this signal

Figures 6 and 7 illustrate

what happens at the tuner and AGC outputs

FIGURE 6

TLH7509 – 10

FIGURE 7

Figure 7 there is no AGC output until the tuner output

equals the wide band threshold At this point both SW2 and

SW1 are closed and the AGC holds the tuner output in

Fig-

ure 6 relatively constant

Another simple case to examine is that of the single out-of-

band signal Here there is no AGC output even if the signal

exceeds VWB There is no output because the ceramic fil-

ters prevent the out-of-band signal from getting to the input

of the IF With no signal at the IF input there is no meter

output and SW1 is open which means No AGC

Figures 8 and 9 illustrate what happens at the tuner and

AGC outputs when the strength of an in-band signal is var-

ied in the presence of a strong out-of-band signal (ie

greater than VWB) which is held constant at the tuner input

For this example the in-band signal at the tuner output will

be referred to as VD (desired signal) and the out-of-band

signal as VUD (undesired signal)

Figure 9 we see that there is no AGC output until the

tuner output exceeds the narrow band threshold VNB At

this point Vm l 1V and SW1 closes Further increase of the

desired signal at the tuner input results in an AGC current

that tries to hold the desired signal at the tuner output con-

stant This gain reduction of the tuner forces the undesired

signal at the tuner output to fall At the point that VUD reach-

es the wide band threshold no further gain reduction can

occur as Vo would fall below VWB (refer to Figure 5 ) At this

point control of the AGC shifts from the meter output

(narrow band loop) to the out-of-band signal (wide band

loop) Here VUD is held constant along with the AGC

FIGURE 8

TLH7509 – 11

Prime indicates referenced to tuner input

FIGURE 9

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