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LM1865 Folha de dados(PDF) 10 Page - National Semiconductor (TI) |
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LM1865 Folha de dados(HTML) 10 Page - National Semiconductor (TI) |
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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 In 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) In 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 10 |
Nº de peça semelhante - LM1865 |
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Descrição semelhante - LM1865 |
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