LM1865 datasheet(12/16 Pages) NSC

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Application Notes (Continued)

WIDE BAND AGC THRESHOLD ADJUSTMENT

There are a number of criteria that determine where the

wide band threshold should be set If the threshold is set too

high protection against IM3 will be lost If the threshold is

set too low the front end under certain input conditions

may be needlessly gain-reduced sacrificing signal-to-noise

performance Ideally the wide band threshold should be set

to a level that will insure AGC operation whenever there are

out-of-band signals strong enough to generate an IM3 prod-

uct of sufficient magnitude to exceed the narrow band

threshold Ideally this level should be high enough to allow

for a single in-band desired station to AGC the tuner only

after the maximum signal-to-noise has been achieved

In order to insure that the wide band loop is activated when-

ever the IM3 exceeds the narrow band threshold VNB de-

termine the minimum signal levels for two out-of-band sig-

nals necessary to produce an IM3 equal to VNB Then ar-

range for the wide band loop to be activated whenever the

tuner output exceeds the rms sum of these signals There

are many combinations of two out-of-band signals that will

produce an IM3 of a given level However there is only one

combination whose rms sum is a minimum at the tuner out-

put IM3 at the tuner output is given according to the

equation

IM3 e aVUD12 VUD2 (assuming no gain reduction)

(1)

where a e constant dependent on the tuner

VUD1 e out-of-band signal 400 kHz from center frequen-

cy applied to tuner input

VUD2 e out-of-band signal 800 kHz from center frequen-

cy and 400 kHz away from VUD1 applied to tun-

er input

In general due to tuned circuits within the tuner the tuner

gain is not constant with frequency Thus if the tuner is kept

fixed at one frequency while the input frequency is changed

the output level will not remain constant

Figure 12 illus-

trates this

It can be shown that for a given IM3 the combination of

VUD1 and VUD2 that produces the smallest rms sum at the

tuner output is given by the equations

VUD1 e 112

A2A1 IM3aJ

(2)

VUD2 e 0794

A22

IM3

(3)

Therefore in order to guarantee that the AGC will be keyed

for an IM3 e VNB we need only satisfy the condition

VWBs

0V 2

(A1) (112)

A2A1VNBaJ (2a A2(0794) A1

A22

VNB

J (2(4)

The right hand term of equation (4) defines an upper limit for

VWB called VWBUL VWBUL is the rms sum of all the signals

at the tuner output for two out-of-band signals VUD1 and

VUD2 as expressed in equations (2) and (3) applied to the

tuner input

TLH7509 – 14

Define A e tuner gain at center frequency

A1 e tuner gain at f o a 400 kHz

A2 e tuner gain at f o a 800 kHz

FIGURE 12

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