LM1865 datasheet(13/16 Pages) NSC

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Application Notes (Continued)

In order to make the calculation in equation (4) the con-

stants a A1 A2 must first be determined This is done by

the following procedure

1 Connect together two RF generators and apply them to

the tuner input Since the generators will terminate each

other remove the 50X termination at the tuner input

2 Connect a spectrum analyzer to the tuner output Most

spectrum analyzers have 50X input impedances To

make sure that this impedance does not load the tuner

output use a FET probe connected to the spectrum ana-

lyzer The tuner output should be terminated with a ce-

ramic filter

3 Disconnect the AGC line to the tuner Make sure that the

tuner is not gain-reduced

4 Adjust the two RF generators for about 1 mV input and to

frequencies 400 kHz and 800 kHz away from center fre-

quency (

Figure 13 )

5 Note the three output levels in volts

6 Knowing the tuner input levels for VUD1 and VUD2 and

the resulting IM3 just measured ‘‘a’’ is calculated from

the formula

a e

IM3

VUD12 VUD2

(5)

where all levels are in volts rms A typical value for ‘‘a’’

might be 2 c 106

7 A1 and A2 are calculated according to the following for-

mulas

A1 e

VIN

f o a 400 kHz

(6)

A2 e

VIN

f o a 800 kHz

(7)

If the wide band threshold was set to VWBUL then when a

single in-band station reached the level VWBUL at the tuner

output AGC action would start to take place For this rea-

son it is hoped that VWBUL is above the level that will allow

for maximum signal-to-noise If however this is not the

case consideration might be given to improving the inter-

modulation performance of the tuner

The lower limit for VWB is the minimum tuner output that

achieves the best possible signal-to-noise ratio in the recov-

ered audio In general it is desirable to set VWB closer to

the upper limit rather than the lower limit This is done to

prevent AGC action within the narrow band loop except

when there is a possibility of an IM3 greater than VNB

The wide band threshold at the pin 20 input to the LM1865

is fixed at 12 mVrms Generally speaking if pin 20 were

driven directly from the tuner output VWB would be too low

Therefore in general pin 20 is not connected directly to the

tuner output Instead the tuner output is attenuated and then

applied to pin 20 Increasing attenuation increases the wide

band threshold VWB

Pin 20 has an input impedance at 107 MHz that can be

modeled as a 500X resistor in series with a 19 pF capacitor

giving a total impedance of 940X Kb58 Thus an easy way

to attenuate the input to pin 20 is with the arrangement

shown in

Figure 14

Notice that pin 20 must be AC coupled to the tuner output

and that C1 is a bypass capacitor R1 adjusts the amount of

attenuation to pin 20 The wide band threshold will roughly

increase by a factor of (R1 a 940X)940X

AGC CIRCUIT USED AS A CONVENTIONAL AGC

If for some reason the dual AGC thresholds are not desired

it is easy to use the LM1865 as a more conventional

LM3189 type of AGC This is accomplished by AC coupling

the pin 20 input after the ceramic filters rather than before

the filters Thus as with the LM3189 only in-band signals

will be able to activate the AGC

TLH7509 – 15

TLH7509 – 16

FIGURE 13 Spectrum Analyzer Display of Tuner Output

FIGURE 14 Wide Band Threshold Adjustment

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