Since the muting control input pin 6 is connected to the base of the emitter grounded transistor (through a protection resistor of

500

Ω in series), the voltage between pin 6 and GND is about 0.6 V when the control current is applied. In actual cases, the

muting operation is accomplished by applying a voltage drive type muting drive output (pin 14 to pin 6 through a high resistance

(up to 20 k

Ω)).

The muting drive output comes in three types:

1)

Hole detector output which develops a voltage when C/N (carrier-to-noise

ratio) lowers under weak signal input conditions.

2)

A reversed output of the signal strength indicating output (output at pin

15)

3)

A bandwidth limited muting drive output which develops a voltage when

the AFC output becomes higher than

±V

All these outputs are led to an OR circuit and the processed output is developed at pin 14. Of the above muting drive outputs,

descriptions on the hole detector output and the bandwidth limited muting drive output will be omitted, since they are the same as

those used in conventional quadrature detector ICs (such as LA1230, LA1231N).

The inverted output of the signal strength indication output is obtained as illustrated below.

constant equal to 4.1 V (typ) for medium or lower signal input levels, where the muting drive output is required. Since the V15

increases proportionally to the increase of the input signal strength, I1 will also increase. Therefore the V14 will decrease with

increasing signal strength. Thus the required muting drive output can be obtained by selecting proper values of R15-G and R16-G.

For example, the muting drive output moves toward strong input signal level

if the R16-G is decreased, or the muting drive output becomes zero due to the

increased to infinity (namely pin 16 is opened). However the muting drive

output caused by a whole detector still exists in this case. Increasing R15-G

decreases the slope of the curve for the muting drive output vs. antenna signal

input level, or decreasing the R15-G increases the slope of the curve.

Furthermore, varying the value of a resistor connected between the muting

drive output (pin 14) and the muting control input (pin 6) changes the value of

the muting control current required to obtain the same muting drive output,

accordingly the slope of the curve for muting attenuation vs. antenna signal

input level is also changed. These characteristics investigated by using an

actual receiver are shown on the curves below.

The general method to adjust the muting circuit of the LA1140 is: to set the signal input level required to actuate the muting

circuit with the R16-G, to adjust the slope of the curve for the muting attenuation vs. antenna signal input with the R15-G, and to

adjust the maximum muting attenuation (determined by setting the noise level at no signal) with the R5-G. The slope of the curve

for the muting attenuation vs. antenna signal input level can also be adjusted by the resistor connected between pins 14 and 6 in

addition to R15-G, however, selecting a resistor too high does not allow the muting control current flowing into pin 6 to reach

120 µA even through the maximum muting drive output (V14) is applied, namely the muting attenuation does not reach its

maximum value. Accordingly a recommended value of the resistor between pins 14 and 6 is about 22 k

Ω.

ANT Input

Unit (resistance:

Ω)

Inverter

Inverter

Inverter

(AGC output)

(Signal meter output)

(Muting drive output)

(Offset)

LA1140

No. 729-4/15