Noise Limiter fig

a. Limiter Circuit. The triode sections of Tube JAN-6N7 (V9) are connected in parallel to form a single triode (fig. 30) which is used as a shunt-type limiter. The heater voltage is reduced to approximately 3 volts by 4-ohm series resistor R31 (fig. 56). The control grids (pins 4 and 5) are connected to 0.05-mf capacitor C42, which is grounded at one end, and through 1-megohm (meg) resistor R49 to the most negative point (A) on the diode resistance. The cathode (pin 8) is connected through LIMITER OFF-ON switch SW5 to a point of higher potential (B) with respect to point (A), and the plates (pins 3 and 6) are connected to the point of highest potential (C)

R48 100 M

xAAA

C45-50MMF

TO 5 ON E4

R48 100 M

xAAA

C45-50MMF

IC43

2,000il

TO 5 ON E4

R 24 50MMF R25 75M 50M

WVW-VNAA

R 30 250M

VWH,

50MMF

OUTPUT "="

TO A-F STAGES

NOISE LIMITER V9 JAN-6N7

OUTPUT "="

TO A-F STAGES

NOISE LIMITER V9 JAN-6N7

figure 30. Radio Receiver BC-779-B, functional diagram of detector and noise limiter stages.

figure 30. Radio Receiver BC-779-B, functional diagram of detector and noise limiter stages.

with respect to point (A). Thus, with the LIMITER OFF-ON switch at ON, the voltage drop between points B and C is the plate-to-cathode voltage of limiter Tube JAN-6N7.

b. Limiter Operation. (1) Development of bias. When a signal of constant amplitude is received, the average (that is, the d-c) voltage between B and C, which is also the limiter plate-to-cathode voltage, remains constant. Similarly, the voltage drop between A and G remains constant and charges capacitor C42 through R49. For a given signal level the grids are held at a potential more negative than the cathode by the voltage drop across resistor R48. This voltage drop, which is applied as bias to the grid, and the plate-to-cathode voltage are so proportioned that the tube is biased to or beyond cut-off. Consequently, the limiter tube has no effect on the input signal.

(2) Noise reduction. Assume that a constant amplitude signal is received and that a very short noise pulse voltage of high amplitude is superimposed on the signal. The noise pulse causes an increased detector current and, hence, an increased voltage drop across the diode load resistance. As the plate and cathode are connected to points B and C, the plate-to-cathode voltage of the limiter is increased. Although there is also a corresponding increase of the negative voltage at A, the grid is not made more negative. Because of the 0.05-second time constant of R49-C42, capacitor C42 cannot charge and/or discharge rapidly enough to follow instantaneous voltage changes at point A. The cathode, however, becomes more negative because of the increased drop across resistors R24 and R25. If the noise pulse is a strong one, the cathode will be negative with respect to the grid. This positive bias in conjunction with the increased plate-to-cathode voltage causes current flow through the noise limiter tube. The limiter is then the equivalent of a low-resistance (approaching a short circuit) shunt from B to C on the detector load resistance. As this condition starts at the point where the noise pulse amplitude begins to exceed the signal amplitude, the peak of the noise pulse is clipped and the noise output cannot become much greater than the signal output. (With the noise amplitude held to a level only slightly greater than the signal, an operator can read signals that would be unreadable without the limiter.) Immediately following the pulse, the original bias voltage, across resistor R48 again biases the limiter, and normal reception is restored quickly. In practice the limiter is effective in reducing the effect of noise pulses, such as those produced by the ignition system of an automobile. If the amplitude of the noise pulse is lower than the signal amplitude, the limiter has no effect. In other words, the limiter can only prevent the noise output from rising appreciably above the signal output.

(3) Changes in signal amplitude. The limiter stage will affect signals, such as amplitude-modulated voice and music transmissions, which contain amplitude changes. Increases of amplitude above the average level cause the same action that is produced by noise pulses and, therefore, the limiter distorts the signal by clipping the peaks. Clipping and, hence, the distortion may increase considerably if the degree of modulation is high.

(4) Changes of average signal amplitude. The time constant of R49-C42 is long enough to prevent bias changes when noise pulses are received. The time constant is short enough, however, to enable automatic readjustment of the bias (voltage of C42) when the average amplitude of the signal changes. For example, if the level of the received signal increases or decreases because of fading, or if the receiver is tuned to a signal of given amplitude and retuned quickly to a stronger or to a weaker signal, the charge of C42 readjusts itself for proper operation under the new conditions.

c. Circuit Differences. The noise 'limiter stage is identical in all the receivers.

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