(from TM 11-866)
17. NOISE LIMITER.
a. The noise limiter tube (V9) is a class B twin triode with its two grids and two plates connected in parallel to secure the lowest possible impedance. When the LIMITER switch (SW5) is closed, the relative potentials of cathode, grids, and plates of the limiter tube (V9) depend on the d-c current flowing in the load circuit of the 2nd detector diode (V8), which in turn depends on the IF carrier voltage impressed on the diode plates. The potential of the grids of V9 is controlled by the filter made up of a 1 megohm resistor (R49) and a .05 microfarad capacitor (C42). The time constant of this combination is one-twentieth second, which is long enough to prevent the grids of V9 from following the carrier variations due to normal modulation, and yet short enough to follow the variations due to fading. This arrangement provides automatic adjustment of the noise limiter circuit for widely different carrier levels at the second detector.
b. With the LIMITER switch (SW5) turned to ON, and a steady carrier being received, the cathode of the limiter tube (V9) assumes a negative voltage with respect to ground (chassis) equal to the drop across resistors R24 and R25 in series. At the same time the grids are held at a potential more negative than the cathode by the drop across resistor R48, and the plates at a positive poten- tial equal to the drop across resistor R30. Under these conditions, with the control grids of limiter tube V9 appreciiably more negative than its cathode, its plate-to-cathode resistance is high and very little conduction takes place as long as the carrier remains unmodul- ated. On high peaks of modulation this balance is upset and some conduction takes place, resulting in distortion of the modulation envelope. This distortion is negligible for modulation percentages up to about 50%, but increases rapidly as the modulation approaches 100%.
c. When the current through the diode load is suddenly greatly increased by the arrival of a pulse of "noise" voltage, the balance described above is changed completely. Due to the time constant of the filter (R49, C42), the grids of the limiter tube (V9) remain at their original potential, while the cathode goes more nega- tive and the plates more positive. If the pulse is a strong one the cathode will be negative with respect to the control grids, and the plate-to-cathode resistance will fall to a low value. This low-resistance plate-to-cathode path is in shunt with the greater part of the diode load (R24, R25, R30). Therefore the current flowing in resistor R25 due to the noise voltage is much less than it would be with the LIMITER switch turned OFF.
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