(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.