Voltage Discriminator

11-6660-204-25

Figure 23.

Cathode follower and clipper amplifier,

simplified schematic diagram.

b. Coupling capacitor C303 isolates the grid of

cause the high value of resistor R309 does not

tube V302B from the dc component of the cathode

allow the space charge on the grid to leak off

voltage of the preceding stage. The waveform

easily. The small value of capacitor C344 allows it

across resistor R310 is shown at J308 (fig. 23).

to be effective as a bypass on the very high fre-

The tube has no externally applied bias; high

quency components of the voltage wave without

plate current is prevented by the large value of

affecting the low frequency comp orients.

plate load resistor R311. High grid current on

positive signal swings is prevented by the high

27. Voltage Discriminator

value of resistor R309; this resistor limits the

(fig. 2-4)

positive grid-cathode voltage to less than 1 volt,

a. The voltage discriminator is a miniature

and the remainder of the signal voltage is taken

twin-triode tube, V303A and V303B. The clipped

up across the resistor. On a negative signal swing,

and partially shaped output signal of tube V202B

plate current cutoff is reached at approximately

triggers the voltage discriminator between two

--5 volts; any signal voltage in excess of this

stable states to produce an output signal that is a

value has no effect on the output voltage from the

square wave of constant amplitude and the fre-

stage.

quency of which is the same as the original input

c. To operate properly, the clipper amplifier

signal. Potentiometers R314 and R317 set the dc

tube must have a relatively large signal voltage

operating levels of the cathode and grid. Resistor

applied from the preceding stage. As a result, the

R312 is a grid-current limiting resistor, The out-

potential at the plate of the tube alternately rises

put of this stage is taken from the resistor R318,

to 300 volts during plate current cutoff, and drops

which is the plate load resistor of V303B. Test

to a few volts throughout the positive grid swing;

jacks J310, J311, and J312 check respectively the

the wave is approximately square (fig. 23), and

cathode, grid, and plate waveforms.

can be checked at Jack J309.

b. Tube V303B conducts under a no-signal con-

dition because the grid of the tube is connected to

d. The pulsating signal current in resistor R309

a positive voltage through a voltage-divider ar-

flows through R310 and, because capacitor C303

rangement consisting of R315, R316, and R317

and resistor R310 form a self-rectified bias, is cut

between +300 volts and ground.

off, and exists only during signal reception. Under

no-signal conditions, a slight bias is present be-

Current flowing through common cathode resis-

2-6