Circuit Functioning

TM 11-5820-554-12

the requirements for noise blanker gain setting.

the blanking pulses are short, while the greater noise

Whenever the noise level appears to have risen, reduce

bursts develop longer blanking pulses. Transformers

the noise blanker gain, and readjust for the threshold

T1 and 12 and the gating diodes are arranged in a

condition described above. If the noise blanker fails to

balanced modulator configuration so that any noise

reduce the noise level, turn it off. The repetition of the

which results from the gating action is canceled and

noise pulses may be too rapid for the blanker to gate, or

prevented from entering the receiver circuits. Any

a strong adjacent channel carrier may be causing erratic

discontinuity of signal resulting from the gating action

blanking.

is compensated for by tuned circuit restoration in the

following stages of the receiver. Both sections of V4

6-6. Circuit Functioning (fig. 6-2)

serve to isolate the noise-operated gate circuit from the

The first three RF amplifier stages are connected as a

receiver circuits. Vif isolation amplifier V4A provides

40-MHz tuned RF amplifier. The gain of the RF

only enough gain to compensate for the small loss in the

amplifier is controlled by the gain control. The output

gate circuit, so that overall gain through the noise

of the third RF amplifier (V3A) is limited by the action

blanker is approximately unity. Filament power, B+

of limited diode CR8. The output of the third RF

power, and bias voltage are taken from the Collins

amplifier is detected by noise detector CRT. A

KWM2/2A power supply.

resistive-capacitive network determines the length of the

6-7. Limitations

blanking pulse. The audio component of the noise

detector is limited by limited CR2 and applied to the

The noise blanker circuitry has the following three

first pulse amplifier V3B). Any negative portion of the

limitations which decrease the blanking efficiency:

output waveform is clipped by limited CR 4. The

a. Noise pulses which have no energy distribution

positive portion is applied to the second pulse amplifier.

at 40 MHz will occur in the frequency spectrum of the

The output of this amplifier is applied to the third pulse

radio receiver range. The noise blanker will not

amplifier. Pulse shaper CR3 is connected between the

generate a blanking pulse and will permit passage of

output and input of V1B. The output of V1B is applied

these noise pulses.

through threshold gate CR7 to the center tap of T1.

b. A very strong signal in the passband between

Variable IF input from J22 in the transceiver is applied

the first and second mixers can be modulated by the

through variable intermediate frequency (vif) cathode

blanking pulses. This modulation process will cause

follower V4B to the primary of transformer T1. The

sidebands in the passband which result in decreased

bias applied to threshold gate CR7 keeps it cut off and

blanking efficiency. To minimize this modulation

at high impedance to the low level pulses, but high level

effect, a blanker on-off control and blanker RF gain

pulses overcome the bias and pass into the gate circuit.

control are provided on the Collins KWM-2/2A front

Gating diodes CR5 and CR6 are biased to conduction

panel.

for normal free operation. However, when a high

c. Some corona noise and static disturbances have

amplitude noise burst occurs, the positive-going pulse

a repetition rate in excess of one hundred thousand

passes through threshold gate CR7 and cuts off both

pulses per second. The blanking efficiency decreases as

CR5 and CR6. This action effectively disconnects the

the pulse repetition rate exceeds five thousand pulses

variable IF signal for the period of the blanking pulse.

per second.

The length of the blanking pulse varies from a few

6-8. Operator Maintenance Instructions

microseconds to a maximum of 30 microseconds.

The noise blanker has no operator controls.

Blanking pulse length is governed by the magnitude of

operator maintenance is required.

the noise pulse appearing at the noise blanker antenna.

For short duration noise disturbances in the variable IF,

6-9. Deleted.

Change 6 6-2