SQUELCH Switch

TM 11-5820-401-34-3/0967-LP-432-3060

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RECEIVER SQUELCH FUNCTION (CONT)

When no rf signals are received, the radio's components generate internal noise frequencies. The

noise is quieted by rf reception. One of the most pronounced frequencies, 7.3 kHz, is used to switch

the squelch relay through a series of actions involving the following components.

SQUELCH Switch

In the OLD ON position, the SQUELCH switch selects a special set of circuits that respond particularly

to 7.3 kHz. The circuits are located in the Squelch Filter A5300.

Squelch Filter

The squelch filter contains circuitry to filter out 7.3-kHz signals. The 7.3-kHz noise is applied to the

Squelch Amplifier A5200. A feedback loop applies the noise (many frequencies including 7.3 kHz) to

the squelch filter. The squelch filter attenuates 7.3 kHz, and feeds back all other frequencies degen-

eratively to the ac amplifier, Therefore, the ac amplifier responds well to 7.3 kHz, but degenerates

sharply for any other frequencies.

The degenerative feedback is important to insure that the receiver remains squelched in response to

the presence of 7.3-kHz internal noise, which is the best possible frequency of internal noise to

indicate the absence of a valid rf input. Other noise frequencies can be present even when rf is

received, in which case the radio would remain squelched if it were not for the selectivity of the

squelch filter.

The ac amplifier increases the signal amplitude of the 7.3-kHz sine wave. The sine wave is then

rectified and applied to the OLD ON DC amp. Application of the rectified signal to the dc amplifier

results in a ground path being opened at one of the amplifier's transistors. The ground path is the

enable circuit for the squelch relay. With no ground path, the relay cannot energize.

Squelch Relay

When the relay's enable circuit is ungrounded at the OLD ON DC amp the relay deenergizes, switching

16 vdc off. With no 16-vdc power available, no audio can be coupled to the amplifier; therefore, the

radio is silent.

As soon as rf at 0.5-microvolt level or above is received, the 7.3-kHz noise disappears, a ground is

created by the dc amplifier in the A5200, and the squelch relay energizes, applying 16 vdc to the audio

amplifier.

NEW SQUELCH

In the NEW SQUELCH position of the SQUELCH switch, 16-vdc power to the audio amplifier is available

through the squelch relay only when a 150-Hz new squelch tone is received along with the rf carrier.

The 150-Hz tone is used to switch the squelch relay through a series of actions involving the following

components.

SQUELCH Switch

In the NEW ON position, the SQUELCH switch selects a special set of circuits that respond particularly

to 150 Hz, The circuits are located in the Squelch Filter A5300.

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