Fig. 5, Diode in series with AGC line provides fast-attack slow-decay action for use with SSB and CW signals. Waveforms produced by CW character **Rn illustrate the action. (Then detector voltage goes negative at start of each code element. capacitor voltage follows almost instantly. When detector voltage goes positive* diode cuts off and only discharge path for capacitor is the high-value resistor shunting it. This permits AGC control voltage to rise toward ground level, but much more slowly. Re* ceiver gain is restored eventually, but not before next code element arrives* In pauses between words or sentences, however, full gain is available.

the amount of variation by the percentage of modulation on the original signal, and in the speed of their variation by the upper-frequency limit imposed on the audio before transmission.

Impulse noise, on the other hand, is un-alfectejd by either of these limits, It may be—and usually is—much stronger than the signal, and may swing much more rapidly. Since it does differ from the modulation on our desired signal in both these key characteristics, either characteristic may be used to distinguish between signal modulation and imp]use noise in order to reject the noise and pass only the desired modulation.

If amplitude is the characteristic chosen to operate the rejection circuit, the resulting circuit may be called a "clipper" or a "limiter'. If the speed of variation is the characteristic chosen to distinguish, the circuit may be called a "silencer". These labels are not strict, however. For example, the "rate of change' limiter circuit originally popularized in these pages some 7 years ago operates, as its name implies, upon the rate of change in signal, yet was termed a limiter.

he Lamb noise silencer, similarly, operates upon amplitude of the signal yet is called a silencer.

Amplitude-limiting circuits, or clippers, operate by setting an arbitrary limit for audio output and restricting all audio to that limit or less. An impulse noise pulse is permitted to reach the limit, but not to exceed it. The limit is derived from the dc voltage, which in turn is derived from signal strength, so that the limit always bears a fixed relationship to the average signal strength. Normally, the limit is set at two times average signal strength. That is, the audio is permitted to rise to a level twice as great as the zero-audio carrier-only voltage. This permits reception of 100-percent modulated signals without distortion, but cuts off noise pulses at the 100-percent-modulation level.

This approach works primarily because noise pulses are so brief compared to the desired audio signals, and cannot mask the desired audio unless they are many times tr *

Stronger—whicl j. in fact, they normally are. By simply cutting the noise back to the same level as the signal, the signal is given a fighting chance.

Rate-of-change devices operate differently. The control voltage in a rate-of-change device is derived from the audio signal rather than from the carrier,, and is filtered through a resistor-capacitor network which permits the control voltage to follow the audio signal level at frequencies below about 3 kHz or so. This is unlike the filtering for clippers or for AGC", where no audio is permitted to remain on the control-voltage line.

The audio is applied to one side of a switching diode and the control voltage

Was this article helpful?

0 0

Post a comment