2.5-555M Regen-Superregen - Radio World

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In CMOS digital logic this may be 0V and 10V, or for RS232 signals from a computer ┬▒6V. Amplitude shift keying, that is two different amplitudes, either of the carrier or of an audio modulating signal is uncommon, partly because nearly all interference mechanisms are amplitude related and better immunity can be achieved by other methods. Frequency shift keying is popular, the carrier having two frequencies separated by 170Hz for narrow shift or 450 and 750Hz wide shift typically used commercially.

The amateur band, intermodulation can occur between them. It should be noted that these signals may be outside the bandwidth of the later stages of the radio receiver but inside the band covered by the RF front end. The IMPs occur in the overloaded front end (normally in the mixer as this is a non-linear stage), producing extra signals which may well be close to, or on top of, the wanted signal. This degrades the receiver performance if the products are within the pass-band of the receiver. The use of high-gain RF amplifiers in receivers can be a major source of this problem and may be countered by reducing the gain by using an input attenuator.

Morse can be regarded as amplitude modulation with only two values, 0 or 100% modulation. Simplistically, there would be sidebands separated from the carrier dependant on the keying speed. That is not the whole story however. The rise and fall of the transmitted envelope greatly affect the overall bandwidth. 53 shows the Morse keying with fast and slow rises and falls of the keying and thus the transmitted RF envelope. The fast rise can be regarded as a small part of a high frequency audio signal; a slower rise time being part of a lower frequency audio signal.

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