RF Noise vs. Signal

The oscilloscope screenshot, below left, shows the linear (analog) output of an LC tuned ASK (Amplitude Shift Keying) RF superregenerative receiver module in the top trace and its digital output in the bottom trace.
The linear signal goes to one comparator input. A capacitor on the other comparator input charges to the average signal strength. When the signal is greater than the charge on the capacitor, the digital output is high. Otherwise, the digital output is low. AGC circuits also respond to signal strength which can be seen in the slope of the response to the long lead-in pulse (just left of center). The overall effect is to raise the threshold when the signal is strong. Closely spaced pulses affect the AGC and the capacitor on the comparator input differently than widely spaced pulses. The effects can be seen in the expanded screenshot below. The red marker is at the same point in both screenshots.
As a result of the AGC and dataslicer (the comparator circuit), the RF receiver makes mountains out of molehills. It dynamically adjusts to signal strength and, in the absence of a signal, outputs a constant stream of noise on the digital output as shown left of center in the above screenshot.

A relatively strong signal results in a clean digital output while a weak or marginal signal will not. The screenshot on the right shows a noise pulse, just left of center, that is a bit higher than the charge on the dataslicer capacitor resulting in an errant digital output pulse.

Measuring the amplitude of the linear signal pulses gives a relative indication of signal strength. However, accuracy and comparisons are limited because the AGC circuit lowers gain when the signal is strong.

It is much harder to distinguish signal from noise without a wide start pulse to act as a marker. The screenshot below shows a signal (preamble + code sent by Pronto TS3000) emerging out of the noise at the receiver only after multiple copies of the preamble set the receiver AGC and threshold. On the far left, the threshold is low and there is continuous noise. By the end of the third copy of the preamble, the threshold is high enough that the noise is suppressed.

FSK (Frequency Shift Keying) has better noise immunity. Two slightly different frequencies represent logic 1 and logic 0. A Carrier Detect output indicates when a signal is being received. Combining FSK with manchester encoding results in robust communications but at a cost that is usually 4-5 times the cost of ASK modules.

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