Detection, or demodulation, is the process by which information is recovered from a modulated RF signal. For AM, demodulation works very much like modulation. Recall that AM was generated by mixing the carrier and information signals in a non-linear device. The non-linear device had the following outputs:
The carrier frequency
The modulating frequencies
The sum of the carrier and modulating frequencies (upper sideband)
The difference of the carrier and the modulating frequencies (lower sideband).
In general, the output of a mixer contains the sum and difference of the two input frequencies . If we put an AM signal into a non-linear device and mix the carrier and the two sidebands, we get the following outputs:
1. The carrier
2. The upper sideband
3. The lower sideband
4. The sum of the upper sideband and lower sideband
5. The sum of the carrier and upper sideband
6. The sum of the carrier and lower sideband
7. The difference of the upper and lower sidebands
8. The difference of the carrier and the lower sideband (the original modulating signal)
9. The difference of the upper sideband and the carrier (the original modulating signal)
Of the 9 output signals, 2 are the original modulating signal (audio), while the others are RF signals of much higher frequency. By filtering the output of the non-linear device through a low pass filter, only the modulating (audio) signals will pass through to the next stage. The diagram below shows the input and output signals from a non-linear device.
This relatively complex process can be realized by a very simple device: the diode. The diode, which only allows current to pass in one direction, is a very non-linear device. By rectifying the RF signal with a diode and filtering the output, audio can be recovered from an AM RF signal. This is exactly the type of detector used in the earliest receivers, the crystal sets. The metal cat's whisker that touched the surface of the galena crystal formed a Schottky diode (although it was not called that at the time - German physicist Walther Schottky did not publish his work on metal-semiconductor diodes until 1938). The capacitor paralleled across the headphones provided the low pass filtering that recovered the audio.
The diode detector is very simple and is still used widely today for AM reception. However, it does have some drawbacks:
A diode detector has no gain - in fact losses through a diode detector can be 6 dB or more, requiring more amplifier stages ahead of the diode.
A diode detector is so non-linear that additional distortion is introduced into the recovered signal.
The requirement for a low pass filter at the output limits the diode detector's ability to handle wide bandwidth waveforms.
To address these drawbacks, the synchronous detector was developed. The synchronous detector is a specially designed mixer whose inputs are the AM signal and a locally generated signal of the same frequency and phase as the carrier signal. The outputs of this mixer are:
The difference of the carrier signal and the local signal = 0 Hz
The difference of the upper sideband and the locally generated signal = the original audio signal
The difference of the locally generated signal and the lower sideband = the original audio signal
The sum of the locally generated carrier and the AM signal = an AM signal at twice the frequency of the input AM signal.
The additional AM signal generated in the synchronous detector is at a much higher frequency than the original carrier and can be filtered out quite easily. The remaining outputs are the audio signals that were present in the sidebands of the original signals.
The synchronous detector can provide gain, and can also demodulate wideband signals. Distortion levels of less than 1% can be realized through careful design. While this detector cures most of the drawbacks of the diode detector, it requires significantly more complex circuitry and is generally used only where wide bandwidth is required. For most other applications, the diode detector is used. The diagram below shows a basic synchronous detector and the input and output signals.
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