How do high speed digital transceivers which do not share a common clock synchronize? Particularly ones which do not have an ADC front end?
Would you just oversample the signal using a digital input, essentially a 1 bit ADC? It seems this would be likely to glitch.
To give an example, USB 2 HS which runs at 480 Mbps, doesn't appear to have a front end ADC but it does send a preamble which leads me to believe the receiver is sampling at a rate higher than the bit rate.
The key is that although the devices do no share a common clock, they each have their own similar clocks. Although these clocks are not the same, they have a specified frequency with some tolerance.
When the transmitter sends a message, the receiver uses the preamble to determine the frequency offset between the sender clock and its own clock. The receiver uses this information to perform frequency and phase synchronization, often in the form of a digital PLL.
Sampling at more than one sample per symbol can make synchronization more reliable, although the main advantage to doing this is to perform symbol timing synchronization, so you are usually sampling near the center of the symbol.
I don’t know the specifics of USB2, but most communication channels perform frequency and timing synchronization of some sort, and a preamble gives the receiver a chance to figure out the frequency and timing of the transmitted signal so it can synchronize before the data begins.
If you are wondering about USB specifically, the specs are freely available for you to read how it works in detail.
In short, any serial data stream sent without a separate clock is not sent blindly as just raw data bits one bit after another, but encoded with a line code or framed with a protocol which ensures that there is enough transitions to help the receiver to recover the clock or regenerate it and keep the receiver synchronized to the bit stream.
Essentially such encoded streams can be said to contain embedded clock or be self-clocking.
The line code may also contain special symbols or bit patterns that can be used to determine for example which bit is the first bit of a byte or frame.
For example USB3 uses 8b/10b line coding transmit data.
