E1 Clock Notes¶
The Rx and Tx clock on a E1 line are completely separate and must not be alighed / in phase / identical.
Any receiver will always recover the clock for the received signal from the received signal itself.
Most transmitters (E1 interfaces) can be configured to use either the recovered/received clock from one particular line, or a local reference for transmit.
GSM BTS / E1 Clock¶
All E1 based BTS we know of use the clock recovered from the E1 line as reference clock to discipline their internal reference oscillator (often an OCXO). This internal oscillator is what's used to derive the GSM carrier clock from, and hence high precision is required.
Implications for our own board¶We want to include a GPS-DO in our design, to ensure operation of the BTS at proper carrier frequency. That GPS-DO should have
- some GPS receiver with 1PPS output
- some local, voltage-controlled oscillator (VCTCXO)
- the frequency should permit simple division to generate the 2.048 MHz required for E1 (otherwise we'd need a PLL)
- some DAC to steer the voltage-controlled oscillator
- some circuitry to count the number of of oscillator cycles between 1PPS edges
- some software to compensate for 1PPS jitter, loop filtering, possibly temperature compensation, etc.
- The Lars DYI GPS-DO
- doesn't actually count clock cycles but charges a capacitor with pulses and uses the resulting voltage?