Wednesday, December 18, 2013

About External Clocking in USRP : GPSDO or OCTO-Clock or OCTO-Clock-GPS or GPSDO

If all frequency and phase and time relations are exactly known, this
simplifies the algorithms. If they are only approximately know this
will   narrow the needed search space for synchronization loops.

The best setup is with several USRPs connected with cables to a single octoclock-GPS.

The second best setup is several (unconnected) units with GPSDO.

An alternative would be clocktamer-GPS modified by adding a 1 PPS
output, but this will have lower performance then GPSDO.

Please see descriptions below for the advantages and disadvantages of
the different possibilities:

1. WIth each USRP unit having a Clocktamer-STD (without GPS, without 1 PPS)
you have no frequency, no phase and no timestamps locked. You would need
to do all synchronisation in software. This will impact the computational load greatly.

2. With each USRP unit having a Clocktamer-GPS standard (without 1 PPS) you
can only have frequency locked, but not phase or timestamps. Note that having no timestamps lock means that your algorithms may need to search though long timespans (several msecs) to look for the right packet. This will impact the computational load greatly.

3. With each USRP unit having a CLocktamer-GPS, and you have the
Clocktamer-GPS modified to also have a 1 PPS output, then you can have frequency and timestamps locked, like with GPSDO. There will be temporary frequency offsets and the phases will not align. Because the clocktamer-GPS uses a Frequency locked loop (not Phase-Locked-Loop) the frequency and phase deviations will be larger then compared to the GPSDO.

4.  With each USRP unit having a GPSDO (includes 1 PPS and 10 MHZ refclock
locked  to the GPS signal) The timestamps and frequency between the different units can be synchronized. There will be temporary phase differences because the reception of the GPS signla and the the lock to the clock to GPS is not perfect. There can also be temporary very small frequency deviations because of this. The timestamp differences will still be small. All differences will be much smaller then compared to systems that do not have their clock locked with PLL to GPS.

5. With octoclock-GPS and 10 Mhz refclock and 1 PPS pulse cables to each unit you can have frequency, phase and timestamps exactly synchronized. This is the perfect setup.

Note that there are no mountingholes in USRP-2 for the GPSDO so you would need to add these or use the GPSDO externally (Needs 5 V DC supply, can also be used without the serial interface attached).

Note that this GPSDO cannot be used with USRP-1.

Clocktamers without GPS do not have the continuous GPS calibration, but do have a much more precise clock then the internal clock of a USRP-2 or a USRP1. The frequency accuracy is just enough for OpenBTS use. They can be used with a USRP-2 when programmed to output 10 MHz.

They can be used with USRP1 when programmed to output 52 MHz or 64 MHz.

A USRP-1 needs a number of modifications to be used with an external clock. This means moving and replacing (soldering) a number of small SMD components, creating a powersupply for the clocktamer and adding a clock input connector. Note that the USRP1 FAN output cannot supply enough power for a clocktamer without modifications. This modifications can be done by an experienced electronics engineer. You whould not have these performed by someone who does not have experience in SMD soldering.

Reference : Mail Conversation with Martin Dudok van Heel

1 comment:

  1. Hello sir,
    I want to implement full duplex transceiver using usrp B200 in gnu radio please guide me thank you in advance