[time-nuts] time-nuts] CPDO?

Christopher Hoover ch at murgatroid.com
Mon Mar 12 12:13:06 EDT 2007


> So, what's it take to make a Cell Phone Disciplined Oscillator?
> I've seen some timing products based on CDMA cell systems, but haven't
> seen any that use an actual cell phone.

An old (older=better) CDMA phone can be made to work as an experimental
platform, but the whole thing is not trivial.  I did this years ago, but not
for time.  You need to feed the baseband into ADC's and then into a DSP or
FPGA.  A microcontroller is handy for implementing the slow control loops.

For time acquisition, you only have to demodulate the pilot and sync
channels.  The pilot tone is not modulated, or more precisely modulated with
all zeros (Walsh code channel 0).  It gives you the short code which is used
by the handset to demodulate the sync channel.  The sync channel gives the
current time at the next PN code rep (which is aligned to the GPS second to
<= 1 us).

The EIA IS-95 spec is the reference for all of this.

-ch





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