[time-nuts] DS-1 from 10MHz
jdschoedel at verizon.net
Mon Jul 5 13:17:26 UTC 2010
Definitely a basement project.
Bob Camp wrote:
> A lot depends on weather this is a one off basement project or a commercial endeavor. If it's commercial, there are people who will sell you a packaged part that will do the 10 MHz to T1 conversion.
> On Jul 4, 2010, at 10:38 PM, Hal Murray wrote:
>>> I would like to generate a DS-1 timing reference from 10 MHz, e.g a
>>> T-bolt. Thought someone here might be able to suggest a starting point.
>> One approach is a PLL. You will have to divide 1.544 MHz by 193 and 10 MHz
>> by 1250. If you want to use the TAPR Clock-Block, I think you will need
>> something like a divide by 10 between the 10 MHz and the Clock-Block to get
>> the numbers within range.
>> I wonder if you could use a VCXO and fit all the logic in a tiny micro. It
>> would probably need a few external parts to filter the PWM output. Maybe the
>> filtering inside the VCXO would be good enough.
>> Another approach is to use a DDS. Analog Devices makes the whole thing in
>> one package, but the numbers don't work out exactly. How close to you need
>> to be? With a 32 bit (binary) adder, you get 1544000.001158 MHz. With 48
>> bits you get 1543999.999999989825.
>> But you don't have to use a binary adder. You have 10000000 Hz and you want
>> 1544000 Hz, so you need to multiply by 1544000 and divide by 10000000. That
>> reduces to 193 / 1250. So add 193 each cycle using modulo 1250 addition.
>> Each time it overflows, kick out a pulse. If you want a square wave output,
>> divide by 625 and toggle the output on each overflow. That sort of logic
>> fits well in a FPGA/CPLD.
>> That will give you a clock that's locked to your input clock but with lots of
>> jitter. (up to 1/2 clock off in each direction, so 100 ns peak-to-peak)
>> You can run that through a low pass filter and/or feed the top bits into a
>> sine table and on to a DAC. Note that isn't the standard ROM. You have to
>> make a new table for each modulus and if you are using standard ROMs with
>> binary addressing you will waste up to 1/2 of each ROM.
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