[time-nuts] DS-1 from 10MHz

J.D. Schoedel jdschoedel at verizon.net
Mon Jul 5 09:17:26 EDT 2010


Definitely a basement project.
J.D.

Bob Camp wrote:
> Hi
>
> 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. 
>
> Bob
>
> 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.
>>
>> -- 
>> These are my opinions, not necessarily my employer's.  I hate spam.
>>
>>
>>
>>
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>>     
>
>
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