[time-nuts] An easy to generate 5, 1 MHz from 10 MHz

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Jul 28 21:43:52 UTC 2009


Rex wrote:
>
> John Green wrote:
>
>> A cheap and easy way is to use a 74HC14, 74HC390. Capacitively couple
>> the 10
>> MHz into one of the 6 inputs of the HC14 to square it up, out of that
>> into
>> one of the divide by 2 inputs of the HC390, that goes to another
>> input of
>> the HC14 to act as a buffer which provides the 5 MHz output. Next,
>> into the
>> divide by 5 input of the HC390 to give 1 MHz out. This also goes
>> through one
>> section of the HC14. There is another section of the HC390 if you
>> wish to
>> divide down farther. I made one of these to feed an old Marconi service
>> monitor that requires 1 MHz instead of 10 MHz as an external reference
>> input. I also have 5 MHz and 100KHz available if I need them. True,
>> the HC14
>> isn't a proper buffer meant to drive low impedance loads, but it
>> seems to
>> work OK for me. I laid it out in Eagle and routed out a board with the
>> T-Tech here at work but there is no reason you couldn't do it with
>> wire on
>> perf board.
>> _______________________________________________
>>  
>>
>
> If you need a stronger output driver, you could look at what was done
> in the TAPR TADD-2:
> http://www.tapr.org/kits_tadd-2.html
>
> Schematics are available in the documentation.
>
> My first htought was that the TADD-2 might work with a modified
> version of the PIC code to give lower division outputs (5 or 1 MHz).
> It might work for generating the 1 MHz, but I think you would still
> need hardware for the 10 MHZ to 5 MHz division. For your task, it may
> be easier to do it all in hardware as mentioned above.
>
Don't copy this circuit slavishly.

Using a single AC04 to buffer 2 different output frequencies is a bad
idea as ground bounce within the AC04 package creates significant
crosstalk between the 2 outputs.
As long as each AC04 is dedicated to a single output frequency (and
preferably load) the crosstalk between outputs will be small.

Bruce




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