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

[Tom Duckworth]

For a 50 ohm driver for your outputs: come off the 74HC390 divide  5/2 
output and buffer the signal with a 74ACTQ240. Then add a 39 ohm series 
resistor followed by (2) 1N4148 diodes (one to ground the other to +5V), to 
limit the output, and you have a 50 ohm driver for your outputs.

Tom
Tom Duckworth
tomduck at comcast.net
----- Original Message ----- 
From: "Bruce Griffiths" <bruce.griffiths at xtra.co.nz>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Tuesday, July 28, 2009 2:43 PM
Subject: Re: [time-nuts] An easy to generate 5, 1 MHz from 10 MHz


> 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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