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
[email protected]
----- Original Message -----
From: "Bruce Griffiths" <[email protected]>
To: "Discussion of precise time and frequency measurement"
<[email protected]>
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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