[time-nuts] Clock Driver Design

Charles Steinmetz csteinmetz at yandex.com
Fri Sep 27 10:53:23 UTC 2013


Tom wrote:

>One of my first applications is to use a 10MHz output to phaselock a 
>VCXO master clock in a radio transceiver.  *   *   *   Next I went 
>to IDT to find the best logic buffer I could find.  I am looking at 
>the IDT 74FCT38072 2 channel clock driver for PPS.  It can drive 
>about 50mA if needed with 1nS rise and fall times.  The one I am 
>looking at for 10MHz is the ICS553 4 channel clock driver.  This one 
>is good for 25mA drive and they actually give a typical output 
>impedance spec of 20 Ohms.  With a 3.3V supply, it has 1nS rise and 
>fall times and a little faster with a 5V supply, 0.7nS and 35mA 
>drive.  To make a sine wave should I use one of the 4 ports on the 4 
>port driver to input to the filter or should I try to hook the 
>filter input directly to the clock driver input?
>Are there tried and true 10MHz filter circuits or is that a non issue?
>After the filter would come the video amp set up for a 50 Ohm drive 
>and into a splitter.  That sound simple enough.

I strongly agree with Magnus that distributing square waves is asking 
for trouble and that converting to sine is preferable unless there is 
some very good reason not to.

IIRC, you said the source is CMOS.  So you can do all of your fanout 
digitally, then filter each output (I believe that is what Bob had in 
mind).  Or, as you appear to be contemplating based on your comments 
above, you could convert to sine immediately and then do the fanout 
in the analog domain with a video DA or whatever.  One reasonable 
filter type to hang on a CMOS output is an L-C-L "tee" filter (there 
is really no reason not to add one more shunt C at the end, for 
L-C-L-C).  This filter needs some termination at all times -- the 
open circuit output voltage can be pretty high.  But you can usually 
get away with an internal termination of ~1k or so.  If you need more 
current to get the output level you want, parallel several CMOS 
outputs (all on the same hex buffer chip, preferably).  There is no 
need for very fast edges, particularly if you are filtering to sine 
wave.  Nothing exotic is necessary.

The same is true even if you decide to distribute square waves.  The 
fewer higher harmonics you have, the better off you will be.

Best regards,

Charles






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