[time-nuts] Clock Driver Design

[Tom Minnis]

I haven't even begun to look for video amps yet.  I may not need one if 
I filter an output of an high powered 5V buffer.  What I hear is a 
simple passive low pass filter will do.  That being the case, I may put 
them on all the outputs and make it a jumper option.  The other project 
brewing here is developing a precision time stamp transceiver which 
needs the fast edges as opposed to the synthesizer reference which needs 
the accurate frequency aspect.  Thanks again for all your helpful ideas.
Tom

On 9/27/2013 3:53 AM, Charles Steinmetz wrote:
> 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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