[time-nuts] Clock Driver Design
Tom Minnis
Tom_minnis at att.net
Fri Sep 27 18:23:53 UTC 2013
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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