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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