I checked Mini Circuits and they have 2 10MHz low Pass filters for
$8.95. I think I will try and lay out a discrete T band pass which can
easily be turned into a low pass if it gets too tweeky. This should be
less than $3. I think I can use a simple SPDT switch to option the
outputs for either sin or square wave output.
with regard to noise specs of oscillators, I was confused to see
Symmetricom spec. the CSAC module they supply at -70dBc at 10Hz and then
see Jackson implement a design using that part and put the 10Hz noise at
-90dBc!
Tom
On 9/27/2013 1:16 PM, Bob Camp wrote:
Hi
Two very conventional RF chokes (couple of uH each) and an NPO cap (couple
hundred pf) are all you really need for the square to sine filter. It's
probably a good idea to put a blocking cap on the thing as well. If you want to
get fancy, put a three resistor 6 db pad on it as well. That way your cable
will be rationally terminated over a fairly wide bandwidth.
Bob
On Sep 27, 2013, at 2:23 PM, Tom Minnis <[email protected]> wrote:
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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