Hi,

Square wave outputs can be rather higher maintainance, with the whole distribution system from the PCB design and layout through all the wiring to the terminations and beyond all has to be up to the fast IC's/transitions. 10MHz sine waves are much more forgiving. 
Square waves are useful for some things, but I tend to avoid them like the plague, or immediately turn them to sine, unless I need them. It just depends what it's wanted for.

Angus.

From: "Bob Camp"
To: "Discussion of precise time and frequency measurement"
Sent: September 27, 2013 12:44 AM
Subject: Re: [time-nuts] Clock Driver Design

Hi

Ok, the SN74LVC1G125 is 35 cents each at Mouser if you buy at least 10 pieces. That's going to be $3.50. LM78L05 is 26.5 cents if you buy 10 pcs. You will need some bypass caps and resistors, I'd assume you already have them.

Say you want 10 channels, that's 11 logic id's. At one IC per output that's almost $4 plus the 27 cent regulator. Still under $5 for all the parts. It's roughly $10 if you double up on all the output channels.

Do a quickie PCB from any of the usual outfits for $10 or so on a small run / one day turn basis.

Total cost without connectors or case < $20 in very small quantities.

Bob

On Sep 26, 2013, at 5:05 PM, "John C. Westmoreland, P.E."  wrote:

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Thanks for all your thoughts on the subject.  Let me play back what I
have learned and how it may apply to my challenge.  One of my first
applications is to use a 10MHz output to phaselock a VCXO master clock
in a radio transceiver.  The VCXO is the Christek CVHD-950 which has a
noise floor of -164dBc and is -86dBc at 10Hz.  The source I want to use
is the Jackson Labs GPSTCXO which has a noise floor of -155dBc and is
-73dBc at 1Hz and 103dBc at 10Hz.  i did a quick survey of the phase
noise specs on various Jackson products that claim to be ultra low phase
noise and found similar numbers.  One was -100dBc at 1Hz but only
-145dBc at 100KHz.  Another was down -90dBc at 1Hz and -160dBc at
100KHz.  It would appear that even the best parts I could find quickly
would not merit the fancy analog gizmo and that a good stiff logic
buffer would work.  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.  What am I missing?

Tom

On 9/26/2013 3:05 PM, Bob Camp wrote:

Tom,

OK - one question for now if that is OK - your video amp - is it base-band
or broadband?

Regards,
John W,

On Thu, Sep 26, 2013 at 11:11 PM, Tom Minnis Tom_minnis@att.net wrote:

Tom wrote:

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

Hi

If you want sine waves with the logic ic's just put Tee filters on the outputs. A simple low Q design will knock the harmonics down > 60 db without messing anything up to much on the IC end.

Bob

On Sep 26, 2013, at 9:40 PM, Angus not.again@btinternet.com wrote:

Hi

Rise and fall times are not the thing to worry about on the gates. Look at the propagation delay. That's what will vary.

On Sep 27, 2013, at 2:11 AM, Tom Minnis Tom_minnis@att.net wrote:

The phase noise out of a TBolt is roughly -165 to -170 floor and -155 to -160 at 100 Hz. (plus spurs of course)

Rise and fall times are not the thing to worry about on the gates. Look at the propagation delay. That's what will vary. If a 3 ns delay varies 1% (30 ps) over 1,000 seconds that's going to give you 3x10^-14 in your ADEV. Are your sources good to 3x10^-14 at 1,000 seconds? If not, don't worry about it.

Simply use a three element Tee on the output of a logic gate. Run one per output. Don't split for multiple output. That way you will have much better isolation (which very much does matter).

Bob

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:

Wenzel has low phase noise amps the will meet your needs and with prices from about $150-$300 the are an excellent value.

Thomas Knox

apitech has a range of wideband amplifiers with guaranteed phase noise
specs eg,:

http://micro.apitech.com/pdf/amplifier/lophase/TM9128PM.pdf

http://micro.apitech.com/pdf/amplifier/lophase/QBH-1423PM.pdf

http://micro.apitech.com/pdf/amplifier/lophase/TM9333PM.pdf

Bruce

Richard Karlquist wrote: