[time-nuts] Clock Driver Design

Magnus Danielson magnus at rubidium.dyndns.org
Sat Sep 28 03:42:33 EDT 2013


On 09/27/2013 12:53 PM, 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.
That comment came out of another thread. Yes, there are issues with
square distribution. If you want to stick with it, then you need to take
care of that square. Square can have a benefit over sine thought,
smaller amounts of reflection can be ignored and not cause much shift as
they will be voltage-shifted out from the comparator
trigger-point/amplifier mid-point. This means that with some care, the
next step can remove it. A square signal is easy to diagnose with a
scope, just toss it in there and you have reflections showing up on the
scope. Sine require a little more subtle methods to figure out what is
wrong. So, it's a two-edged sword.
> 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.
Yes and no, depending on what plagues your installation. Sine is the
perfect waveshape of converting additive noise (hum for instance) into
jitter if it meets a comparator. The low slewrate will be the problem
with the sine, so increasing the through-zero slew-rate reduces this
problem. The jitter conversion follows this formula:

t_rms = n_rms / S_signal

Where n_rms is the Noise (including hum and other unwanted signal) RMS
value (in V),
S_signal is the signal slew-rate (in V/s) and
t_rms is the jitter time (in s)

This have bitten me many times.

You can avoid having additive noise transform into jitter by not feeding
it into a comparator but rather linearly gaining yourself out of the
situation. Yes that gain-stage will saturate at the ends. See the TADD-2
input stage, as adapted from the Wenzel page on the subject. Such input
can also handle square signals.

Not all inputs is made to handle sine signals properly, so you can have
some additional noise on your reference as you lock up your instrument.
This also means that some receivers is best served by square, even if
square is tricker to handle to some degree. Others can be best served by
sine.

In the end, there isn't one way which is always right, rather you need
to adapt to the receivers needs.

It would be nice to have a little standard board with a good quality
sine squarer such that it can be put inline at or in the instrument.

Cheers,
Magnus


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