[time-nuts] Clock Driver Design

[SAIDJACK at aol.com]

Hi Tom,
 
yes, the GPSTCXO has a pretty good typical phase noise above 100Hz or so  
for being "just" a TCXO, competitive to or even a bit better than some good  
SC-cut OCXO's. Compare that to the Trimble Mini-T for example which has a  
noise floor spec of only -145dBc/Hz according to the  Trimble datasheet..
 
But there is a big difference between -73dBc/Hz at 1Hz, and -100dBc/Hz at  
1Hz. These are certainly not similar numbers, not remotely..
 
That's what makes the difference between a low cost TCXO and a  
high-performance SC-cut double oven OCXO. In fact you can probably expect an  OCXO that 
is specified at -105dBc/Hz at 1Hz to cost twice as much as one  specified 
at "only" -100dBc/Hz at 10MHz..
 
There is also some specmanship here, because of the difference between max  
noise and typical noise. Max noise means all of the units shipped must be 
at or  better than x, typical means the 1-Sigma noise numbers of a number of  
production units should be at or better than x. Some may be way better, 
some may  be worse. There will be a cost difference between the two for the 
same  number.
 
In terms of signal buffering, I think this thread is sometimes  slightly 
over-complicating the issue, take a handful of fast single CMOS  gates  
especially if you already have a CMOS source, then run them at 5V,  and then do 
some more or less complicated low pass filtering to generate 50 Ohms  10MHz 
sine waves. You can probably even get some low-cost Mini Circuits BNC  type 
low pass filters of-the-shelf to do the job for you. But layout  and power 
supply design is of critical importance to achieve the best noise  performance, 
I would not use a simple LM78M05 regulator, I would use a low noise  LT or 
Analog Devices unit because any low-frequency 1/f type power supply  noise 
is going to be modulated right onto your signal unless you do a true  
differential design (way too complicated). With the above mentioned design you  can 
assume to achieve better than -130dBc/Hz noise at 1Hz, and a floor of about 
 -162 to -165dBc/Hz for the amp. That matches the Crystek VCXO performance 
you  will get pretty well.
 
All that said, in my experience there are really few applications where you 
 truly need a -100dBc/Hz at 1Hz noise performance in a radio transceiver, 
other  than for maybe Radar type applications that analyze data down to a 1Hz 
offset  from the carrier.. But this is time nuts after all, so every dB 
matters.
 
Bye,
Said
 
 
In a message dated 9/26/2013 23:15:16 Pacific Daylight Time,  
Tom_minnis at att.net writes:

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