Termination of AC coupled 100MHz clok to generate low jitter 50MHz clock?

[Nial Stewart]

I have a requirement to terminate an AC coupled 100MHz clock and produce
an LCVCMOS (3.3V) 50MHz clock with low jitter (the amount of jitter isn't
specified).

The input is specified as an "AC coupled signal of 1mW into 50 ohms".

I don't have any experience with low jitter clock distribution so am
unsure how to approach this. I currently have the clock terminated then
ac coupled and biased into a LT1715 (150MHz comparator) feeding a single gate
d type flip flop but am not confident this is the best approach.

Once you've all stopped rolling around laughing, I'd appreciate any
pointers to a better approach.


Thanks in advance,


Nial.

 

[Nial Stewart]

The LT1715 won't go faster than 150MHz

Bill,

Is this a problem if the input clock is 100MHz?

The Analog Devices AD96685 might be worth looking at, if you can live
with ECL outputs.

As usual my request wasn't fully spec'd. I'f prefer to power whatever I'm
using just 3.3V, I can provide 5V if I _have_ to.

John Larkin's point about a tuned circuit in the front end is a good
one. I once used a half-wavelength of delay line between the inverting
and non-inverting inputs of a comparator to get a little more voltage
drive, but three feet of even minature coax could be a bit too bulky
for comfort.

Space is fairly tight


Thanks,

Nial.

 

[Joerg]

My inner Diogenes tells me that 'low jitter' with no specification means
one of two things: they'll never check, so you can do whatever you want,
or no matter what you do they'll come back and complain that it jitters
too much.

Press for a jitter spec on your output clock, and one for the input
clock, too. Best case you'll just be able to do the comparator thing and
call it good. Worst case you'll have to make some fancy PLL circuit to
clean up the jitter while tracking the input phase and frequency. It all
depends on what you need and what you're starting with.

Or use the voodoo method, build an oscillator and do injection locking.
That's almost guaranteed to produce some jaw dropping in the design
review

 

[Nial Stewart]

Thanks again for the feedback Bill.

The curve in the data sheet is for a typical device, not worst case,
and it shows you needing 5mV swing at the inpot to get a logic swing -
2.5V - at the outputs, a gain of about 500.

The data sheet doesn't give upper and lower limits on maximim toggle
frequency, but it does give worst case propagation delay ranges from
66% of nominal to 150% of nominal, which presumably scales with
maximum toggle frequency. Your 100Mz sits on the worst case maximum
toggle frequency that one might guess on this basis.

More details of the design....

Hmm. The 1mW into 50ohm input I'm told produces about 0.6V (not
what I calculated but a real world measurement from the client).

The maximum temp this will operate a is 70 Deg, with a 3.3V supply
directly driving a clock buffer with max Cin of 6pF.

The performance characteristics on page 5 (typical) would suggest that
I'm no-where near even the 150MHz limit, even degrading this to 66%
means I'm still OK.

No?


Nial.

 

[Fred Bartoli]

Joerg a écrit :

Or use the voodoo method, build an oscillator and do injection locking.
That's almost guaranteed to produce some jaw dropping in the design
review

Used that once to sync a local small uc to the main ref clock of a real
sensitive lock-in sync detector and IF board.
Run the ref clock track near the uc crystal and voilà.

 

[Joop]

Or use the voodoo method, build an oscillator and do injection locking.
That's almost guaranteed to produce some jaw dropping in the design
review

At the time I was not aware of the term or concept "injection
locking", but for personal use I created a PIF12Fxx base serial output
frequency logger. Besides running on its own usual setup crystal I
wanted to try and see if this minimalist design could be made to work
on my 10MHz reference as well. The simplest thing I could thing of was
feeding it 10MHz directly to the crystal oscillator input via a few
KOhm plus a cap. It was fun to see it move the required tens of Hz
when the reference was connected.
No switches required. It runs free when standalone and locked when the
BNC is plugged in. Cool..

Joop