# super fast divide-by-N

Discussion in 'Electronic Design' started by Thomas Magma, Aug 16, 2005.

1. ### Thomas MagmaGuest

Hello,

I am looking for a fast divide-by-N chip with a high bandwidth. I'm hoping
for an input frequency of 1 GHz but would probably settle for one around 80
MHz. Duty cycle doesn't matter for the DSP application I have in mind but I
do need to run at 3V.

Thanks to anyone who knows of one and cares to share,

Thomas

2. ### Jim ThompsonGuest

http://www.azmicrotek.com/

...Jim Thompson

3. ### Thomas MagmaGuest

Thanks Jim,

Nice parts but I can only see a divided by 2 and divided by 4 from that
company. I'm kind of looking for a divide by N. Maybe up to 32 or so.

Thanks,
Thomas

4. ### PeteSGuest

Once you have divided by 2 or 4, you are in the realm of fast logic
(look at the various fast logic families).

One search I would suggest is 'fast prescalers'.

Jim has set you on the right path

Cheers

PeteS

5. ### Jim ThomasGuest

Above, you imply that there are only 32 values of N (1-32). Can you

6. ### Jerry AvinsGuest

That's a pretty broad range, Thomas. Like me asking a bank for a loan of
\$1,250 and saying I'll settle for a dollar.

Jerry

7. ### Thomas MagmaGuest

Can you multiply by 1/N instead?

Ah...yes?... I don't quite know what you mean here. I can see how they are
equal mathematically, but I don't know how to achieve this with electronics.

8. ### Jim GranvilleGuest

Look at OnSemi, under high performance counters, you'll find ones that
go 1.4GHz. You will pay for this performance
If a few hundred Mhz is OK, then look at any modern 32 macrocell CPLD
[ Xilinx / Lattice ], and they are closer to \$1
-jg

9. ### Thomas MagmaGuest

Ha. Let me give you a bit of back ground then. I am really pushed for space.
I need different clock frequencies thru-out my board. My highest frequency
is a Fox RFXO running around a GHz. I would like to derive all other clocks
from this one if possible. Next lowest frequency is around 80 MHz. Then a
few after that. I hate PLLs and I don't want a bunch a xtals on my board. So
I thought I would just divide down from my highest frequency. One chip
solution would be the best.

Thomas

10. ### Chris JonesGuest

The important number is what is the minimum divide ratio that you need. If
the minimum N is a reasonably large number then you can use the
conventional approach used in RF synthesisers, where you have a fast
prescaler (e.g. divide by 8 or 9) and then a slower block of logic that
decides when to switch the prescaler into divide by 8, and when to use
divide by 9.

You might be able to find a RF synth chip where the divider output is
available at a pin. Look at the Analog Devices ADF4111 for example which
can mux out the divider output to a pin.

Chris

11. ### Jerry AvinsGuest

It's a good approach, and the reasoning seems sound, but I was struck by
the very large spread between "need" and "want".

Jerry

12. ### Paul CarpenterGuest

On Tuesday, in article <[email protected]>
Have you looked at ICS <http://www.icst.com/>. You might find this useful
or not

<http://www.icst.com/icscs/PartSumma...f-b504-bb110514c174&name=ICS889874&mode=short>

Upto 2GHz input and various divide outputs 1,2,4,8,16

They also do other dividers and buffers. Usually I find the costs, very
low in small quantities. However distributing all the clocks divided around
the board will be fun. ICS also do small PLL chips that work well and are
cheap, small and could run off of one lower clock drive. Unless of course
you all the clocks synchronised to the highest frequency.

13. ### John FieldsGuest

---
You should be ashamed of youself! <G>

When you ask for a super-fast divider around here, (excuse me for
seeming to be territorially authoritative) without specifying that
it's a clock frequency divider, you raise the spectre of a machine
which can grind out quotients in arbitrary timeslots, the ideal
being loading the dividend and the minuend on one edge of the clock
and then spitting out the quotient on the next edge.

Clock dividers? Tell us what frequencies you need, your source
frequency and the skew you can stand, and you'll get an an answer
soon enough.

14. ### MacGuest

There are DDS's that can run off of a 1 GHz clock. The Analog Devices
AD 9858, I think it is. That might be massive overkill for what you are
doing.

Instead, you could divide the 1 GHz down by 4 or 8 using some kind of
prescaler, and then feed the resulting 250 MHz clock to a CPLD or
something which could implement whatever kind of divide-downs you want,
and produce multiple copies.

You don't say anything about jitter.

I dropped comp.dsp, because it seemed to have more to do with hardware
design.

--Mac

15. ### Jon HarrisGuest

I think he is talking about dividing a clock signal down by N, down dividing a
digital value by N (e.g. in a DSP). I missed that point the first time
too--reading this in a DSP group, I assumed it was a DSP question, but notice
the cross-posts. It wasn't until I read some of the responses that I realized
what he was trying to do.

16. ### Rene TschaggelarGuest

MC100EP016A :
3.3V ECL 8bit synchroneous Binary up counter,
operating frequency > 1.30GHz, LQFP32, [email protected]

Rene

17. ### Paul KeinanenGuest

While a divide-by-N could be implemented with an up-counter and using
some gating to detect N and asynchronously reset the counter, this
will usually have some timing glitches. Usually a divide-by-N is
implemeted with a presettable down counter, when "0000" is detected, N

If you do not need the divide by 1, some synchronous presetting could
be used, i.e. the gating detects the "0001" state and let the next
clock pulse do the actual presetting.

This could be done with up-counters, but now the gating would have to
detect the N-1 condition to perform a synchronous reset at the next
clock pulse.

Paul

Rene

19. ### Guest

Thomas Magma skrev:
I'd think that whenif you find something that will divide 1GHz to
80MHz, it'll be bigger more expensive and more trouble than just adding
a 80MHz oscillator ...

-Lasse

20. ### Jerry AvinsGuest

Lasse,

Nice to hear from you again! I think the OP might want a coherent
division. Maybe not.

Jerry