OT - Generating a 20MHz clock that can be adjusted by +- 2%

N

Nial Stewart

Guest
This is off topic for the news group, but I know there
is a vast array of experience here, hopefully someone
can help.

I've an application where I need a 20MHz clock which can
be adjusted by +/- 2% (rough requirements, the client doesn't
actually know what's required). The timebase of frequency
adjustments isn't critical. I haven't been given any
jitter specs.

This has to be incorporated in a working prototype in
3 weeks so the solution must be simple and fairly
stable.

The VCXOs that I've found are adjustable by at most
200ppm, so this isn't an option.

I'm thinking of using a 74LS624 with a single pin DAC
driving the frequency control input. The data sheet
specifies 20MHz as the maximum output frequency, but
the performance graphs show outputs at up to 30MHz.


The prototype board I'm using has an Altera Cyclone
(EP1C6).

Ideal requirements...
20MHz +/- 2%, easily adjustable.
Simple (single device ?)
Operates from 3.3V supply
Can be sourced from distributor in a couple of days.



Thanks for any ideas,


Nial Stewart
 
Nial Stewart wrote:
This is off topic for the news group, but I know there
is a vast array of experience here, hopefully someone
can help.

I've an application where I need a 20MHz clock which can
be adjusted by +/- 2% (rough requirements, the client doesn't
actually know what's required). The timebase of frequency
adjustments isn't critical. I haven't been given any
jitter specs.

This has to be incorporated in a working prototype in
3 weeks so the solution must be simple and fairly
stable.

The VCXOs that I've found are adjustable by at most
200ppm, so this isn't an option.

I'm thinking of using a 74LS624 with a single pin DAC
driving the frequency control input. The data sheet
specifies 20MHz as the maximum output frequency, but
the performance graphs show outputs at up to 30MHz.


The prototype board I'm using has an Altera Cyclone
(EP1C6).

Ideal requirements...
20MHz +/- 2%, easily adjustable.
Simple (single device ?)
Operates from 3.3V supply
Can be sourced from distributor in a couple of days.
Click to expand...
LTC6900, LTC6903 from linear technology

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
 
Rene Tschaggelar wrote:
Nial Stewart wrote:

This is off topic for the news group, but I know there
is a vast array of experience here, hopefully someone
can help.

I've an application where I need a 20MHz clock which can
be adjusted by +/- 2% (rough requirements, the client doesn't
actually know what's required). The timebase of frequency
adjustments isn't critical. I haven't been given any
jitter specs.

This has to be incorporated in a working prototype in
3 weeks so the solution must be simple and fairly
stable.

The VCXOs that I've found are adjustable by at most
200ppm, so this isn't an option.

I'm thinking of using a 74LS624 with a single pin DAC
driving the frequency control input. The data sheet
specifies 20MHz as the maximum output frequency, but
the performance graphs show outputs at up to 30MHz.


The prototype board I'm using has an Altera Cyclone
(EP1C6).

Ideal requirements...
20MHz +/- 2%, easily adjustable.
Simple (single device ?)
Operates from 3.3V supply
Can be sourced from distributor in a couple of days.


LTC6900, LTC6903 from linear technology
Click to expand...
Also, Maxim have VCO & clock chips, and Cypress have
a range of programmable clock generators.
Even a Cygnal uC will come close : They have
an internal current injection OSC, with a 8 bit DAC,
that is nominally 24MHz, but trims 16-24 MHz IIRC.
You can also build one using a LC osc and a varicap :)

-jg
 
The question isone of adjustable range (2%) vs. inherent stability.
A VXCO gives you very good stability, but limited range (as a result of the
high Q value that is responsible for its stability)

Resistor-based oscillators, like the ones from Linear Technology, offer a
wide range, but are much more affected by Vcc,temperature etc.

If you want both stability and wide range, you are in for complex circuitry,
like DDS phase accumulators etc, Max acceptable jitter then becomes the
issue...
Peter Alfke
 
0f@news.sunrise.ch> <BCAEDB27.6009%peter@xilinx.com>
Organization:
Keywords:

How close does the clock have to be? Is this a one-shot job or
a high volume run.

If it's just a one shot job, how about a collection of crystals.
Just insert the right one. Order them now and you will have them
by the time you could design and debug a fancy circuit.

Peter's "DDS phase accumulators etc" suggestion reminds me that
radios commonly synthize frequencies. I assume they use a PLL
locked to a crystal. There are probably some neat chips for this
purpose, but I'm not familiar with them. Radios generally want
a reasonably stable signal so they probably have low enough jitter.

--
The suespammers.org mail server is located in California. So are all my
other mailboxes. Please do not send unsolicited bulk e-mail or unsolicited
commercial e-mail to my suespammers.org address or any of my other addresses.
These are my opinions, not necessarily my employer's. I hate spam.
 
Nial,
Got a Xilinx with a DCM spare?
Try this crap idea.

Frequency in = Fin

From N= 20 to 32, use a DCM to give you

Fout = (N/(N-1))*Fin
Gives you a range of multipliers from 1.052 to 1.032 in 12 steps
Scale the frequencies accordingly to fit the DCM specs, i.e. use CLKFX at
(say) double your 20MHz. Of course, this gets a lot harder to do on the fly,
but might work for compile time.
Do I win Ł5?

Cheers mate, Syms.

"Nial Stewart" <nial@nialstewartdevelopments.co.uk> wrote in message
news:40892522$0$31708$fa0fcedb@lovejoy.zen.co.uk...
I've an application where I need a 20MHz clock which can
be adjusted by +/- 2% (rough requirements, the client doesn't
actually know what's required). The timebase of frequency
adjustments isn't critical. I haven't been given any
jitter specs.

Thanks for any ideas,


Nial Stewart
Click to expand...
 
Peter Alfke wrote:

The question isone of adjustable range (2%) vs. inherent stability.
A VXCO gives you very good stability, but limited range (as a result of the
high Q value that is responsible for its stability)

Resistor-based oscillators, like the ones from Linear Technology, offer a
wide range, but are much more affected by Vcc,temperature etc.

If you want both stability and wide range, you are in for complex circuitry,
like DDS phase accumulators etc, Max acceptable jitter then becomes the
issue...
Click to expand...
Or something with a 'Medium Q' - the pull-ability/Stability trade off
of an OSC is determined by the Q, and there are other resonance
solutions with intermediate Q's.
Inductors can come in a range of Q's, I thing the highest are silver
plated helical resonators, and SAW resonators can be
made with moderate Q's...
-jg
 
Thanks for all the replys guys, I should have stipulated
that the frequency has got to be adjustable on the fly.

LTC6900, LTC6903 from linear technology
Click to expand...

This looks like my best bet, apart from....

Can be sourced from distributor in a couple of days.
Click to expand...

I can get them from the LTC web site, does anyone have
any experience of their delivery times?



Thanks again for all suggestions,


Nial
 
Nial Stewart <nial@nialstewartdevelopments.co.uk> wrote:

: Thanks for all the replys guys, I should have stipulated
: that the frequency has got to be adjustable on the fly.

: > LTC6900, LTC6903 from linear technology


: This looks like my best bet, apart from....

: > > Can be sourced from distributor in a couple of days.


: I can get them from the LTC web site, does anyone have
: any experience of their delivery times?


The delivery time is very short!

--
Uwe Bonnes bon@elektron.ikp.physik.tu-darmstadt.de

Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt
--------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------
 
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