[time-nuts] Frequency divider design critique request

Magnus Danielson magnus at rubidium.dyndns.org
Thu Jul 10 18:06:30 EDT 2008


From: "David C. Partridge" <david.partridge at dsl.pipex.com>
Subject: [time-nuts] Frequency divider design critique request
Date: Thu, 10 Jul 2008 21:30:56 +0100
Message-ID: <C2B66AD4CD8D4E64A0C47D3C29559EA6 at APOLLO>

David,

> As I've mentioned before, I've been working on the design of a frequency
> divider to go with my TB.
> 
> The idea is 10MHz sine in from TB, output 2.5Vp-p 50% duty cycle square wave
> into 50R (5V into 1M), at 10Mhz, 5MHz, 1MHz and decade selectable 100kHz
> down to 1Hz.  All rising edges synchronised to the 10MHz clock rising edge
> (or as near as I can get with 74AC logic).

Good initial concept.

> With a considerable amount of constructive criticism from Bruce Griffiths
> (thank you again Bruce) I believe the design now to be complete.   
> 
> The aim is to have as low a level of nasties as possible (i.e. fit for
> time-nuts).
> 
> All faults are my own - no blame attaches to Bruce!

The one thing I would do is to hook caps over at R24 to R26, say 10 nF, to
make the thumb wheel leads less susceptible to noise and less of areal for
the edges from the CMOS. The thumb-wheel either keep them floating in one
end or didged hard to +5V. To avoid both E and H fields, a series-resistor
should be included.

> I've not yet subjected this design to the ultimate simulation tool (PCB,
> parts and solder) yet, and I have no means to test it for levels of jitter
> (phase noise) or similar nasties.

I am sure we can come up with some arrangement for that. Several handy
time-nuts around.

> I think that it's now the right time to open the design up for critique from
> a wider audience before I commit it to copper.
> 
> I'm therefore attaching the design as a PDF file for your comments.
> 
> A few comments are in order:
> 
> 1) The 5Mhz and 1MHz outputs are re-clocked TWICE deliberately to delay them
> by one clock cycle so they line up with the 1MHz and lower outputs.

Neat.

> 2) The selected output (at the '4051 mux) from the ripple counter chain is
> re-clocked to 1MHz before re-clocking to 10MHz as the worst-case delay in
> the chain of '4017s is large enough that the lower frquencies wouldn't
> reliably re-clock directly to 10MHz.  

Good thinking!

> I have also done a PCB layout (4-layer) and I'm happy to send a print of the
> top/bottom layers to anyone who feels that they want to comment on that
> (inner layers are ground and power).

... and you say they are not of interest?

> Let the brick-bats be thrown!

I have a lots of bricks around me (my summerhouse is build with old handmade
bricks) but I wont toss them.

So far, only the thumbwheel is the only minor flaw that I could come up with.

I need to check some more...

I rather have a few questions on why you did not include certain features...

In my experience, having a few extra 10 MHz signals to feed Ext Ref on
instruments is a good thing. That way you can keep these others at hand for
various setups you need to do.

I would consider a dedicated 1 PPS output.

I would consider a synchronise feature with a PPS/synchronise input. It should
be wise to not directly wire it to the counter resets, but provide an arm
button and maybe a very simple arrangement to indicate "left", "on mark" and
"right" with red, gren, red LEDs. Just a tought. The arm button could also
have an electrical input, but I am running into creaping featurism here.
I think however that synchronisation might be a good thing. That way you can
shift the phase of the signal to fit your need. Pulling and inserting the
10 MHz cable is a very crude way of doing it.
Maybe it would be just too much fuzz for too little gain, what do I know, but
I know I would enjoy seeing it.
A pulse-add/pulse-swallow technique (with a shift in initial divide by 10)
could be used to provide inc/dec functionality for a manual movement of phase.

Cheers,
Magnus



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