[time-nuts] Frequency divider PCB: Current status on"pre-orders", and pointers to documentation.

Bob Camp lists at rtty.us
Thu Mar 18 16:46:14 UTC 2010


Hi

I think I'd just take the design over to a reasonable CPLD and be done it,
if you are trying to improve it's floor. Having everything on a single
"lump" of high speed silicon takes care of a lot of issues. 

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of David C. Partridge
Sent: Thursday, March 18, 2010 8:41 AM
To: 'Discussion of precise time and frequency measurement'
Subject: Re: [time-nuts] Frequency divider PCB: Current status
on"pre-orders", and pointers to documentation.

I've just had in interesting offline chat with Warren S. 

Basic outline is that this divider is plenty good enough to use with e.g. a
TBolt (which was my design target), and probably also for the less than
totally committed time nuts.

To quote him:

"What I'm saying is does not meet the Nut cases description of a low noise
buffer / divider."
For good performance, do make absolutely certain that the power supply for
this board isn't fed from the same PSU as your Rb or GPSDO, or if it is,
make sure you take the regulated 12V from that supply and feed this board
through another stage of cleanup using a 5V linear regulator and decoupling
caps right close to the power input connector.   Linear supply rather than
switched (I admit that mine is switched, then post regulated).  Star
grounding of course.   

But for the ultimate time nuttery level of low jitter (low pS level), the
following changes would NEED to be made.

1) Transformer coupled input clock shaper with multi-stage filtering to
restrict input fed to comparator to only the intended single input frequency
(10MHz).

2) Change the power supply to the board to be (say) 12V or 9V.  Then use
several 5V on board regulators to feed:

 a) the clock shaper
 b) the main '163 divider
 c) the 4017 divider chain
 d) each 74AC541 output driver and related FFs.

Did I get that right Warren?

This ensures that if e.g. the switched output is set for 1Hz, that the 5V
supply for the other parts of the board doesn't twitch in sympathy when the
'541 switches.

3) Maybe use BIG earth bond terminal and feed +ve supply in on single
header.

Warren, please jump in to add further points

Warren is of the view that using COGs and thin film resistors isn't
necessary except possibly to use thin film parts for R4 & R5

Regards,
David Partridge
Email:david.partridge at perdrix.co.uk

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of David C. Partridge
Sent: 17 March 2010 18:28
To: TekScopes at yahoogroups.com; TekScopes2 at yahoogroups.com;
hp_agilent_equipment at yahoogroups.com; TestEquipTrader at yahoogroups.com;
'Discussion of precise time and frequency measurement'
Subject: [time-nuts] Frequency divider PCB: Current status on
"pre-orders",and pointers to documentation.

The current situation is that I have almost enough statements of intent to
get to the magic 50 which will allow a price of GBP14.50 per board plus
delivery.   For the avoidance of doubt, this is the price for a bare PCB,
not for a kit, and definitely not for a made up board.

I intend to "keep the book open" until 18:00 Zulu (UTC or GMT) on Sunday
21st March, I will then count up what I have and order that many boards (and
maybe a few over to get a nice round number).

I've received numerous reqeusts for the design documentation, schematic, and
a bill of materials

They can all be downloaded from my website, but there's no way (yet) to
navigate to them (a round tuit problem).

Write up:

<http://www.perdrix.co.uk/FrequencyDivider/Frequency%20Divider%202.pdf>

Schematic:

<http://www.perdrix.co.uk/FrequencyDivider/Frequency%20Divider%202%20Schemat
ic.pdf> 

and BOM:

<http://www.perdrix.co.uk/FrequencyDivider/Frequency%20Divider%202%20Bill%20
of%20Materials.pdf>

The schematic and write up have both been updated today, and the BOM is new
today.

For those who worry about SMT soldering, you don't need a reflow oven, it
can all be done with tweezers, a small tipped iron, fine solder wire, and
liquid flux (or a flux pen).  A good pair of strong reading glasses helps
too!   See:

<http://www.curiousinventor.com/guides/Surface_Mount_Soldering/101>

I've also had questions on part pricing:  Back in 2008, the cost to populate
one PCB using a MAX999, thick film resistors, and standard (X7R) chip
capacitors was about GBP28 including Molex headers and SMB sockets.   I
don't expect it to be massively different now.   I'm afraid I don't have
full parts kits, and the necessary up front costs to do so is more than my
finances allow at present.

FWIW, the ADCMP600 is a bit pricier than the MAX999, and is supposed to be
"better", though I'm not sure in what respects it is better.

If you want the lowest possible level of phase noise, you would follow the
bill of materials recommendations and use thin film resistors and C0G
capacitors in the clock shaper part of the circuit at the very least, but
this adds considerably to the cost (for example 100nF C0G 1206 capacitors
are about 1 pound each, while an X7R part is only a few pence).

Regards,
David Partridge


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