[time-nuts] Generating a solid PPS from 10Mhz source

Bob Camp kb8tq at n1k.org
Fri Jan 15 23:09:53 UTC 2016


The astonishing part of this “new world” is that a very complex chip that is made in high
volume is cheaper than a handful of less popular (but far less complex) chips.

To do this with old style logic, you need to divide by 7 decades. If you do three dual decade
counters, it’s probably easier to use half of another one as to get a different chip. 

Mouser will sell me a SN74LS390 in a DIP package for $1.47 each. The four chips will come
out to $5.88. Yes, there are other chips out there, but not a whole lot. The 74AC390 is no longer
around. The 74HC390 would be cheaper ( 0.67 each) , but lower performance. 

From the same source, a DIP package MCU is $1.37. If I put things in sockets, the costs go up, one
8 pin socket is a lot less than four larger sockets. Same issue with bypass caps, board space and 
all the other “stuff” that goes into a design. 

Putting things in surface mount would save some money, but make the gizmo a bit less “basement 
builder” friendly. There are a *wide* range MCU’s if you real the package to include SMT stuff. You can save
about 40 cents on the AVR by going to SMT. 

At least as big a question comes up when you build the design. Do you do a pc board? If so you likely 
spend more money on that than any of the things above. Do you put it in a box? Again, likely more money
than the board plus the parts. How about BNC connectors? They are not at all cheap. Power costs money
if you buy the wall wart ( and a connector to go with it). 

Even with all the this and that, this is a gizmo that likely costs less than the Pizza Guy will be looking for 
when dinner arrives. I’m not real sure that optimizing the cost is as significant as simply getting off one’s 
back side and actually building one up. We all seem to spend a lot more time talking than doing ...


> On Jan 14, 2016, at 6:44 PM, Morris Odell <vilgotch at bigpond.net.au> wrote:
> Am I missing something here?
> I understand the ease and fun of programming up an AVR as much as anyone but surely this task could be accomplished  easily with a chain of fast synchronous TTL or CMOS dividers. A resynchronising FF could also be added at the end to  clean up the 1 pps if required. You wouldn't need to worry about the maths or OBOBs either.
> Admittedly you would need more PCB real estate but that shouldn't be a big deal in most cases.
> Morris
>>> If anyone is interested in the equivalent functionality using an ATTiny25 (for instance, if you’re already heavily invested in AVR instead of PIC, like I am), ping me. I’ve privately written code to solve almost the same problem and it could easily be adapted into doing the same job.
>>>>> Is there an easy circuit to build that can consistently deliver a 1 PPS from a 10MHz source with excellent resolution and repeatability?  My first application is to test different 10MHz oscillators without a TIC always attached and then compare the PPS output change over time against a master GPSDO PPS with an HP53132A.
>>>>> The circuit used for PPS generation would have to deliver consistent PPS output with preferably not more than 100ps noise or jitter, assuming a perfect source.  I'm totally guessing that for this resolution, the PPS would have to be generated and accurate to within 0.001Hz every second.  If this is too difficult, maybe the integration time can be increased to generate one pulse every 10second or every 100,000,000.00 cycles?
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