[time-nuts] Frequency Divider
hmurray at megapathdsl.net
Thu Apr 2 09:09:59 UTC 2009
> Start with a buffer amp and then a decent Schmidt trigger.
If you have a clean input signal, a Schmitt trigger doesn't solve any
problems. It does help if you have a slowly rising signal such that noise
might be significant while the signal is near threshold. A 10 MHz sine wave
is slow relative to AC logic.
Since we were recently speaking of LPROs, their user manual has a section on
how to convert 10 MHz sine waves into TTL signals. None of their suggestions
used Schmitt triggers.
This feels like the sort of thing that should have been hashed out here by
now. Is it time to start a FAQ?
My straw man would be to capacitive couple into a 74AC00 that's biased
halfway between VCC and GND. That's clean and simple. A transformer would
break ground loops. A differential input chip might reduce jitter from noise
on the power supply.
> Feed it to a symmetrical divide by 2 for 5 Mhz, and a symmetrical
> dive by 10 for 1 Mhz.
> It seems the crowd is against 7490s, and 74390s - and I would like to
> know what the crowd recommends as suitable.
Dividing by 10 is simple. Doing it with symmetrical output takes a bit
more/different logic than comes prepackaged in a single DIP, or at least not
any that I'm familiar with.
Plan A would use a 4 bit loadable counter and load it with 3 when it reads 12
so the top bit would be off for 5 cycles, 3 through 7, then on for 5 cycles,
8 through 12. That's reasonable to implement in old TTL DIPs. 12 is easy to
decode, just a 2 input gate since states 13-15 won't happen. 74xx163 and
Plan B would be to use a PAL or CPLD. I don't know of any that are available
in DIP, have free design software, and are easy to program without a fancy
programmer. There could easily be something I don't know about. I know that
Xilinx CPLDs have free software (WebPACK) but they don't come in DIP. A
friend has written software to program them, but he's a wizard so I don't
know if mortals could do it. WebPACK may do the programming if you have a
gizmo. One is available at a reasonable price from Digilent.
This technology is too handy. There is probably some hobbyist friendly setup
out there. You may have to build a programmer.
These are my opinions, not necessarily my employer's. I hate spam.
More information about the time-nuts