[time-nuts] Frequency Divider
bruce.griffiths at xtra.co.nz
Thu Apr 2 02:49:32 UTC 2009
Ideally one would use something like a Johnson counter or a Grey code
counter, decode the required outputs and then resynchronise the decoded
outputs using a set of dedicated flipflops.
Since such counters only change the state of one flipflop at a time it
is possible to avoid decoding glitches.
The resynchronisation flipflops minimise the clock to output delay and
its associated tempco as well as minimise the jitter.
Although the decoders can have glitch free outputs there will be some
edge delay patterns that repeat every ten input clock periods.
If one is using a set of retiming/resynchronisation flipflops then
decoder deglitches are relatively unimportant as they are eliminated by
the retiming flipflops.
One could use a 74HC4017 and produce the 5MHz output as:
5MHz = Q0 +Q2+ Q4 + Q6 + Q8
which will be glitch free.
However resynchronisation will be necessary to minimise periodic
variations in edge delay for this output.
1MHz = Carry Out
Brian Kirby wrote:
> Maybe we all could come up with a separate new board to take 10 Mhz and
> give us 5 Mhz and 1 Mhz out.
> Start with a buffer amp and then a decent Schmidt trigger. Feed it to a
> symmetrical divide by 2 for 5 Mhz, and a symmetrical dive by 10 for 1
> Mhz. These out puts could be buffered with 74AC04 for TTL. Another set
> of outputs could be derived and filtered to give sine wave outputs.
> It seems the crowd is against 7490s, and 74390s - and I would like to
> know what the crowd recommends as suitable.
> Brian KD4FM
> John Ackermann N8UR wrote:
>> Hi Bruce --
>> Good point; I'll leave it to the software guys to figure that out. :-)
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