[time-nuts] Frequency divider design critique request
bruce.griffiths at xtra.co.nz
Fri Jul 11 19:32:29 EDT 2008
Magnus Danielson wrote:
> Hej David,
>> Magnus Danielson:
>> 1) Please could you clarify what you're proposing with the series resistors?
>> I get the idea about 10nF in parallel with R24-R26, though I'm not sure what
>> the benefit is? Those MUX control pins are going to sit pretty hard on 5V
>> or pulled down to ground.
> OK. I was thinking EMC here. You may think that this is a DC part of the design
> but infact you have "DC-signals" hitting a board with sharp edges, so you could
> very well have transmitter antennas here. Also, the succeptability of those
> inputs to E or H field disturbance will affect the output of the mux, so
> attempting to reduce the efficiency as E and H field antenna should be a
> benefit for the stability. Bypassing the input connection from the push-
> button to ground with caps just where deas leads hit the board will make the
> E-field susceptability low, but the low loop area over to the resistors and
> chip will also make the H-field susceptability low. Trying to analyze such a
> problem would take either a good feeling about things, luck or experience to
> quickly pinpoint. It is cheap enought to reduce the problem, even if I don't
> think the risk is imminent.
> Do you follow my lines of thought?
The attached circuit should be a little easier on the relatively
delicate thumbwheel switch contacts (particularly if the capacitance of
the filter caps is large) whilst still providing adequate filtering.
The 74HC14 is merely indicative of logic with a low dc input current.
The circuit is essentially a low pass filter which rejects any high
frequency pickup in the switch wiring.
Typical thumbwheel switches are have either closed or open contacts.
When closed the associated wiring acts as a loop antenna picking up any
hf magnetic fields.
When the switch is open the wire acts as a short whip antenna terminated
by the pullup (or pulldown) resistor picking up any hf electric field.
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