[time-nuts] Happy Holidays

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Dec 26 10:28:51 UTC 2008


Bruce Griffiths wrote:
> Richard H McCorkle wrote:
>   
>> Ho Ho Ho
>>
>> Merry Christmas from the far north to all the
>> Time-Nuts. As a gift for the holidays I have
>> created a free version of the PICTIC for you
>> and Didier has kindly posted the files at:
>>
>> http://www.ko4bb.com/dokuwiki/doku.php?id=precision_timing:pictic
>>
>>
>> And to all a good night,
>>
>> Richard
>>
>>
>>   
>>     
> Richard
>
> Your analog interpolator needs a little work.
> The switching speed of the 2N3906's is so slow because the 2N3906's that
> switch the current to ground or into the 470pF cap are saturated.
> They will switch much faster if used as current mode switches.
> In fact you can just replace the 2N3906 whose collector is connected to
> the 470pF cap with a fast switching diode and the current source will be
> switched much faster.
> The 2N7000's have a relatively high non linear output capacitance
> replacing them with something like 1/6X 74HC05 would be better.
> The 470pF silver mica caps will have relatively large dielectric absorption.
> NP0/C0G caps would be a better choice, particularly if higher input
> pulse rates were to be used.
>
> Have you actually measured the linearity of the interpolator?
>
>
> Bruce
>
>
>   
The quickest fix is to drop the differential drive to the 2N3906's and
bias the base of the 2N3906 whose collector is connected to the 470pF
cap at +2.5V.
This will ensure the 2N3906's switch much faster.
The maximum voltage on the 470pF cap should be about 1V to avoid
saturating the transistor whose collector is connected to it.
The opamp gain network should then be adjusted to produce the required
output swing.
If the maximum charging time of the capacitor is 400ns then the charging
current should be about 4.7E-10/4E-7 or about 1.2mA.
The 2N3906's will switch a bit faster if the current is increased to 5
or 10mA in which case the 470pF cap should be replaced by a ~2nF cap
(with 5mA charging current).
Use an NP0/C0G for low dielectric absorption.

Bruce




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