[time-nuts] Happy Holidays

Richard H McCorkle mccorkle at ptialaska.net
Fri Dec 26 23:46:04 UTC 2008


Bruce,

Thank you for your critical evaluation of the interpolator circuitry.
I have no doubt that further improvements to the design are possible
and hope to inspire other amateurs on the list to come up with an
improved interpolator design. The circuit shown provides good results
with very low temperature sensitivity, and I recommend building one
and testing it your self before saying it needs further work to be
useable. What I found in 7 months of testing different variations of
the interpolator (including biased and commutating diode versions as
you have suggested) were:

1. A high-gain constant current source provides the best ramp linearity.
2. Larger capacitors with higher currents produced better stability
   with less effect from stray capacitance.
3. A saturated differential switch had the least effect on ramp
   linearity of numerous switching arrangements tested.
4. With a saturated switch the voltage span across the capacitor can be
   maximized so minimum amplification of the sample voltage is required.
5. Minimum amplification of the sample voltage minimizes the temperature
   sensitivity as voltage variations due to capacitance changes with
   temperature are not amplified.
6. A differential switch provided faster turn-off switching and less
   sample “droop” at turn-off than using a high-speed commutating diode.

  The transistor switching time is not as critical in this design as
there is always a sample at least 1 clock cycle in duration. Using a
saturated switch insures it has minimum effect on the charge current
delivered to the capacitor by the high-gain constant current source.
Slower turn-on times reduce the minimum count returned, reducing the
data offset, but as long as the turn-on time is consistent and less
than 1 clock cycle in duration the switch is fully saturated and the
ramp is linear at the beginning of the sample period and has little
effect on the data. A slower turn-off time due to saturation increases
the maximum count returned but as long as the transistor turn-off
time is consistent and the maximum sample voltage is below current
source saturation the data span is linear and consistent.
  When a high-speed switching diode was used instead of the
differential switch the diode conducted for a longer period than
the switch transistors and reduced the sample voltage from its
peak value. With similar transistors in the differential switch
the voltage ramp stops abruptly with no turn-off “droop” evident
in testing. Choice of capacitors is left to the user but 5%
tolerance NPO/COG or mica caps should be used for best temperature
stability. The discharge switch can be replaced with any suitable
device but the 2N7000 switches in 10ns, is readily available, low cost,
and with large sample capacitor values has minimal effect on the data
returned. Testing of the charge linearity without the differential
switch was performed on a similar design with the results shown in
the attached file. The linearity with a saturated differential switch
in the actual interpolator is similar but has not been documented yet.

Richard


>> Bruce Griffiths wrote:
>> 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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