[time-nuts] Wilkinson TDC

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Apr 29 18:51:37 UTC 2012


The circuit for the Tek 2440 is in the manual.
However, it isnt that well executed.
Relying on the overload recovery of an unclamped jfet input opamp limits 
the recovery time and performance as does the unisolated input 
capacitance of the opamps used to control the current source transistor 
emitter currents, the Wavecrest interpolators which incorporate several 
refinements to improve the transient response of the current sources are 
far better in this respect.
The Wavecrest interpolators also have sub picosecond resolution although 
their noise is around 3-6ps.

Bruce

paul swed wrote:
> Do you have an actual circuit?
> It looks a lot like the  old hp5360 counter interpolator.
> Regards
> Paul
>
> On Sun, Apr 29, 2012 at 3:51 AM, Bruce Griffiths<bruce.griffiths at xtra.co.nz
>    
>> wrote:
>>      
>    
>> The essentials of a Wikinson TDC can be simplified to the attached circuit
>> which only requires the addition of a zero crossing comparator to monitor
>> the voltage across the capacitor C1.
>>
>> A few refinements to improve the capacitor charging current switching
>> transitions and the addition of an upper voltage clamp together with the
>> use of faster transistors may be useful.
>>
>> Apart from level shifting to drive the npn and pnp longtailed pairs only a
>> 2 bit shift register is required for the synchronisers reducing the number
>> of external (to an FPGA or CPLD) logic packages required.
>> The jitter of the count logic etc., isn't critical and can be implemented
>> in an FPGA or CPLD.
>>
>> With a 100MHz synchroniser and counter clock a resolution of 10ps can be
>> achieved with a 1000:1 ratio between charge and discharge currents.
>>
>> Bruce
>>
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