[time-nuts] PICTIC II ready-made?

David davidwhess at gmail.com
Fri Apr 27 21:44:35 UTC 2012


On Sat, 28 Apr 2012 08:01:43 +1200, Bruce Griffiths
<bruce.griffiths at xtra.co.nz> wrote:

>The Wilkinson TDC (dual slope) has been successfully used for decades in 
>nuclear instrumentation.
>One problem is in switching the discharge current on and off 
>sufficiently quickly.
>This can be largely circumvented by having it on all the time.
>One drawback is the slow conversion speed (100us for a 10,000:1 ratio of 
>charge to discharge current).
>However they can have superb differential linearity.

All of the designs I have looked at leave the discharge current turned
on.  The higher resolution ones adjust it during self calibration.

I prefer to use integrating converters (charge balancing, single
slope, dual slope, and delta-sigma) where possible because of their
noise rejection.

I actually looked into a GPSDO design using a charge balancing DAC to
drive the VCXO because of the low parts count and simplicity.  I still
may try it.

>The problems associated with the jitter associated with an FPGA can be 
>circumvented by using external logic for the critical circuity 
>(synchroniser and current source gating).

I had already concluded that the clock, trigger, and strobe paths need
to be outside the FPGA for minimum jitter.

>Using a FET input comparator is advisable to avoid problems (linearity 
>and stability) associated with the comparator input bias current.

I wonder if substituting an LF311 would be good enough.  I am inclined
to follow the integrator design from the Tektronix 7T11.

>It may be feasible to implement the synchronisers in a small CPLD, but 
>careful selection to avoid those that use an internal preload state 
>machine whose clock runs continuously and not just during startup will 
>be required.

I will have to watch out for that.  Thanks for the warning.

One of the FPGA delay chain implementations I read about got down
below 50ps with heroic self calibration and mentioned that ANY I/O
activity during the measurement significantly reduced the accuracy.

>David wrote:
>> On Fri, 27 Apr 2012 16:30:11 +0200, Attila Kinali<attila at kinali.ch>
>> wrote:
>>
>>    
>>> On Wed, 25 Apr 2012 23:30:45 -0500
>>> David<davidwhess at gmail.com>  wrote:
>>>
>>>      
>>>> If you add a second lower current source or sink, then you can get
>>>> away with a LM311 class comparator and one fast timer channel in the
>>>> microcontroller.  The input pulse width charges the capacitor and the
>>>> timer counts how long it takes to slowly discharge.  Since the
>>>> conversion is integrating instead of sampling, it has better noise
>>>> immunity.
>>>>        
>>> Yes, a dual slope time strecher would work too. I'm not sure, but
>>> i would guess this aproach would be a lot more limited by the noise
>>> and device variations.
>>>      
>> It would be a lot more immune to noise.  Both integrating and sampling
>> designs suffer from the same device variations which can be removed
>> through self calibration.
>>
>>    
>>> Usually a timing input of an uC runs with a counter in the region
>>> of 100MHz max, ie +/-5ns resolution. To get to 50ps, one would need
>>> to stretch it by a factor of 100 at least, better 1000 to get some
>>> headroom for calibration in software. This means that the currents
>>> have to have a factor of 1000 in between. Using a charge current
>>> somewhere between 10 to 100mA would yield to a discharge current
>>> between 10 to 100uA. Keeping the two current sources stabile
>>> enough for the ratio to stay stable would be already quite an
>>> acheivment. Also keeping the leakage currents at bay would be
>>> quite some feat...
>>>      
>> That is about the performance level of the Tektronix 2440 delay time
>> counter.  The counter only runs at 40 MHz but both edges of the 500
>> MHz sampling clock are used with two integrators so that metastability
>> can be detected and resolved.  The charge current is fixed at about
>> 25mA and the discharge current is set during self calibration to
>> maintain a 1250:1 ratio at about 20uA.
>>
>> Stability should not be a problem in the analog design when self
>> calibration is used and that is required at higher performance levels
>> anyway.  Even the high offset voltage and bias current of the bipolar
>> technology LM311 only contributes offset and gain error which is how
>> they got away with 100pf of integration capacitance.
>>
>>    
>>> In contrast to that, a 16bit ADC is dirty cheap and a 24bits are readily
>>> available. I haven't had a look at it yet, but if the capacitive charge
>>> redistribution ADCs simplifiy the circuitry that much as Bruce has said, then
>>> you could get "easily" 16-18bit resolution. Combine that with a 100MHz
>>> reference clock, then you get a nominal resolution 150-40fs(!).
>>> Acheiving 10ps resolution should be then a piece of cake and 1ps possible.
>>> (yes, i know that 10ps is not that easy...)
>>>      
>> Charge redistribution ADCs by design have a built in sample and hold
>> which can simplify external circuitry and like delta-sigma converters,
>> they can be built on a digital logic process.  In this case, the
>> simplification is in comparison to non-sampling converters where the
>> signal level has to be constant during the conversion cycle for valid
>> results.
>>
>> The advantage with the dual slope design is that it is integrating so
>> high frequency noise is ignored.  Controlling noise in a
>> microcontroller sampling ADC even at the 10 bit level is a significant
>> challenge.  In a conservative design, I usually start by figuring the
>> loss of one bit do to DNL and another bit do to noise.  If you want
>> better performance, the ADC either needs to be integrating or external
>> where noise can be better controlled.
>>
>> I have been looking at a better than 10ps performance design but not
>> primarily for GPS timing applications.  I am more interested in
>> equivalent time sampling and high bandwidth sequential or random time
>> sampling.  The later can not use an integrating converter because of
>> sampling rate requirements.



More information about the time-nuts mailing list