[time-nuts] 5 MHZ PIC PPS Divider?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Apr 15 19:42:42 EDT 2008

Stanley Reynolds wrote:
> Thanks Pete not sure my wife will let another big box in the door. I was thinking of some thing like this :
> http://www.onsemi.com/PowerSolutions/product.do?id=MC100E137FN
> a ECL 8-Bit Ripple Counter. And a 1Ghz oscillator make the measurement of  differences. This should get me to the 1ns level ?
> But looking at the sampling heads may be close to what I want, I will look for some manuals on the web. I would want to transfer the data to a PC, would I need a GPIB Interface and some software ? I'm trying to eliminate the extra stuff like the scope tube and it's power supply and end up with a small box to interface to the computer.
> Stanley,
You can achieve subnanosecond timing resolution much more easily using a 
pair of ADC's sampling a quadrature pair of 10MHz sinewaves on the PPS 
leading edge using a Linear technology LTC1407A-1 dual simultaneous 
sampling ADC (available  from LTC on a demo PCB complete with FPGA all 
you need is store and process the data). With care a resolution 
significantly better than 100ps is possible. This only has a 100ns range 
which should be more than sufficient for use with a timing receiver like 
an M12M or M12+T. Logic to count cycles can easily be added. The 
simplest and most reliable method being to synchronise a version of the 
PPS signal to both 10MHz zero crossings and use these synchronised 
outputs to sample a synchronous counter clocked by the 10MHz signal. One 
of the counter samples will be reliable and the other suspect due to 
metastability issues. The associated ADC samples can be used to work 
(calculate the 10MHz phase angle at the unsynchronised PPS leading edge 
and hence decide which synchroniser output meets the synchroniser 
flipflop setup and hold time constraints) out which of the 2 count 
samples is reliable.

Another technique is to use a triggered ramp as to interpolate the time 
to the next 10MHz zero crossing.
This is very easy to do particularly when a modern capacitive charge 
redistribution ADC is employed - its input capacitance just becomes part 
of the ramp generator circuit.
Resolution of 200ps or better is readily achieved without needing to use 
any expensive ECL parts.

Either method is far cheaper and has higher resolution than sampling a 
counter clocked at 1GHz clock.
A synchroniser clocked at 1GHz is essential for reliable operation when 
sampling the count of a 1GHz counter.

Using a ripple counter is a particularly bad idea, guaranteeing reliable 
sampling is likely to be difficult to impossible unless the counter is 
capable of reliable operation at several GHz.
The problem being the ripple clock propagation delay from one flipflop 
to the next. For this counter the input clock to output transition delay 
is typically over 4nsec whilst the clock to Q0 delay is about 1.7ns a 
difference of 2.3 cycles at 1 GHz.


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