[time-nuts] Basic question regarding comparing two frequencies

[Bruce Griffiths]

Hal Murray wrote:
>    
>> There is another way to compare two frequencies, relevant when they   are
>> very close together. I divide a reference down to 100KHz and use it to clock
>> a phase detector made of a pair of D flip flops. The unknown (divided to
>> 100KHz) is fed into the circuit and an output   that is proportional to the
>> phase difference appears on the output as a changing mark-space ratio.
>>      
> I like it.  Thanks.
>
> How did you pick 100 KHz?
>
>    
>> Using CMOS and a precise power supply (because under no load, CMOS
>> output is precisely rail to rail), the averaged output (100ms RC filter) is
>> fed to a strip chart recorder.
>>      
> Has anybody checked the edge cases and/or linearity of a setup like this?
>
>    
>> The recorder shows the changing phase difference and folds back each time
>> a whole cycle passes. A 12 bit analog data logger resolves 2.5ns of phase
>> and gives data for further analysis.
>>      
> Is 2.5 ns good enough?  What would you gain by using a 16 bit DAC?
>
>    
A ratiometric ADC where the ADC uses the (low pass filtered) CMOS supply 
as its reference is probably advisable when using high resolution ADCs.
A high resolution sigma delta ADC that aloows an external reference to 
be used may be useful for this application.
>
>
>
>
> If 2.5 ns is good enough, I'll bet you can do the whole thing in digital
> logic.  Just get a fast FPGA/CPLD.  I haven't done a serious design, but a
> quick check at some old data sheets shows it's not silly.  You could probably
> bump it up by another factor of 2 with some external (p)ECL chips.
>
>
>    
If one used an FPGA with an internal 500MHz (use the internal PLL 
available in some FPGAs) clock and dual edge clocking or a 1GHz internal 
clock, 1ns resolution should be readily achievable. However it may be 
advisable to use something like LVDS inputs to alleviate the effects of 
ground and Vcc bounce.
If you need more resolution then one could always sample the outputs of 
an internal tapped delay line using internal gates as delay elements.
With a suitable FPGA a resolution of a few hundred ps is feasible.
If the delay line delay is more than 1 clock period then an embedded 
calibration of the delay line is possible from the coarse (1ns) count 
and the fine count from the internal tapped delay line.


Bruce