[time-nuts] Question about precise frequency / phase measurement

[Bruce Griffiths]

Using a dual mixer time difference system (either the digital dual mixer 
time difference (DDMTD) or the analog variant (DMTD)) can easily achieve 
the required resolution.
The DDMTD is relatively cheap to implement however it requires an offset 
oscillator to beat against the 2 signals being compared.
However a DDMTD can use a 5MHz offset oscillator can be used with 5MHz, 
10MHz, 15MHz ... input signals whereas a DMTD requires a 10MHz offset 
oscillator to be used with 10Mhz input signals.

The DDMTD uses a pair of shift registers clocked by the offset source 
where each of the 2 signals being compared is connected to the data 
inputs of its shift register.
The time difference between beat outputs of the 2 shift registers is 
then measured with relatively low resolution.
Some digital filtering of the shift register output transitions is 
usually required.
A pair of 74HC164's will typically have a equivalent input jitter of 
around 10ps or so, a 74AC164 will be about 4x quieter.

With a 5.000055MHz offset oscillator and 10MHz inputs the shift register 
output beat frequency will be 110Hz.

It is usually advantageous to use an FPGA to implement the digital 
filtering, timestamping and even the shift registers (although external 
shift registers will have less crosstalk).

Bruce

skywatcher wrote:
> Hello @all,
>
> my name is Wolfgang and i'm new to the list.  :)
>
> I browsed through the list archive, but i didn't find the infos i 
> need, so i decided to join the list
> and to ask the experts directly.  :)
>
> I want to measure the frequency difference between a 10 MHz OCXO and a 
> 10 MHz Rubidium.
> I think that's what many people here have done many times... but i 
> don't want to use expensive
> equipment like time interval counters with picosecond resolution etc. 
> I would prefer a cheap and
> easy solution. I also would like to have an update rate of more than 1 
> measurement per second,
> or even more.
>
> My first approach was to use a simple XOR phase comparator. I tried a 
> 74HCT86 and a 74HCT4046.
> It works, but it's very noisy, so i don't get better than about 10 mHz 
> frequency resolution.
> If i look at the lowpass-filtered output i don't see a nice sine or 
> triangular wave, but it looks more
> than a triangular wave with round tops and some bumps between them. 
> Another problem is that the
> difference frequency gets very low when the frequencies are very 
> close, so it's not enough to look
> only for zero crossings of the difference signal.
>
> Does anybody know a possibility to get a resolution < 1 mHz ?
>
> Best regards,
>   Wolfgang
>
>
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