以下是我征求意见的方法。我其实做了一个改进的尝试：原来两个输入都是50欧，而我把一个设置成1m欧后，稳定度又提高了一倍。
i found a way to measure/compare two frequencies to 1e-12
in 1 second by sr620.
this is desirable in order to measure the allan deviation
for tau = 1 second or adjust frequencies in real time.
by surfing the net i found an article from srs describing
a way how this can be done at:
http://www.thinksrs.com/downloads/pdfs/manuals/prs10m.pdfthis is the prs10 manual, refer appendix b for details.
the principle is:
if rs620 is used to measure frequency, the rms jitter is
is typically 100ps. however, the jitter will be reduced to
25ps if we measure time interval. if we take 1000 samples
per second and average the result, the rms jitter will be
less than 1ps (25/sqrt(1000)).
i tried this method several time before by feeding the a
and b input the same 10mhz source, and the result is not
as good as promised. it always jump more than 25ps and
sometimes more than 50ps. when i look at the jitter:allan
(for every second), it something like 20ps. until today
i found a way to modify it by:
1) tee the input again and use this 3rd 10mhz as the
external reference for sr620 (i use sr625 as reference
before, which is wrong)
2) make the input a none 50 ohm input.
when i look at the jitter:allan again it says <9ps!
and the mean reading is very much stable to <2ps!
i'm confident (sigma=2) that the consecutive pair will not
change more than 1ps because about 95% of the reading
will be changed not more than 1ps. here is the short video
to demonstrate this (6.0mb):
http://www.dl-car.com/~2038/time/dscn5103.movtherefore, if we replace input b with another 10mhz, we
can observe the drift of a few parts in 10^12 in 1 sec.
if we can record 100 readings (in 100 seconds) and make
some calculation (in excel, say), we will get the allan
deviation down to perhaps 1e-12.
here is the example of my calculation:
video (9.6mb):
http://www.dl-car.com/~2038/time/dscn5104.movcalculation:
http://www.dl-car.com/~2038/time/adev.xlsinput a: hp58540a, holdover
input b: trimble thunderbolt locked to gps
any comments of corrections?
