Bruce Griffiths wrote: > Martyn Smith wrote: >> Hi, >> >> Two questions for all of you today. >> >> 1st Question >> >> I can measure 10 MHz frequency to an accuracy of about 2 parts in >> 10E-12 in a one second gate time. >> >> I use a SR620 time interval counter and make a timeAB measurement. >> >> I make two measurements of phase (between my UUT and my reference) >> and then calculate the frequency. >> >> I have some nice software that controls the SR620 and does all the math. >> >> I'm sure most of you understand my measurement technique, since it's >> as old as the hills. >> >> Does anyone have a good application note explain this standard >> procedure? I'm trying to explain it to a friend, and can't find a >> nice application note with some diagrams, formulas etc. I know HP >> did one and Standard did as well, but can't find any quickly. >> >> 2nd Question. >> >> I really need to measure 10 MHz to an accuracy of 1 part in 10E-13 / >> second (ignoring the accuracy of my workshop standard). I've tried >> multiplying the 10 MHz to the GHz frequencies, but never manage to >> improve on the technique I mention in my first question. >> >> Any ideas for a relatively cheap way of doing this. I actually >> represent a company that can measure to parts in 10E-15 in one >> second, but their boxes costs $50k. >> >> Best Regards >> >> Martyn >> >> > Martyn > > To achieve this you need: > > 1) A frequency standard that has an ADEV < 1E-13 for Tau = 1 sec. > Such sources tend to be rare and expensive. (eg hydrogen maser, BVA > OCXO etc). > The frequency of the standard also has to be known to better than 1E-13. > > 2) Heterodyne techniques can easily achieve the required resolution > but achieving the low noise and drift isnt easy: > a) The zerocrossing detector shaping the mixer beat frequency > output has to be designed to amplify the slope whilst minimising the > noise. > The naive design approaches often advocated are inadequate. > Low frequency ground loops can easily corrupt the measurements. > b) The temperature of the mixer has to be held constant to better > than 0.005C. > > c) The zero crossing detector temperature has to be held to > constant to better than 1C. > A naive zero crossing detector design may require temperature > control to 0.001C or better. > > d) A low noise stable offset frequency source of accurately known > frequency is required > > Even when all of the above conditions are met the measurement range > may only be around 0.1Hz or so. > > Bruce >
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