[time-nuts] Frequency counter questions

[Jerry Hancock]

This is an exercise more than anything.  It started when I noticed the counter in a 3586B I purchased was +1 count unless I dropped the input frequency .25hz.  I was using my GPSDO reference for it and the 3336 it was counting.  I then took my GPSDO and tried reading that directly and I see the same thing.  This is annoying more than anything and I assume there is a slight gate phase related issue.  I don’t have anything to read the jitter on my GPSDO which is a Lucent RFTG-U with the REF1 and REF0 but since I am using it as the reference and the signal under test, it shouldn’t matter at .1hz.  If I run the count over 3 minutes, the count is .923hz high consistently. To put a finer point on it, reading 10,000,000.00hz from my GPSDO, It will read consistently 10,000,000.1 or 10,000,000.0 with the distribution of each to average 10,000,000.923 over 5 minutes.  If I drop the input frequency by .25hz, it will read 10,000,000.0 without a 10,000,000.1 until the cows come home.  I’ve let it run about a day to check. 

So I was thinking, o.k., let’s put a frequency counter on the back to read the F0 output signal rom the 3586B and see what it reads but I then realized I don’t have anything that accurate.  I do have a lot of micro processors around, everything from the lowly PIC thru the DSPIC to the STM32F7’s, F4’s and if I have to, I will get a FPGA and go from there.  I could purchase an HP 12 digit counter if I could find one locally as I am tired of paying shipping but haven’t found one to date. 

So no really pressing reason for the counter other than just curious as to what is causing the issue with the 3586B.  By the way, when I read any of the WWV signals they count +.1hz as well under constant signal.  I am pretty sure the 3586B is recognizing my reference input as the OCXO is dialed to 10,000,000.0 and the standard oscillator is a little high at +.4 (if I disconnect the OCXO strap) and I haven’t looked into it yet.

Thanks for the interest and help

Jerry