DIY calibration of new-to-me Rubidium Oscillator - Page 1
Forum section: electronics / general

author: tkamiya
author_rank: Super Contributor
timestamp: February 03, 2019, 04:42:11 am
content: I am attempting to calibrate a Rubidium Oscillator.I have a GPSDO that has been continuously running over 3 months. This is my current house-standard.I fed this 10 MHz signal to clock-in port of 12 digit counter, HP53131. Gate time is set to 10,000 secondsWith this setup, I can see 100 microsecond digit flickers about plus-to-minus (not plus-AND-minus) 5 counts from time to time, during 24 hour period, but short term it's rock steady. In fact, it's rare to change gate to gate. At this gate time, it takes 2.78 hours to finish counting.I think this is the best I can do, given available equipment. I'm thinking this kind of stability and accuracy is more than I would ever need as a serious hobbyist. My plan to buy a hydrogen-maser has been denied by my-girlfriend....

author: tkamiya
author_rank: Super Contributor
timestamp: February 03, 2019, 04:48:14 am
content: Plus, how do you, the community use Rb standard?From stone cold to physical lock takes about 3 minutes. At this point, it is still fluctuating.At 30 minutes to 1 hour point, fluctuation slows and narrows.At 6 to 12 hours point, it seem to have reached the ultimate stability.I kept it on for 2 days and didn't see any material difference from 12 hour result.Do you keep it off?Do you keep it on?Use GPS disp. Oven XO for usual practice and use Rb when additional stability is needed? (My Rb is stand alone, and no GPS)

author: edpalmer42
author_rank: Super Contributor
timestamp: February 03, 2019, 05:50:14 am
content: The 53131 has a resolution of 1 ns. Why not use it? Stop trying to measure frequency. Measure Time Interval.Feed your Rb to Channel 1. Feed your GPSDO to Channel 2. Measure the Time Interval from the rising edge on Channel 1 to the rising edge on Channel 2. Square waves should be used if possible. They will give you much better results (lower noise) than sine waves. Your counter should be set so that it only makes *one* measurement at a time. No averaging. I don't remember if the 53131 can do this.Connect the 53131 to a computer and feed the data to a program like Timelab. Watch the frequency graph. If the frequencies are identical, the graph will be a flat line. Otherwise, the graph should show a straight line that slopes up or down. If the line has a curve to it, one or both frequencies are changing. You'll probably find that your Rb takes a few hours or more to really stabilize its output frequency, even though it *is* locked. How cool is it that you can really see and measure that?If you don't have any way to connect the 53131 to a computer, you can still do the work by watching the counter to see if the time interval is changing. If the time interval is decreasing, it means that Channel 2 is catching up to Channel 1 so you need to increase the Rb's frequency. If the time interval is increasing, it means that Channel 1 is running away from Channel 2 so you need to slow the Rb down.Your counter's 1ns resolution theoretically allows you to detect a frequency difference of 1 PPB in one second and 1 PPT (1e-12) in 1000 sec. *** Of course, since your reference is a GPSDO that has some wobble to it, your actual results won't be this good.No, you probably don't need to adjust your Rb this close, but since you can, why not?EdEdit:*** I should know better than to try and explain complex things late at night. I forgot to state that this math only works for a one second measurement interval. The easiest way to obtain this is if both the Device Under Test (DUT) and the reference have a 1 PPS output. That gives you a one measurement every second, a zero to one second measurement range, and the stated measurement resolution.If your DUT only has a 10 MHz output, it works better if you make the 1PPS output of your GPSDO the start signal, i.e. Channel 1, and the Rb becomes Channel 2. This maintains the one second measurement interval and gives you a zero to 100 ns range of measurement. Reversing the channels reverses the 'sense' of the frequency difference. That is, if the time interval is decreasing, it means that the Rb is catching up to the GPSDO so you have to slow the Rb down and if the time interval is increasing, the Rb is falling behind and you need to speed it up.

author: awallin
author_rank: Frequent Contributor
timestamp: February 03, 2019, 09:30:19 am
content: the graphs here give you some idea of the averaging times required:https://www.thinksrs.com/products/fs740.htmlanything below a few hours the Rb will be better than a typical GPSDO.the GPSDO probably has quite strong 12-hour periodical movement...about a week or two of recording the time-interval between your Rb and the GPSDO will give a reliable average frequency offset between your Rb and GPS.

author: ArthurDent
author_rank: Super Contributor
timestamp: February 03, 2019, 03:39:46 pm
content: There are a couple of ways I check oscillators out. For a quick check I like the visual method of triggering my scope from one of my GPSDOs and connecting the oscillator I’m testing to the input channel. I can set the sweep rate down to 2ns/div and watch the trace for drift with left high, right low. With 2 channels I can watch 2 oscillators at once.For long term drift/stability I have a Trimble Thunderbolt that had a defective oscillator so after removing the defective oscillator I brought the gnd/efc/10Mhz lines out so I could connect them to test oscillators. If I feed the 10Mhz from a free running oscillator under test into the Thunderbolt I can use the Lady Heather software to plot the drift data compared to the GPS 10Mhz over a long length of time. Here is a plot of a Rb oscillator I was adjusting. The plot is for 8.5 hours and the purple line shows the drift and corrections I made to get it close.