[time-nuts] Looking for good SR620 setup to compare GPS and rubidium
Hi , I have looked through the archives occasionally, and finally decided to subscribe... I compare 10 MHz frequencies using an SBL-1 mixer and a RS232 enabled DVM. Connect the two frequencies to be compared to the IF and RF ports, bypass the LO port for RF, and measure the DC at the LO port. The DVM can be used with software to either save, or graph the results. Tweak it and come back in a few hours to see how you did. The LO port must be DC coupled. TUF-1, SBL and many others from MCL fit the bill. If you don't see a very slow change, then you're not very close. A dual trace O'scope can be used to get close in a hurry. Just put each signal into its own channel, and trigger from one channel. Adjust to make the traces stand still. My scope is not very fast, so I go to the mixer from this point. If your scope is good enough to see one cycle at a time, or so, you might do better. I find this method very easy to use. If the voltage is low, you might try an opamp to boost the DC a little. I have not found it necessary. Recently I compared the Pyro-Joe offering against the Shera controller and 10811. Fun stuff ! I put this together from the junk box so expense was zero. Accuracy is limited by your patience. Mike KD7TS ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
[time-nuts] Looking for good SR620 setup to compare GPS and rubidium
Folks, We have a rubidium oscillator in a laboratory here that is ripe for calibration against a primary standard. We have installed a Datum 9390 GPS receiver next to it as well as an SR620 counter. Can any of you recommend a good operating mode to make the SR620 reveal the rubidium drift rate in a reasonably short time a couple days is OK) so that we may adjust it to near zero? We have both 10 MHz and 1PPS available from both sources. We don't currently have a logging computer connected to the SR620, but we can do that if needed. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Looking for good SR620 setup to compare GPS and rubidium
Hi David: The most straight forward way is to make a time interval measurement between the two 1 PPS signals. It's good to use a BNC-T connector at the SR620 front panel and a scope so that you can set the trigger levels at 50%. If both sources are TTL (0 to 5 volts open circuit) and you use the 50 Ohm termination in the SR620 then the pk-pk will be 2.5 volts and a trigger level of 1.25 volts works well. By setting SAMPLE SIZE size to some number like 500 (seconds since your have 1 PPS input) you can then DISPLAY either MEAN or JITTER. The latter is a very good indication of what's going on. Have Fun, Brooke Clarke w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com David Forbes wrote: Folks, We have a rubidium oscillator in a laboratory here that is ripe for calibration against a primary standard. We have installed a Datum 9390 GPS receiver next to it as well as an SR620 counter. Can any of you recommend a good operating mode to make the SR620 reveal the rubidium drift rate in a reasonably short time a couple days is OK) so that we may adjust it to near zero? We have both 10 MHz and 1PPS available from both sources. We don't currently have a logging computer connected to the SR620, but we can do that if needed. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Looking for good SR620 setup to compare GPS and rubidium
David Forbes wrote: Folks, We have a rubidium oscillator in a laboratory here that is ripe for calibration against a primary standard. We have installed a Datum 9390 GPS receiver next to it as well as an SR620 counter. Can any of you recommend a good operating mode to make the SR620 reveal the rubidium drift rate in a reasonably short time a couple days is OK) so that we may adjust it to near zero? We have both 10 MHz and 1PPS available from both sources. We don't currently have a logging computer connected to the SR620, but we can do that if needed. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts . This is a similar measurement problem to what I'm doing at home. Count the rubidium oscillator's output down to 1 pulse per second, and feed that 1 PPS into a length of 50-ohm coax. (I use a PTS-50 distribution amplifier for this; it has a divided-by-N output, for which I set N such that it provides 1 PPS.) Attach that 50-ohm cable to an oscilloscope, uwing a 50-ohm terminator at the oswcilloscope input. Observe the waveform, and pick a voltage level that will be convenient for triggering the scope (soon, the SR620 counter) such that the trigger point will not be susceptible to any errors from ringing on the waveform. Now set Set the SR620's A trigger level to that same voltage, and move the cable from the oscilloscope to the SR620's A input. (You can remove the 50-ohm terminator now, and use the SR620's internal 50-ohm terminator.) Using a similar procedure with the 1 PPS output of the GPS receiver. Use the oscilloscope to determine a suitable trigger level, then connect the 1 PPS from the GPS receiver, via a length of 50-ohm coax, to SR620's B input. Set the SR620's B trigger level to the level you determined with the oscilloscope, and enable the B input's 50-ohm terminator. Set the SR620 to its +time mode, to measure the time interval between the reference 1 PPS (counted down from your rubidium oscillator) and the 1 PPS from the GPS. This is your relative phase measurement. Set the sample size (I'll call it N) on the SR620. For a quick look, set N=1, and configure the SR620 to graph its output on your oscilloscope. (The SR620's rear panel X and Y outputs go to two channels of your oscilloscope, with the oscilloscope set to display the output in XY mode, with 1 volt per division on X and 1 volt per division on Y.) Set the SR620 display its strip chart of relative phase. With N=1, it takes 250 seconds to fill the strip chart, after which it starts re-painting the strip chart. Now increase N, to give you a longer time to obswerve the trend of the relative phase measurements. The slope of the graph will tell you the frequency error. If your rubidium oscillator is high in frequency, the slope will be positive; if low in frequency, the slope will be negative. My rubidium oscillator (a Stanford Research Systems model PRS-10) has a 10 MHz output. The 10 MHz from the PRS-10 goes to the 10 MHz input of the PTS-50 distribution amplifier. One of the PTS-50's 10 MHz outputs goes to the rear panel external frequency reference of the SR620. And, as I mentioned before, the 1 PPS output of PTS-50 goes to the A input of the SR620. If your rubidium oscillator has a 5 MHz rather than 10 MHz output, it can still be used as an external reference for the SR620; just configure the SR620 accordingly. I hope this helps. I didn't describe everything, as the SR620 manual tells you how to do all this. - James Maynard, K7KK Salem, Oregon, USA ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
