Rb vs TCXO vs OCXO - Page 1
Forum section: electronics / projects

author: lincoln
author_rank: Regular Contributor
timestamp: April 17, 2015, 12:35:45 am
content: My apologies to Lightages, I seemed to have tuned your thread into a pissing contest about oscillators. Here is some data I have collected :This is a comparison of an TS 2700 in holdover/free running vs a reasonable production VC-OCXO vs a good VC-TCXO.Both the OCXO and and TCXO were allowed to come up to temperature. A control voltage was supplied to center the frequency to the nominal of the crystal as close as I could get it. The control voltage supply is using a LT1236 10v reference / dekapot KVD / buffer driver. The data was back processed to remove the frequency offset but not drift for the OCXO and TCXO.The Time Source 2700 CDMA disciple RB oscillator. In my area, this means that it is effectively disciplined to GPS. It had been up and running for about 6 months. We started the measurements, and disconnected the antenna. The unit went into holdover and then into free running mode.Our house reference is a GPS disciplined OCXO, all the counters use it as their time base.This is showing TDEV of the three.This is the frequency error or the TS2700. The first 6mHz spike is the change over form locked to hold over. The second spike is the unit going form holdover to free running. No offset or drift compensation was applied to this data.This is the frequency error or the OCXO. You can see even though the oscillator has a single oven there is still a temperature dependance. All the measurements were under regular building HVAC. Notice the magnitude of the spike of the Rb oscillator vs the scale in this graph.

author: Vgkid
author_rank: Super Contributor
timestamp: April 17, 2015, 12:59:16 am
content: What gear are you using to graph that.

author: lincoln
author_rank: Regular Contributor
timestamp: April 17, 2015, 01:59:27 pm
content: We have a couple of Agilent 53220a and my favorite SRS Sr620 counter.

author: Dago
author_rank: Frequent Contributor
timestamp: April 17, 2015, 03:58:06 pm
content: One thing to note with rubidium frequency standards is that for some models (at least the popular FE-5680A) the phase noise of the 10 MHz output is very poor, no good for sensitive RF applications (graph fromhttp://www.ke5fx.com):

author: lincoln
author_rank: Regular Contributor
timestamp: April 17, 2015, 04:51:54 pm
content: Hello Dago,Yep, it depend on what is between the output jack the the rb cell. I don't have the equipment to measure close in phase noise correctly. We are mostly concerned with wander. Our TS2700 has a SRS PRS10 in it. I'm interested to get one of the FEIs and do a comparison. It would be a stretch to delicate a counter to it for that amount of time...

author: Lightages
author_rank: Unknown
timestamp: April 17, 2015, 04:55:33 pm
content: Quote from: lincoln on April 17, 2015, 12:35:45 amNo problems on the "pissing contest". It was additional information that was basically on topic. I learned from it and that is the point of the forum I think.

author: kjs
author_rank: Regular Contributor
timestamp: April 17, 2015, 07:04:38 pm
content: Rb standards (the cell itself) have very poor short term stability. They jitter by design. This makes them totally unsuitable for a short term time base reference (like in frequency counters). Their big thing is long term stability which is the reason they are used as "time counters" in many military applications where the time has to be correct after a long time without synchronization.To make them better many references lock an OCXO (or TCXO) to the Rb output frequency which gives them a better short term stability (= phase noise). Unfortunately many of the Rb reference manufacturers don't see a need to use a prime quality OCXO there any often have far to much loop bandwidth which means they still have short term issues.As for most lab applications the real long term stability (as in months) isn't important the better reference is a GPS disciplined high quality OCXO or if the stability requirements aren't as tight a very stable (as in +/-0.1ppm TCXO). We have GPS-Rb's and GPS-OCXO's and GPS-TCXO's in use and only use the Rb's for long term stuff.What many overlook is that an OCXO based reference should never be turned off as power cycling is the fastest way to ruin an OCXO. That makes it a bit painful for the home lab as you always burn the power consumption of the heater. We have some TCXO based units from a Swiss company which gets very close to the stability of a good OCXO based version. That thing can be power cycled "at will" and doesn't age faster that way. Only needs about 10mins to be within spec so it's ideal for the occasional or service/mobile use. Only needs <15mA (+GPS current) which also allows long use with battery power.Just to be complete: the low phase noise (or short term stability) is naturally also very important if you use it as a reference for high frequency synthesizers as we do.

author: edpalmer42
author_rank: Super Contributor
timestamp: April 17, 2015, 10:53:25 pm
content: Quote from: lincoln on April 17, 2015, 01:59:27 pmWhat software are you using to collect the data and create the graphs? They look good.Ed

author: lincoln
author_rank: Regular Contributor
timestamp: April 18, 2015, 01:33:23 am
content: Quote from: edpalmer42 on April 17, 2015, 10:53:25 pmI'm a yob.. Symmetricom's (now MicroSemi) Time Monitor. Its not the most intuitive package but it can do a lot. We also use python and gnuplot for our caned reports.