10MHz OCXO Frequency Reference Project - Page 1
Forum section: electronics / projects

author: wyphy
author_rank: Contributor
timestamp: August 06, 2014, 03:09:37 am
content: Greetings all.Been following the blog for years, figured it was time to join up and start posting some stuff.Anyway, on to the good stuff.First up, 10MHz OCXO reference.Got the used OCXO cheap and wanted to make it into a reasonably accurate and stable 10MHz reference.Apparently it was used as a reference for a microwave transmitter or some such.Schematic is attached, and mostly speaks for itself I'm sure.As I'm sure you can tell, I basically "designed" this by researching a few other sites (http://gerrysweeney.com/racal-dana-199x-diy-high-stability-diy-timebase-hack-for-under-25/), and whacking the datasheet reference designs into a final circuit.Bridge for polarity protection. I know it's a bit inefficient, but it's only a 1.1V drop, and there will be plenty of headroom on source.Also thought about allowing AC input but figured I'd need too large a capacitance to fit in the smallish case I'm using to get a reasonable ripple, so I left it out.Was going to use a 7805, but was concerned about the 4W or so (7.7V @ .5A) on initial heating of the OCXO being a bit much with the oven generating it's own heat, so I went to the 78SR regulator.As the precision voltage reference setup was just a few bucks, I thought the extra stability would be good.Not much else to say, I guess, so what do you all think?Good, bad, suggestions for changes?Cheers.

author: Vgkid
author_rank: Super Contributor
timestamp: August 06, 2014, 03:36:27 am
content: What model OCXO are you using?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 06, 2014, 03:53:06 am
content: Maybe AC couple the output? Or have it as an option? Keep the straight output DC for logic levels, and the attenuator path AC coupled? Ya know, if you ever plug that into a spectrum analyzer without a DC block, that won't be good.

author: mij59
author_rank: Frequent Contributor
timestamp: August 06, 2014, 03:58:51 am
content: Hi,The 87SR is a switch mode voltage regulator, it will inject noise into the circuit, best is to use a linear regulator.You still need to calibrate the circuit, what are you going to use as a reference ?Things you may want to consider : multiple outputsadjustable output leveloutput isolation transformer to eliminate ground loops

author: yramgu
author_rank: Contributor
timestamp: August 06, 2014, 07:58:46 am
content: Divided output? Like 10MHz => 1MHz => 100kHz. Could always be useful.

author: daqq
author_rank: Super Contributor
timestamp: August 06, 2014, 08:25:33 am
content: Are you sure you want the OCXO directly driving the output? Some kind of output protection would be nice if nothing else - clamp diodes and such so you don't blow the driver when you accidentally short it or connect it to something nasty. As was said - add some (optional) dividing - making a 1pps or /10^n with logic/microcontroller ain't such a problem.

author: daqq
author_rank: Super Contributor
timestamp: August 06, 2014, 08:28:02 am
content: Oh, and optional AC coupling is very useful on occasions.

author: dannyf
author_rank: Super Contributor
timestamp: August 06, 2014, 10:56:47 am
content: is R1 wired correctly?

author: wyphy
author_rank: Contributor
timestamp: August 06, 2014, 06:19:46 pm
content: Thanks for the suggestions and questions, I've answered them in the order posted.I don't have anything specific in mind for this project, it's mostly just an interesting project because I thought it would be neat, and I got the OCXO cheap, because it's used (and 14 years old if the date code is 2000/19wk as it appears).I'm also finishing up a frequency counter kit, so this is also something to help tinker with that.I also plan to use the counter as a reference to check the alignment of some RF transmitters and make sure they're where they're supposed to be.I'm not huge into electronics, and you can see a bit more background if you check my post in the "introduce yourself" thread.The OCXO unit says:QuoteSomeone told me that it might be made by Vectron International, so I sent them a mail to see if they can tell me more, but it is supposed to be a 5VDC +20dBm sinewave out into 50 ohms. Person I bought from said he checked them all to make sure they worked; I haven't fired it up yet.AC coupling... See, this is exactly why I knew it was a good idea to post this here.Obviously I need a cap, so what value should I be using?I wondered about the switchmode regulator, but was worried about the dissipation in a linear regulator. Would be close to 4W (~7.7vdc at 500mA for initial heating, plus LED and Vref chip), plus whatever heat the OCXO generates, and it's a somewhat small case I'm using.Maybe I should use the 7805 with a pass transistor for the current; something in a TO-3 case? Can get some lower profile heatsinks for those.As for getting it aligned, I'll be doing this :Don't think I want multiple outputs, but other divisions might be handy. I will probably just build this as it is now, with any needed fixes, then maybe modify or rebuild it later with more options, or do a new design if I find another cheap OCXO.No, R1 is not wired correctly, but it will be later.Thanks for all the suggestions and info, I'll take all you care to give.

author: wyphy
author_rank: Contributor
timestamp: August 12, 2014, 12:52:54 am
content: Okay, I've had more time to work on this and I got the OXCO at least partially tested, and watched Dave's thermal design tutorial to see if I can get this under control with a linear regulator.I've included a spreadsheet with info on current draw over time and rough temperature estimates if you want to see all the math I did.Also, forgive the crudeness on the current graph as I don't have a datalogging ability so it was just done with a clock.I had many of the parts in the schematic already so I wired up the input through the bridge and switching regulator to get some measurements.Bridge drops 1.5V, and the OXCO draws a peak of just over 600mA, but only for one minute (tested 2-3 times), before ramping down to about 200mA long term.For temp reference, I ran the unit in the case that will house the project and wrapped it in some towels for extra insulation.Room temp was 26*, and temp inside case was 33* after 13 hours, so I went with 45* for initial ambient on the spreadsheet to try and estimate for the additional heat in the case from the regulator itself.Using the heatsink datasheet, I added 1.5* to the estimated sink-air figure and went with 2* for case-sink based on Dave's video even though I will use compound, so reality should be much lower than 2*C/W, and I trusted Fairchild's junc-case spec.Long story short (too late!), looks like the regulator should be around 100*C for that first minute then slowly drop to around 63*C.I think I probably went too high with the initial ambient temperature and changing to values closer to what I think they may really be (13.8 Vin, 40* amb, 3.5* sink-air, 1* case-sink) I come up with about 83* for the first minute, slowly dropping to around 54* long term.As the spec on the regulator is 125*C, I'm at least 25* under even in my worst case estimate, but you all tell me if this looks reasonable or not.Thanks.

author: wyphy
author_rank: Contributor
timestamp: August 12, 2014, 12:53:18 am
content: Still have a couple questions about outputting the signal.I do want to have it AC coupled, so a cap is obviously in order.Am I just looking for a .1uF here? .01uF? Something else?I'm thinking that having the output variable might just be better than a switchable 20dBm/0dBm output, though I may do both (and it kind of depends on if it actually outputs 20dBm), but would a standard pot wired as a divider work okay for this? Seems like it should, but I obviously don't have the experience to know.Presuming it will work, suggested values for the pot? 5k? 10k?Also, I started thinking about this a couple days ago, but I'm not sure about my output termination.I saw Gerry Sweeny's conversion of the Extron video box and he changed the 75R to 50R to get the proper output impedance.As my usage would be similar to his, this is probably something I should do as well, yes? 50R to ground, and should it be before or after the attenuator/variable section?I tried looking for some info on diode clamping, but not really sure what applies here, could you give me a reference or something?Thanks all for your help and opinions.