Small TCXO on ebay & oscillator testing - Page 1
Forum section: electronics / projects

author: cncjerry
author_rank: Unknown
timestamp: February 17, 2024, 05:51:24 am
content: Hi,I saw one of these:https://www.ebay.com/itm/295873326010on ebay and was wondering what they meant by "ultra low phase noise". It's a nice little package but you'll see why it is a waste of money as I continue. I paid $14.95 plus S/H that came to $20.13USD. The board has a regulator and two outputs, a sine and TTL. I haven't played with the 10-turn but I assume it is attached to the EFC pin.Some observations: decent package, takes 7-12v (has a regulator), dual outputs, LT1009 reference, 10dBm output +/- 2dbm*, calibrated to +/- .01hz**, EFC control, etc. They say it warms up in 5minutes,which it did, but it drew over 450ma*** while doing it. So if the heater cycles at all, it will overload a lot of my small linear supplies. I ended-up paralleling two Tek TM5000 supplies.* How can it be off by 2dBm? I mean, yes, it could, but heck, where's the quality control?** I question the .01hz calibration as mine was off -.02 or more. It would really need to age and warm up over a month or two.*** The specs on eBay rate it at up to 700ma while warming.Then phase noise. I wanted to see what they meant by "ultra low phase noise". I think "Ultra low" is under -170dBc or so. The test is still running but I copied it off as is. note the powers of 10 on the X axis and the little blip at 305hz. That is actually 300hz and it is the harmonic of the 60hz grid. But at first glance it looks really clean. That plot is barely smoothed by the PN software. But the reason it looks clean is the level of phase noise (in PSD). The lighter trace is the residual noise floor of the test system for each plot.Instead of this little board, for $20, if you keep your eye out on eBay, you can get a Wenzel 500 Sprinter series, or 501, both the same oscillator just that one is a custom part and the other is a stock part. Most Sprinters go down around -175dBc or so. They are TCXOs, have EFC pins and also a variable cap. The Wenzels age well, are completely sealed, some have dual outputs and they consume about half the current at 15V compared to that little oscillator mentioned above. The Wenzel ULNs are expensive, start at around $350 or so on eBay. I want a pair for my phase noise test set. But the 26Mhz sprinters I am using as references (two at once) are down below 160dBc at 1k. What I'm saying is that if that oscillator I am testing was cleaner, you would see the grid noise hash in the plot like in the Sprinter plot.I have 20 or so sprinters. The 5Mhz sprinters that were around for cheap pre-covid were a great deal. One of the time-nuts sent a note out about them, might have been Corby. The other great deals on oscillators (none are mine, the only time I sold something on eBay- I forgot and the person got all pissy)https://www.ebay.com/itm/225361082784<- these are cool as they take a 10Mhz input and phase lock it to the 119.5Mhz output.https://www.ebay.com/itm/225550057281<- same thing as above but with lower phase noise. These somewhat one-off frequencies are good for clocking ADCs where you don'twant to generate spurs from the ADC clock mixing with the 10Mhz DUT clock while doing phase noise testing. Using this one, a spurswill show at 500Khz on the plot. Usually by 500Khz the phase noise has flattened out.https://www.ebay.com/itm/275888341169<= this one has been around the block a few times but it would kick the but of that POS I tested. First picture if you forgot.https://www.ebay.com/itm/256309013397<- If I didn't have so many oscillators, I would buy this one just to test it. the shipping is high otherwise it would be a great deal. The oscillatorsin the larger can could be decent. low risk, great reward.Last one:https://www.ebay.com/itm/176184502931<- I've spent a fortune on eBay and they won't let me hit the 'share' button anymore because I hit it so many times in a row to get the list above!!!This is an example of a 501 series and this person knows what they have. You often find 501 for less than $25USD because they look like500 series and the seller doesn't know what they have. As I mentioned, you generally can't get the specs or datasheet for a 500 series asit was a custom part. 501s I guess are stock parts.https://www.quanticwenzel.com/wp-content/parts/501-08772.pdf<- the sheet for the one above is here.A very helpful person named Adrian gave me the info on the Wenzels.Lastly, a good oscillator needs a great regulator. We tested the lt3045 regulators inline after the Tek TM5000 power supplies and they made a marked improvement. The negative versions of that oscillator helped as well on dual rail opamp supplies. Adrian recommends 15,000uf of lump capacitance on these oscillators because you can't run the oven separately. There are a few Wenzels I guess that expose the oven drive but I haven't found any.If the links above don't work, they product is either gone or I didn't copy it correctly. You should be able to just copy and paste in a browser. I didn't insert them as hypers because I didn't want to screw up the nice formatting. The last numbers are the item number. most likely the formatting will be hosed once I hit submit.Anyway, hope you found this useful. Anyone want to purchase a slightly used, not so low phase noise packaged oscillator?Jerry

author: 807
author_rank: Frequent Contributor
timestamp: February 17, 2024, 10:56:58 am
content: Have you seen this thread about these units:-https://www.eevblog.com/forum/projects/cheap-chinese-ocxo-frequency-reference-modules/A lot of them need to have resistors changed on them to trim them to 10MHz.

author: dmendesf
author_rank: Frequent Contributor
timestamp: February 17, 2024, 12:59:30 pm
content: Maybe I can (politely) use this thread to ask a related question: how do you measure phase noise / Allan variation? I understand the result, I just can't clearly see the what the hardware needs to do. Is it enough to connect a 10MHz oscillator (the DUT) to a frequencimeter with a very good reference and get N measurements? (A large N, of course). Does the measurements need to be equally spaced in time? I'm totally wrong? Any directions would be helpful.

author: cncjerry
author_rank: Unknown
timestamp: February 17, 2024, 01:01:31 pm
content: I'm surprised there aren't more threads on those boards because CTI used to make decent oscillators and the packaging makes it look like something. But there are much better deals on oscillators, as I attempted to point out.

author: cncjerry
author_rank: Unknown
timestamp: February 17, 2024, 01:29:03 pm
content: Lots of ways to measure PN, most are very expensive. There are some inexpensive ways that will allow you to measure one vs another oscillator. But if you want accurate charts to very low levels of noise you need a system of some type. I use a digital system with ADC and FPGA that Andrew Holme developed. Look up "Andrew Holme phase noise" and you should find him in the UK. He is a great person to work with, very patient and knowledgeable. His code is on the 23rd version. There are people using a soundcard to digitize an oscillator after downconverting to DC using a DBM to eliminate the carrier but I don't know how well it works.Regarding ADEV from fractional frequency measurements as used in digital PN systems, I have yet to see a correlation with other methods. My testing that is; I can't make it correlate using a number of ADEV testing methods that are repeatable. Don't want to start a war on that point, but I can measure PN pretty accurately and Allan variance accurately, both are very repeatable, but the numbers don't line up. I read the Chang paper on the DOD site and others and it should work for taus less than a second, but I get very different numbers.Jerry

author: cncjerry
author_rank: Unknown
timestamp: February 17, 2024, 01:40:02 pm
content: Quote from: dmendesf on February 17, 2024, 12:59:30 pmBasic method works as you describe. Look up Dual Mixer Time Difference (DMTD) on this site. Corby developed a nice board and TIC that will improve the resolution, vs your counter method, into femtoseconds. You still need a stable oscillator as a reference. I use a Cs beam or Rubidium, you can use a GPSDO. Different references for PN testing vs ADEV, but that is debateable.

author: cncjerry
author_rank: Unknown
timestamp: February 17, 2024, 06:53:18 pm
content: Adding ADEV, frequency delta and phase plots from "CTI" testing last night. Remember, this is a little oven, and yes, the room changes a little at night, but I've never seen a phase plot so perfect. It's like a frown! I put the CTI in quotes as the etching on it reminds me of the etching you see on a fake Asian Benchmade knife. I hade to put the module under a scope to make it out.Jerry

author: sky2city
author_rank: Contributor
timestamp: February 20, 2024, 02:38:08 pm
content: Hi Jerry,For this one,https://www.ebay.com/itm/256309013397<- If I didn't have so many oscillators, I would buy this one just to test it. the shipping is high otherwise it would be a great deal. The oscillatorsI have one too. I don’t think this is good one per my PN test.It has ref input lock to 10MHZ source.I will post my PN test data later on. Just provider some FYI.---------------------------------------------------attached the PN test data.