Distributing lab standards - Page 1
Forum section: electronics / general

author: tkamiya
author_rank: Super Contributor
timestamp: August 14, 2019, 06:02:26 am
content: This is probably DUH! kind of thing for people are experienced. But I started wondering about cabling my lab standard (10MHz) as I go higher and higher in precision.My regular standard is a GPSDO that is running 24/7. It is, for all intensive purposes, a telco grade T-bolt. From there, I go through two Spectracom 8195B that are daisy chained. Cables are all some type of RG58 and connectors are RG58. They are suspended and secured to high up on a wall, so they don't move.My precision is approaching e-13, depending on what I am measuring. In this environment, how does coax, quality and length (they are not all the same) dialectic factors come into play? What about daisy chaining 8195B?I am already aware phases are all messed up. Also, rise time and fall time can't be all that precise/consistent through coax, connectors, and dist amps. What else? What else folks who have masers and rubidium FOUNTAIN type concern themselves?Thanks.

author: tkamiya
author_rank: Super Contributor
timestamp: August 14, 2019, 06:15:22 am
content: I found this:http://www.leapsecond.com/pages/tadd-1/

author: chuckb
author_rank: Frequent Contributor
timestamp: August 14, 2019, 03:12:18 pm
content: Fifteen to 20 years ago the US Naval Observatory upgraded their GPS antenna coax because they noticed a daily timing change from the GPS. I believe they changed to HELIAX model FSJ4. This coax is more phase stable and it works better with the daily outside temperature swings. I installed it to my dual frequency GPS antenna. It feeds a Dual Frequency Ashtech and a T-Bolt that controls a Rb.I was not able to find the article that described their problem and solution. With outside temperature changes of 20C? they needed a better coax. I don't know about inside the house in a controlled environment.

author: babysitter
author_rank: Frequent Contributor
timestamp: August 14, 2019, 06:34:22 pm
content: Everybodys favourite, Teflon, undergoes certain changes in the crystalline structure at normal pressure at temperatures it might be actually exposed to. Nothing special, 0-30°C range. This has an effect on the electric length. Time distribution doesn't like unexpected cable length shift as it rocks the phase.

author: tkamiya
author_rank: Super Contributor
timestamp: August 14, 2019, 07:48:00 pm
content: So..... what IS the perfect scenario, or is there a such thing?

author: thermistor-guy
author_rank: Frequent Contributor
timestamp: August 15, 2019, 02:57:25 am
content: Graph showing the Teflon knee:https://www.everythingrf.com/community/what-is-the-ptfe-kneeSee also:https://www.everythingrf.com/community/minibend-ctr-phase-invariant-cable-assemblies

author: forrestc
author_rank: Unknown
timestamp: August 15, 2019, 05:50:08 am
content: Quote from: tkamiya on August 14, 2019, 06:02:26 amThe missing part of your question is what are you going to be using the reference for. Generally, if you're using 10Mhz as a frequency reference you'll find that cabling doesn't matter all that much as it is pretty difficult to shift the frequency in a length of cable. On the other hand if you are worried about spectral purity and/or phase then things get a lot messier since a cable can introduce a bit of noise for things such as temperature drifts, vibration, and other effects.If you've looked at ADEV plots, you'll notice that they are more accurate over longer periods. This just means that over a longer period, a bit of phase noise doesn't matter a lot. Take for instance GPS which is good to maybe +-100ns. An error of 100ns over a second is pretty bad. Over a day it's quite good. Over a year, it's excellent.

author: tkamiya
author_rank: Super Contributor
timestamp: August 15, 2019, 01:39:22 pm
content: First, let me describe a physical layout of my lab. It's in size of a regular sized master bedroom and I have 5 racks. My frequency standards are on far left. So to reach an opposite corner, it could be a bit long. I have a future plan of clock distribution going to the other side of my 1000sqft house.Since I do this for my hobby, what I would be measuring will be hard to say. Currently, I'm playing with an HP53132, an HP5335A, and a TICC. They are spread between racks.Currently, I can't think of a situation where comparison will be made between two pieces of gear. So my primary goal is to have minimal jitter, temperature variation, and frequency instability (cases where jitter takes place on extreme side of phase at beginning and the end) I mentioned the last because there are two daizy chained dist amps. Because of number of ports needed, this is unavailable.Yes, I am aware of how Adev graph changes over time. I have an immediate goal of having an ovenized xtal oscillator, so short time jitter will be less.I'm going to change a question a bit then. What is the best practice in lab standard distribution? I fear mine is the worst way to do it. It happened due to my lack of knowledge and because my lab slowly grew.

author: forrestc
author_rank: Unknown
timestamp: August 21, 2019, 08:07:56 am
content: Quote from: tkamiya on August 15, 2019, 01:39:22 pmFrom your original message, it sounded like you have a GPSDO in place.... If you have pretty much any normal GPSDO, you pretty much have an ovenized xtal oscillator (OCXO), which is automatically adjusted by the 1PPS signal from the GPS over a long period of time.My thunderbolt is on all of the time, and produces a very robust 10Mhz signal. The Thunderbolt 10Mhz output is plenty good enough for everything I do, except there are some measurements I prefer to use a non-disciplined clock when I want to avoid potential noise from the disciplining algorithm. For those, I have a rubidium which I warm up a few days before I expect to need it for these measurements and then leave on until I know I'm not likely to use it for a few months. I actually use the 10Mhz from the thunderbolt in conjunction with the TICC to verify the stabilty of the rubidium oscillator. Note I said stability, not accuracy.Assuming a reasonably decent set of distribution amplifiers, and a reasonably good set of patch cables, you're not likely to have much of an issue. Pretty much every piece of decent test equipment assumes a certain amount of noise is riding on the 10Mhz signal and uses various techniques to clean it off. Unless you're running a very high end clock source, the chances of the distribution amplifiers and cables being the weak link is minimal. You're usually going to get more noise out of the clock source than the cables/amps. Assuming you took some reasonable care in selecting cables/amps.One note to mention is that many labs will 'distribute' a 10Mhz signal by running a coax from the source to each instrument with a BNC tee at each instrument. The last tee gets a 50 ohm terminating resistor. This works well where the test equipment all has high impedance external reference inputs, not so much when they're 50 ohm inputs, although you can sometimes cheat by effectively using a 50 ohm device as an end terminator. If you're familiar with how thinnet (10Base2) was cabled, that's what I'm talking about. Mainly you want to make sure the cable is terminated at both ends with 50 ohms, often the source will be a 50 ohm source so you don't may or may not need to terminate that end, but the far end definitely needs a terminator or a 50 ohm instrument.My distribution currently consists of a modified Extron video amp (seehttps://www.eevblog.com/forum/metrology/10mhz-distribution-amplifier/for how others have used these, including modifying them for 50 ohms). I sure can't see any additional distortion which isn't being generated by the GPSDO. Each instrument gets their own feed, terminated as appropriate. I plan at some point to add (or replace this with) some TAPR TADD boards to this arrangement such that I have a few more reference clocks available to me and also to do a better job of distributing the 1PPS signal.

author: tkamiya
author_rank: Super Contributor
timestamp: August 21, 2019, 02:33:07 pm
content: Yes, I have a T-bolt variant GPSDO as my main (usual) timing source. This one is on 24/7, and has a dedicated antenna. From there, I have two distribution amps daisy chained.I also have various GPSDO, Rubidium, and old Cesium. These are turned on few days before needed.One thing I never thought of is terminating the line.... I assumed they are internally terminated. So there are no 10base-2 like terminations. Also, I have one equipment per channel. I with there are some standard.... They way it is, I'll have to consult specs for every equipment.I've done some testing, and found out my GPSDO is pretty stable with exception of occasional blips. I'm actually amazed how good thins thing is. It actually outperforms all other GPSDO I have. It's a Nortel branded modified T-bolt. I've added 1pps as well.I basically came to realization that there are lots of things I can improve, but unless I get to tune it to specific use, there is no need to stress too much. One thing I'm going to change is the location. I'll have to move it to bottom of my rack. The current setup exposes it to too much draft from air conditioning outlet.

author: feedback.loop
author_rank: Frequent Contributor
timestamp: August 21, 2019, 11:14:25 pm
content: What are your "intensive purposes"?

author: tkamiya
author_rank: Super Contributor
timestamp: August 22, 2019, 01:35:51 am
content: I am picking up time-nut type activities and that's why I started paying more attention to my lab standard. I am still very much in learning stage.

author: forrestc
author_rank: Unknown
timestamp: August 22, 2019, 06:21:22 pm
content: Quote from: tkamiya on August 22, 2019, 01:35:51 amJust FYI, you should join the mailing list time-nuts@lists.nebo.nethttp://leapsecond.com/time-nuts.htm

author: tkamiya
author_rank: Super Contributor
timestamp: August 22, 2019, 06:23:08 pm
content: I am already a member, but thanks.

author: texaspyro
author_rank: Super Contributor
timestamp: August 22, 2019, 07:29:42 pm
content: Quote from: forrestc on August 21, 2019, 08:07:56 amYou can turn of disiplining of the Tbolt and let the oscillator free run... a lot of people do this when they want a clean signal. Leave disciplining on to get the OCXO on freq and turn it off when you need a cleaner (but potentially less accurate) signal. The Tbolt OCXO should have quite a bit better phase noise than a Rb osc.

author: texaspyro
author_rank: Super Contributor
timestamp: August 22, 2019, 07:38:57 pm
content: For a fun test feed a 1PPS signal into chan A of the TICC that has a "T" connector. "T" a hunk of coax over to the chan B input (with terminator). Set up the TICC to measure the chA-chB time difference. Place the coax in a freezer for a few hours the connect it to the TICC. Watch what happens as the coax warms up.https://www.febo.com/pipermail/time-nuts/2017-April/104923.html

author: FriedLogic
author_rank: Regular Contributor
timestamp: September 06, 2019, 08:30:11 pm
content: One of the main problems that I've had with cabling has been with the connectors and adaptors. The shield connections are often very poor, particularly with BNC and TNC plugs and adaptors.On good quality ones there's usually a raised rounded flange around the end of the shield connections on the plug, like the one on the left in the photo below. This helps it to make a good connection with the inside of the socket. On cheap ones it's often not there, like the one on the right, so the connection can be a bit variable. The contacts on that one did not grip the socket at all. Even big brands and distributors can have ones that don't make a firm connection, so it's best to test them.A reasonable quality socket without the centre pin fitted is useful for testing cables and adaptors, although since the dimensions can vary a bit it's worth checking how representative it is.Some plugs actually grip more tightly than I would be comfortable using where they would be repeatedly connected and disconnected, but at least they're not likely to move during a test.

author: forrestc
author_rank: Unknown
timestamp: September 06, 2019, 11:14:07 pm
content: Quote from: FriedLogic on September 06, 2019, 08:30:11 pmLast year I went to a Time and Frequency Metrology seminar at NIST. One point they were careful to make is that BNC plugs are problematic. As a result, they seem to use SMA pretty much everywhere they can. The real advantage of SMA is that you can tighten fixed connections down.

author: FriedLogic
author_rank: Regular Contributor
timestamp: September 07, 2019, 07:20:57 am
content: I prefer SMA connectors too, but BNC connectors are often used on equipment anyway, and the lack of coverage of issues like poor shield connections makes me think that a lot of their bad reputation is down to poor connectors and adaptors being used.I usually try to avoid cheap SMA connectors too.

author: forrestc
author_rank: Unknown
timestamp: September 07, 2019, 08:07:32 am
content: Quote from: FriedLogic on September 07, 2019, 07:20:57 amI think some of the underlying issues with BNC is the whole spring-loaded component. Even a properly connected and good quality BNC connector will have some movement in the connector pieces, especially side to side. Movement often equates to connector noise and/or intermittent connections. Whereas most of the thread-on connectors (including SMA) tighten down such that there is little connector movement possible.I agree though that a good quality BNC connector in good shape isn't all that bad for most things. Especially for connections which are regularly changed.

author: awallin
author_rank: Frequent Contributor
timestamp: September 07, 2019, 04:59:00 pm
content: that PDF seems to have some numbers:https://ivscc.gsfc.nasa.gov/meetings/v2c_wm1/phase_stability.pdfand thishttps://www.haystack.mit.edu/tech/vlbi/mark5/mark5_memos/069.pdf

author: ArthurDent
author_rank: Super Contributor
timestamp: September 07, 2019, 07:51:02 pm
content: I've used the TNC instead of the BNC in some applications and those connectors are solid as well as taking the same size coax as the BNC series. I also use the SMA with double shielded mini coax.