Twisted pair coax cables - Page 1
Forum section: electronics / projects

author: Cicada
author_rank: Regular Contributor
timestamp: August 29, 2017, 09:09:58 am
content: HelloYou get shielded twisted pair (STP) cables.Are there connectors (such as BNC connectors for coax cables) that can be used with STP to connected the outer shield of the connector to the cable shield and the twisted pair to two separate pins?Or maybe a something like a triax connector. Connect the cable shield to the outer shield of the triax connector, the one side of the twisted pair to the inner shield of the triax connector and the other side of the twisted pair to the inner pin of the triax connector.I ask this because I think one can get better over all shielding to Electric field (coax) and Magnetic field (twisted pair) interference with such an arrangement. Is it true? Would a STP cable provide better overall shielding to magnetic and electric field interference than a triax cable?If so the only question is what connector (coax type) is the easiest/most practical to work with when connecting STP cables? Maybe provide a link to such a manufacturers website.Thank you for your thoughtsCicada

author: Cicada
author_rank: Regular Contributor
timestamp: August 29, 2017, 09:28:15 am
content: MmmWriting a post sometimes help you to get your thoughts in order. It is then easier to do a search. To answer my own question.... maybe this will work.https://www.samtec.com/rf/components/original/bayonet-twinaxPlease comment if you know of better solutions or have opinions about my statements of Triax cables (and what I now know as) twinax cables efficacy to sheild both magnetic and electric field interference.I plan to use such a solution in applications where I have very long cable runs (industrial noisy environment) to measure very small currents i.e. picoamps to micro-amps with pico amp accuracy. The front end electronics cannot be close to the radiation hardened sensors because it will get damaged by radiation. Hence the long cable runs where noise can be picked up.Also look at the attached picture. The pink thick line is where I plan to use this cable. The faraday cup is the sensor. The thick green line represent a very long path back to the ion source where lots of electrical equipment induce noise in this return path. So I plan to connect the cable outer shield at the faraday cup to the ground and not on the electronics side. The inner conductors will be connected to the amp input and the amp ground which I will have floating. I am not sure if floating the amp will work. It might be that floating the circuit ground could cause potential differences between analog ground and local ground that can cause high leakage currents. I will have to experiment.So this is where I want to use the cable. Just to make noise coupling on that part of the circuit as low as possible. The green part i.e. common return to the source I can do nothing about.Post update. I have uploaded a better image in the next post

author: Cicada
author_rank: Regular Contributor
timestamp: August 29, 2017, 09:36:14 am
content: Updated picture

author: helius
author_rank: Super Contributor
timestamp: August 29, 2017, 11:05:56 am
content: Coaxial and twisted-pair cables are different beasts. Their characteristic impedance is specified for a different relationship. In a coax cable, the characteristic impedance is between the core and the shield. A twisted-pair cable specifies the characteristic impedance between the wires in each pair. Shielded twisted-pair (STP) cable does not specify the impedance of the shield in relation to anything else! Twinax cable has a specified impedance between the two cores, and also between the cores and the shield, which makes it quite different from STP.As you know, a coax shield is crimped or soldered to the corresponding shield in the coax connector. STP cable is constructed differently, and usually has a drain wire, a bare wire which is in contact with the foil shield. If desired, the drain wire can be terminated in a modular connector.

author: 4CX35000
author_rank: Regular Contributor
timestamp: August 29, 2017, 01:14:59 pm
content: How long is the distance between beamline and F100 ?

author: macboy
author_rank: Super Contributor
timestamp: August 29, 2017, 03:53:52 pm
content: You are basically describing twinaxial cable. This is often terminated into the same TRB style connector used for Triaxial cable. The TRB connector is usually mistakenly called a "Triax" or "Triaxial" connector even though Triaxial describes the cable, not the style of connector. For examples, look up Trompeter/Cinch brand PL75 connectors. There are many types for many various cable types including various Twinax and Triax cables.

author: Cicada
author_rank: Regular Contributor
timestamp: August 29, 2017, 04:01:51 pm
content: Hi 4CX35000The distance between the F100 and the Faraday cup is ~ 50 to 80 meters.The distance from the Faraday cup to the Ion source is also a guess and will vary from installation to installation. Worst case may 100 to 200m.Look at the attached pic. This is not a picture of the system that I am working on. It is just for illustrative purposes.The return path from Faraday cup to the ion source could be through the beam pipe (shown in pic), beam line optical bench* (not shown in this pic) or there could be separate ground cables running back to the Ion source as the ones seen going into the walkway grating**. I guess I will have to speak to the right people to get an idea of the return path from the Faraday cup to the ion source (proton accelerator).As can be seen from the picture, these type of setups have noise inducing currents that can flow in the metal connecting parts between such a Faraday cup and the ion source i.e. the path shown with the green line in an earlier picture.* beam line optical bench = metal support frames on which the beam pipe, steering magnets and sensors like Faraday cups are mounted on.**I know these are vacuum or cooling water pipes but image they are thick grounding cables.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: August 29, 2017, 04:45:47 pm
content: What is the bandwidth of the signal?STP does have defined impedance for a given cable. Specified in capacitance per foot, for example. E.g.http://www.mogamicable.com/category/bulk/microphone/quad/Henry Ott has a couple of books on noise that seem to be apropos, here. But the signal bandwidth seems to be a key missing spec, IMHO.(I pointed you to microphone cable because it also controls tribolectric effects that will be important in the picoamp range).Randy

author: jmelson
author_rank: Super Contributor
timestamp: August 29, 2017, 04:47:07 pm
content: Quote from: Cicada on August 29, 2017, 09:28:15 amLemo makes a number of 2-pin shielded connectors. We have used digital audio cables made for recording studio and broadcast use with these connectors to transmit analog signals between units. The cable is pretty cheap, the Lemo connectors are a bit expensive, but made very well.Jon

author: Damianos
author_rank: Frequent Contributor
timestamp: August 29, 2017, 07:06:19 pm
content: It seems a very interesting project!I have done this in the past (measuring low level currents) but the distance between the sensor and the current to voltage converter was less than 2 cm!I am thinking how much of the signal current will be absorbed of the cable's insulation! Even if Teflon is used, at these lengths and pA levels, maybe the signal does not arrive at the other end. In this case any external interference is of no meaning.I think that, before the final installation, it would be a good idea to test what you can measure by using a cable (kind/type?) of this length and an accurate source and meter. Do it in an electrical and magnetic "silent" environment. The results will lead you how to manage the continuance.By the way:- What is this F100 unit?- Has it a guard feature/connection and how you plan to use it?- What are the electrical characteristics of the signal source (burden voltage, impedance)?

author: 4CX35000
author_rank: Regular Contributor
timestamp: August 29, 2017, 08:29:24 pm
content: If you look in the manual for the F100, then they will probably recommend a connector and the manufacturer of the connector will recommend a range of cables which are ideal for the connector. Just pick on of those to meet your needs.I'm not familiar with what your doing, but from experience of working in hostile EMI environments, it is normal to keep the distances as short as possible between sensors and measuring equipment and then use the RS232 output with a RS232 to ethernet converter or ethernet itself to get the length required back to the monitoring computer. Length of cable tends to ethernet main advantage, more so if the data being carried is low.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: August 29, 2017, 10:53:55 pm
content: Quote from: macboy on August 29, 2017, 03:53:52 pmI beg to differ: the term 'triaxial' describes a concentric assembly of three conductors and applies to connectors in the same way. That is, the concentric structure is maintained through the connector! Connecting a triaxial cable by a 'twinax-BNC' would be akin to connecting a coaxial cable by means of two single pins = loss of signal integrity and heavy step in impedance!

author: Cicada
author_rank: Regular Contributor
timestamp: August 30, 2017, 08:48:15 am
content: Quote from: RandallMcRee on August 29, 2017, 04:45:47 pmHello RandallMcReeSignal bandwidth is very low. I would say nothing more than few hundred kilohertz in most applications. There might be cases where one would realy want to look at the dynamics of an ion beam with multi channel current measurement devices but even then the dynamics is slow.I will look at Ott's book

author: Cicada
author_rank: Regular Contributor
timestamp: August 30, 2017, 11:58:33 am
content: Quote from: Damianos on August 29, 2017, 07:06:19 pmLeakage current.Not a lot. Using this 50 ohm cable for examplehttp://www.hubersuhner.com/ProdDet/6002827with Rho = 10x10^12 [Ohm-Meter] and the calculation given here forhttps://www.electricaltechnology.org/2015/03/insulation-resistance-of-a-cable.htmlinsulation resistance. You get Rins = 1.4045 Tera Ohm / meter. So for a 100 meter cable you will have Rins = 14.05 Gig ohm. Taking into account that the source (Ionization chamber or faraday cup) can be approximated by a current source and the F100 input is at virtual ground (i.e. kept close to ground) you will never have a voltage on the cable of more than a couple of hundred micro volt. So 100's of micro volt over 14 Gig ohm insulation will be very small leakage current.Dielectric absorptionBy extension of the discussion above it should be clear that dielectric absorption with such low potentials on the signal carrying conductor is also not really a problem. By the way when you go into the electronics it is whole different ball game regarding dielectric absorption in the capacitors used.Quote from: Damianos on August 29, 2017, 07:06:19 pmIt is a current to voltage converter.http://www.ptcusa.com/products/8I am just using this device as a reference. I plan to build a unit with multi channels and a lower current range.Quote from: Damianos on August 29, 2017, 07:06:19 pmNo. Not needed for the type of source used with it. The voltage on the center conductor is very low.Quote from: Damianos on August 29, 2017, 07:06:19 pmAs mentioned earlier these types of sources can be approximated by a current source driving into a virtual ground at the op amp. So as long as the current source sees a low resistance it will not lift its output voltage to maintain the current that it wants to output.

author: Damianos
author_rank: Frequent Contributor
timestamp: August 31, 2017, 07:58:26 pm
content: Thank you for the information!My little knowledge and experience leads me to some thinking (because you stated that you plan to havepA accuracy):- Except of the insulation resistance, the insulators have also some other properties, among them there are the "charge absorption" and the "charge pockets". These will be your "enemies" especially by using long cables. There is relative information in the "low level" application notes of Keithley, as an example, here:https://www.tek.com/search?type%5B0%5D=type%3AdocumentOf course there are other similar documents from other manufacturers that have departments specialized in low level measurements (electrometers, SMUs, pA-meters, nV-meters etc).- In the user manual of the F100 there are some notifications relative to the connections. In the section 9, for example, they say:9.3.1 Typical setupQuoteThere is also the section: "23 Making Low Current Measurements"- The "guard" is not used for the signal wire itself(!), its use is to protect the signal on it and the sensor from the surroundings. There is a high voltage source there (at least one) and you stated that the environment is electrically noisy (probably there exist steady electrical fields). The magnetic fields (steady or variable) is another "story".- ... a current source has some characteristics. Among others there is its equivalent resistance. Because it will drive a "transconductance amplifier", a "current to voltage converter", a "feedback type ammeter" or whatever anyone likes to call this thing, this value is important. If it is low, then any offset voltage will be amplified and "bury" any useful signal. You will find relative information if study a little the documents mentioned above and also on relative notes of semiconductor manufacturers (like AD, TI, LT ... and the old ones like NS, BB etc)....I am expecting any results of your tests and any new info to learn more (I have not any resources to do experiments like this!).