Instrument cables.

[James Luck]

Hello all,

I've asked this on the fusor forums, but the people here seem to be more active then on fusor.

Anyone have a good supplier for quality cables. I'm aware this often isnt a cheap affair. Of course a lesser price than usual is always nice.

I'm now starting the search once more for cables, and reluctantly not going back to the previous retailer, just to see what others are using.


If it helps at all I need multiples, in both BNC-BNC and MHV-BNC.
Previously I've bought some from feild components, with nothing bad to say besides the time it took for me to sort out my order and receive the order. Otherwise the cable works well and has yet to fail me.
Anyhow, just trying to see what others use as a supplier.

James.

[Pavel M.]

As always, that probably depends on your setup. For example, my detector uses two separate lines (HV and signal) to connect the probe with the spectrometer (which contains an external sound card and also all the required electronics). For connecting it to a PC, I have a decently shielded USB cable.
I don't think the quality of the BNC cable matters that much (at least when using it for HV only). I'm using a basic BNC-BNC RG-58 coax (RG-58 has an impedance of 50 Ohms, for RG-59 it's 75 Ohms).
For signal, I'm using a cable with 3-pin GX-16 plugs (which I got with the detector), that one seems to be quite nice, it's a rather thick microphone cable made in Germany by the brand "Cordial" (the CMK line), I'm not experienced with cables, but from what I can tell, I'd probably recommend this brand.
I wanted to experiment with longer cables, so I've recently ordered a bunch of different cheap cables (for ~130USD in total), but they still haven't arrived. I'm wondering if there will be any difference at all. I know that the capacitance is somewhat important.

I would also like to get some useful feedback from others about this topic.

Pavel

[Sparky]

Pavel,

As you say, it depends on your set-up. I use the same type cable (#3 below) for my Ludlum survey meters and my Gamma Spectroscopy PMT adapters. I measured the capacitance of various cables and the results are listed below.

Cable Capacitance tends to dampen the input signal. Whether it is an issue depends on the application. The oscilloscope trace was taken at the low voltage side of the signal input HV capacitor of my Ludlum. It compares the pulse from a GM tube using Cables 1 (yellow) and 4 (blue) below. The threshold for the count is 40mV, so this dampening makes little or no difference on the countrate.

With a scintillator, the pulses are weaker and variable. With my homemade plastic scintillator probe, that coiled cable will cut the countrate with the same instrument by 50%. You can see cables #3 and #4 in my last post to the “Lounge”.
I am guessing that for Gamma Spec, your gain adjustment will be able to compensate the dampening of a longer cable to some extent, but gain has a tendency to amplify noise as well.

Cable 1 - CDV700 - 150 pF
Cable 2 - CDV700 - 146 pF
Cable 3 - 1 meter RG58? - 74 pF
Cable 4 - Coiled - 555 pF

[Sparky]

James,

I bought a couple of 1 m BNC X BNC cables from this ebay seller. I thought the quality and price was good.
He might be able to get different connections. Not sure.

324181797207

[James Luck]

James,

I bought a couple of 1 m BNC X BNC cables from this ebay seller. I thought the quality and price was good.
He might be able to get different connections. Not sure.

324181797207

I saw that listing, more I think about it I might as well go for them. I could always ask if he has other types of connectors, worth a try.

Most of the cables needed are for survey meters, as I use single connector scintillators.

[James Luck]

Pavel,

The issues I've come across was not what type of cabling needed, but rather where to buy.
I tend to not trust ebay after a few cables came as a dead short. From two unrelated sellers.

Most of my instruments are bnc to bnc, from meter to probe. Although I use a few eberline meters that use a mhv on the meter side.
only issues I've found was sensitivity to radio frequency interference on cheaper ones. With the bonus of internal insulation taking a dumb after only a year of use. Which is costly, to say the least.

I might have to just go and buy all of them from the previous supplier I used.

[Sesselmann]

James,

I can make up quality 50 ohm coaxial cables for you, just send me a PM and let me know how many and whet type you need and I shall give you a quote.

Steven

[kotarak]

Hey guys,
I purchased a cable from Tom (iRad) 36" SHV-to-BNC. The cable is really nice - strain relief, thick while quite flexible - it is marked "SYV 75-5-41" 75 Ohm - the cables seems to be Chinese , but very high quality - I have not tested the cable with my VNA yet.
While waiting for the cable to arrive from Tom, I made an interconnect cable myself, using Mil-Spec THERMAX M17/128 RG-400/U cable and high quality (KINGS) SHV and BNC connectors. The cable I used is really the "good stuff" when it comes to 50 ohm coax - it has silver plated stranded center conductor, solid PTFE dielectric, 2 silver-plated shields and PTFE (Teflon) outer insulator - this is as good as it gets when we are talking flexible coax. This cable is rated up to 1900V.
My cable is longer than Tom's by a foot- approximately 120 cm total length. I have no doubt in my cable - I am a Ham radio operator and over the years I installed hundreds of different coax connectors from the tiny IPEX/IPX and SMAs to N-type for some exotic cables including hardline so I am sure the cable I made is good - both for materials and workmanship.
To my surprise, the two cables when compared gave me slightly different plots of the spectrum - I'll try to post some screen-grabs - not sure if this is due to the different impedance (50 ohm vs 75 Ohm) , different capacitance (32pf/m vs. 67pf/m), different length (120cm vs. 90cm), Velocity of propagation 0.69 vs 0.66) etc.. but the difference is pretty apparent - so now I am confused which one should I use :-)
I can run plots on my VNA but Tom's 75ohm cable will always have some reactance as my vector network analyzer is 50ohm one and there will be some impedance mismatch and SWR.
I wonder if delay is a factor but the difference in velocity is not that great. Any ideas?
Currently I am using the cable from iRad under the assumption that it was tested to work with GS.
Best,
Andrey

[kotarak]

This is the plot of the two cables.
The green trace is the RG-400 Mil-spec cable, the blue trace is the 75ohm cable from iRad.
I calibrated using Lu-176 and the iRad cable, adjusting the energy trimmer.
Then just changed the cables and did another run.
Any ideas why the difference? Basically, the energy trimmer had to be moved up about 400 units for the mil-spec RG-400 to get identical results to the Chinese 75 Ohm cable.



Best,
Andrey

[kotarak]

I just did a crude measurement of cable's capacitance, using my LCR bridge (which goes up to 16kHz):
-RG-400, with smaller size dielectric, better shielding and longer physical length measures 130 pF with both connectors.
-The 75Ohm cable from iRad with much thicker dielectric and shorter in length measures 62 pF

I wonder if this causes the difference in plots when cables are directly swapped without re-calibration?