RG58 cable trimmer - Page 1
Forum section: electronics / beginners

author: Deathwish
author_rank: Unknown
timestamp: July 22, 2015, 07:52:05 am
content: I have a couple of RG58 cables to interconnect my Sig Gen and other stuff to each other, I want to plug it into my scope as well, problem is it needs a trimmer , anyone know where to get one or make one ?.

author: apis
author_rank: Super Contributor
timestamp: July 22, 2015, 08:10:23 am
content: What do you mean by trimmer?

author: tron9000
author_rank: Frequent Contributor
timestamp: July 22, 2015, 08:12:20 am
content: http://uk.farnell.com/webapp/wcs/stores/servlet/Search?st=RG58&catalogId=15001&categoryId=700000006706&langId=44&storeId=10151These?

author: miguelvp
author_rank: Super Contributor
timestamp: July 22, 2015, 08:15:11 am
content: Maybe you are referring to a terminator, like a 50 ohm one?

author: Deathwish
author_rank: Unknown
timestamp: July 22, 2015, 08:32:28 am
content: No. the same as is on my scope leads to trim them

author: ElektroQuark
author_rank: Unknown
timestamp: July 22, 2015, 08:33:50 am
content: The variable capacitor od probes?You don't need that.

author: Deathwish
author_rank: Unknown
timestamp: July 22, 2015, 08:39:03 am
content: Why not ?. the waveform coming from my sig gen into my scope on a rg58 lead is sloping like hell.

author: apis
author_rank: Super Contributor
timestamp: July 22, 2015, 08:44:42 am
content: Is the scope input 50 ohm terminated?

author: BravoV
author_rank: Super Contributor
timestamp: July 22, 2015, 08:45:53 am
content: I guess the OP means the 50 Ohm terminator, here, a picture worth thousands of words ...

author: Deathwish
author_rank: Unknown
timestamp: July 22, 2015, 08:46:44 am
content: ahh ha so I need to get a 50Ohm terminator then , thanks.

author: apis
author_rank: Super Contributor
timestamp: July 22, 2015, 08:48:45 am
content: The terminator is just a 50 ohm resistor to ground (ie from inner conductor to the shield) I believe, so you might be able to make something yourself.

author: tomlut
author_rank: Regular Contributor
timestamp: July 22, 2015, 08:49:54 am
content: Make sure your sig gen is set to have a 50 Ohm output impedance as well.

author: Deathwish
author_rank: Unknown
timestamp: July 22, 2015, 08:50:11 am
content: Quote from: apis on July 22, 2015, 08:48:45 amBeats £1.50 for one then. will give it a go later.

author: Deathwish
author_rank: Unknown
timestamp: July 22, 2015, 08:50:40 am
content: Quote from: tomlut on July 22, 2015, 08:49:54 amIt does. Thanks.

author: BravoV
author_rank: Super Contributor
timestamp: July 22, 2015, 08:53:11 am
content: Quote from: tomlut on July 22, 2015, 08:49:54 amMaybe this sounds trivial, and also unlikely looking at OP's experience, but I mention it here anyway, make sure the scope's input isNOTset to 50 Ohm mode.Most low end scope don't have 50 Ohm mode input, and if it does, then you don't need the terminator.

author: apis
author_rank: Super Contributor
timestamp: July 22, 2015, 09:00:06 am
content: There will still be losses in a coax cable especially for high frequencies and longer cable but if you don't terminate it properly the signal bounces when it hits the end of the cable. The reflected signal adds up (superimpose) with the original on the way back and then goes into the signal generator, so you can get very strange results.

author: Deathwish
author_rank: Unknown
timestamp: July 22, 2015, 09:01:04 am
content: My scope is a tek 2225 thank you, soon to be a rigol 1054z today, I hope, if it comes.

author: miguelvp
author_rank: Super Contributor
timestamp: July 22, 2015, 09:05:46 am
content: Quote from: Deathwish on July 22, 2015, 09:01:04 amYou'll still need a 50 Ohm terminator for the Rigol. The DS2000 has a 50 Ohm terminator built in but only accessible via an SCPI command which is a pain because you'll have to reset it back and there is no UI for it within the scope's menu. So your best bet is to get the T connector and the terminator, or a through terminator (more expensive)

author: Deathwish
author_rank: Unknown
timestamp: July 22, 2015, 09:08:52 am
content: I have 3 tee connectors, I used to repair TV's many many years ago. I have never had the desire to leave any kit permanently connected to a scope so was unsure if it should be trimmed.

author: BravoV
author_rank: Super Contributor
timestamp: July 22, 2015, 09:11:43 am
content: Quote from: Deathwish on July 22, 2015, 09:08:52 am.... correction ... , the source of confusion to all of us here.

author: apis
author_rank: Super Contributor
timestamp: July 22, 2015, 09:12:54 am
content: https://www.eevblog.com/forum/projects/home-made-feed-through-terminator-for-bnc/

author: Deathwish
author_rank: Unknown
timestamp: July 22, 2015, 09:13:13 am
content: Quote from: BravoV on July 22, 2015, 09:11:43 amslap accepted. Sorry

author: BravoV
author_rank: Super Contributor
timestamp: July 22, 2015, 09:35:39 am
content: Quote from: Deathwish on July 22, 2015, 09:13:13 amIts ok, it happened to all of us sometimes.When I first read that word "trim", I have this in my mind ....

author: tautech
author_rank: Super Contributor
timestamp: July 22, 2015, 09:50:41 am
content: Quote from: Deathwish on July 22, 2015, 07:52:05 amProper termination is discussed in the first page of this thread:https://www.eevblog.com/forum/testgear/show-us-your-square-wave/

author: Deathwish
author_rank: Unknown
timestamp: July 22, 2015, 09:55:44 am
content: I'm getting old, leave me alone, actually I kept asking people here for a small filing cabinet, they kept bringing me them and i kept saying nope wrong thing, turns out I was looking for a bisely drawer set.... Then blast my eyes if no sooner than I order one for my bits Toploser starts to sell a decent set of drawers on wheels

author: Deathwish
author_rank: Unknown
timestamp: July 24, 2015, 01:55:14 pm
content: Back to this again, would it be of benefit if I added a 33pf variable cap at the end for compensation ?.

author: w2aew
author_rank: Super Contributor
timestamp: July 24, 2015, 05:28:32 pm
content: Quote from: Deathwish on July 24, 2015, 01:55:14 pmIf you're terminating into 50 ohms and your source is 50 ohms, then NO, you do not need or want any compensation capacitor.Note that if you have the scope set to AC coupling and are running a slow square wave, then it is normal to get droop on the pulse top and bottom. It's just a normal RC high pass filter response.Note also (trap for young players) that when you set the "load impedance" on your function generator, you are NOT changing its output impedance! It is still 50 ohms. The only thing that changes is the way the output amplitude level is computed and displayed. I've got a video on that.

author: Gyro
author_rank: Super Contributor
timestamp: July 24, 2015, 06:03:54 pm
content: Through terminators are normally pretty expensive, but as I see you're in the UK, RS Components have reduced their price to less than I've seen them elsewhere (just bought a couple myself) they're 2W rated too which is more than you typically get with the ones that you'd put on a T piece...http://uk.rs-online.com/web/p/rf-terminators/5464695/Price is +VAT of course but delivery is free.

author: miguelvp
author_rank: Super Contributor
timestamp: July 24, 2015, 06:33:29 pm
content: From a post I made regarding this:https://www.eevblog.com/forum/beginners/50-ohm-bnc-adapter-needed/msg677356/#msg677356I got mine from Wallco Industrial Electronics (Edit: not through but with T connectors):Calrad 75-614 BNC Dust Cap w/ Chain ($0.85 each), for when I don't have probes hooked into the scope, watch for the chain since it would be grounded if the scope is powered up, you can always remove the chain, but I don't let my boards that close to the scope so no need.http://www.wallcoinc.com/Calrad_75_614_BNC_Dust_Cap_w_Chain_p/wal22-75-614.htmCalrad 75-540 "T" Style Adapter w/2 BNC Females to 1 BNC Male ($1.50 each), they do have a 75 ohm version for over $3 but not sure if that would fit the scope's bnc (Calrad 75-690 if you want to get those instead):http://www.wallcoinc.com/Calrad_75_540_T_Style_Adapter_w_2_BNC_Females_t_p/wal22-75-540.htmPan Pacific BNC-7864 BNC TERMINATOR JACK, 50 OHM 1% 1/2 W ($1.15 each):http://www.wallcoinc.com/Pan_Pacific_BNC_7864_BNC_TERMINATOR_JACK_50_OHM_p/wal78-bnc-7864.htmPan Pacific BNC-7862 BNC TERMINATOR, 75 OHM 1% 1/2 W ($1.66 each), for analog video signals:http://www.wallcoinc.com/Pan_Pacific_BNC_7862_BNC_TERMINATOR_75_OHM_1_1_p/wal78-bnc-7862.htmDon't recall how long it took them to ship but I probably used the cheaper (fedex ground) because I was in no hurry. I think ordered about 3 of each, maybe 4.Edit: They do have a lot of other things, like all kinds of bnc adapters, switches, knobs, tools, well almost everything you can think off.Oh maybe you want to get some coax cable as well (search for "coax cable rg58" or "coax cable rg174" since they don't seem to carry rg142 which is best since it's double braided)

author: Deathwish
author_rank: Unknown
timestamp: July 24, 2015, 07:28:17 pm
content: Quote from: Gyro on July 24, 2015, 06:03:54 pmthank you, ordered oneEveryone else, all was noted so thank you for the repliesGarry

author: Deathwish
author_rank: Unknown
timestamp: July 24, 2015, 07:36:28 pm
content: Quote from: w2aew on July 24, 2015, 05:28:32 pmCan I have the link to that please

author: Gyro
author_rank: Super Contributor
timestamp: July 24, 2015, 07:41:58 pm
content: Glad it helped - you obviously have the same space issue as I do - trying to fit two T pieces and terminators onto the front of my Tek475A was getting silly for being able to access the knobs.It's pretty easy to overload terminators, 2W gets you to 10V rms if my sums are right. Much less for the smaller ones as I found out while playing with my function generator.Way back while I was working for RACAL the biggest source of friction with the Calibration Dept was repeatedly fried internal terminators on our HP scopes while working on manpack transmitters!Chris

author: Deathwish
author_rank: Unknown
timestamp: July 24, 2015, 07:57:50 pm
content: Are You from Reading then Gyro ?. I used to go out with a girl who worked for Racal and had been having a affair with one of the bosses who lived in eldon square there ....

author: Gyro
author_rank: Super Contributor
timestamp: July 24, 2015, 08:10:02 pm
content: Damn! rumbledYes, Just off the Basingstoke road (Mobilcal/Tacticom), back in the early '80s. Best not to hazard a guess at the identities of the individuals involved (although I can think of several who might have fit the bill)Small world!

author: Deathwish
author_rank: Unknown
timestamp: July 24, 2015, 08:25:54 pm
content: I lived at various addresses in reading from about 75 to 86. So your forgiven.

author: Deathwish
author_rank: Unknown
timestamp: July 24, 2015, 08:52:28 pm
content: bloody norah, i ordered it just now from rs and its enroute already , just been notified with tracking number.

author: Gyro
author_rank: Super Contributor
timestamp: July 24, 2015, 09:13:49 pm
content: They always used to be trade account only but a little known fact that anyone can have an account these days. Still standard next working day delivery. You can use the trade counters too but little point as they're free delivery now. All the good bits that you can't get from CPC

author: Deathwish
author_rank: Unknown
timestamp: July 24, 2015, 09:25:00 pm
content: Sounds good to me, i have accts there and with cpc

author: w2aew
author_rank: Super Contributor
timestamp: July 25, 2015, 02:30:01 am
content: Quote from: Deathwish on July 24, 2015, 07:36:28 pmHere it is:

author: Deathwish
author_rank: Unknown
timestamp: July 25, 2015, 02:46:28 am
content: Much obliged and thank you.

author: Deathwish
author_rank: Unknown
timestamp: July 27, 2015, 10:40:27 am
content: Came today, thanks for the advice, especially Gyro. I always take notice of what I am told so it is not wasted.