RG58 cable trimmer

[Deathwish]

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 ?.

[apis]

What do you mean by trimmer?

[tron9000]

http://uk.farnell.com/webapp/wcs/stores/servlet/Search?st=RG58&catalogId=15001&categoryId=700000006706&langId=44&storeId=10151

These?

[miguelvp]

Maybe you are referring to a terminator, like a 50 ohm one?

[Deathwish]

No. the same as is on my scope leads to trim them

[ElektroQuark]

The variable capacitor od probes?
You don't need that.

[Deathwish]

Why not ?. the waveform coming from my sig gen into my scope on a rg58 lead is sloping like hell.

[apis]

Is the scope input 50 ohm terminated?

[BravoV]

I guess the OP means the 50 Ohm terminator, here, a picture worth thousands of words ...

[Deathwish]

ahh ha so I need to get a 50Ohm terminator then , thanks.

[apis]

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.

[tomlut]

Make sure your sig gen is set to have a 50 Ohm output impedance as well.

[Deathwish]

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.

Beats £1.50 for one then. will give it a go later.

[Deathwish]

Make sure your sig gen is set to have a 50 Ohm output impedance as well.

It does. Thanks.

[BravoV]

Make sure your sig gen is set to have a 50 Ohm output impedance as well.

Maybe this sounds trivial, and also unlikely looking at OP's experience, but I mention it here anyway, make sure the scope's input is NOT set 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.

[apis]

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.

[Deathwish]

My scope is a tek 2225 thank you    , soon to be a rigol 1054z today, I hope, if it comes.

[miguelvp]

My scope is a tek 2235 thank you    , soon to be a rigol 1054z today, I hope, if it comes.

You'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)

[Deathwish]

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.

[BravoV]

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 .... terminated .

.... correction ... , the source of confusion to all of us here.

[apis]

https://www.eevblog.com/forum/projects/home-made-feed-through-terminator-for-bnc/

[Deathwish]

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 .... terminated .

.... correction ... , the source of confusion to all of us here.

slap accepted. Sorry

[BravoV]

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 .... terminated .

.... correction ... , the source of confusion to all of us here.

slap accepted. Sorry

Its ok, it happened to all of us sometimes. 

When I first read that word "trim", I have this in my mind .... 

[tautech]

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 ?.

Proper termination is discussed in the first page of this thread:
https://www.eevblog.com/forum/testgear/show-us-your-square-wave/

[Deathwish]

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

[Deathwish]

Back to this again, would it be of benefit if I added a 33pf variable cap at the end for compensation ?.

[w2aew]

Back to this again, would it be of benefit if I added a 33pf variable cap at the end for compensation ?.

If 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.

[Gyro]

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.

[miguelvp]

From a post I made regarding this:
https://www.eevblog.com/forum/beginners/50-ohm-bnc-adapter-needed/msg677356/#msg677356

I 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.htm

Calrad 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.htm

Pan 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.htm

Pan 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.htm

Don'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)

[Deathwish]

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.

thank you, ordered one

Everyone else, all was noted so thank you for the replies

Garry

[Deathwish]

Back to this again, would it be of benefit if I added a 33pf variable cap at the end for compensation ?.

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.

Can I have the link to that please

[Gyro]

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

[Deathwish]

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 .... 

[Gyro]

Damn! rumbled   

Yes, 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!

[Deathwish]

I lived at various addresses in reading from about 75 to 86. So your forgiven.

[Deathwish]

bloody norah, i ordered it just now from rs and its enroute already , just been notified with tracking number.

[Gyro]

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

[Deathwish]

Sounds good to me, i have accts there and with cpc

[w2aew]

Back to this again, would it be of benefit if I added a 33pf variable cap at the end for compensation ?.

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.

Can I have the link to that please

Here it is:

[Deathwish]

Much obliged and thank you.

[Deathwish]

Came today, thanks for the advice, especially Gyro. I always take notice of what I am told so it is not wasted.

[Gyro]