How do I make a terminated test lead?

[markzb]

Hi everyone

Hope you can help this newbie!

I bought myself a used function generator which is hard to get hold of this side of the world. Its a Leader LFG-1300S and Im so glad to have this toy.

Anyhow, I got a copy of the manual and it says that when you want to do amplifier testing, you need to terminate the line with a 50 Ohm terminator on the amplifier input.

On the generator itself there is a female 50 Ohm BNC connector.

I would like to make a test lead that has a BNC male on one end and a pair of RCA's on the other.

My question is kind folks, how do I build a test lead that incorporates this termination into it.

What bits should I buy?

Many many thanks!

mark

[leadbelly]

Could you perhaps quote parts of the manual in more detail? My 1st inclination would be that the manual is instructing you to terminate the unused input of the function generator when you are using its output for testing amplifiers.

[jackinnj]

you can buy 50 ohm terminators -- Tektronix or Hewlett Packard -- on EBay --

[markzb]

Hi Jackinnj

Thanks for your reply. I am able to buy a terminator here at farnell. My problem I guess is that I am not sure how to use it!!!!!


Would it be ok and correct if I buy a coax cable of say 3 foot length with BNC's on both ends....one end into function gen, and one end into the terminator. Then from this terminator, I hook up a BNC to RCA adaptor to connect to audio gear. Is this correct?

Many thanks guys

mark

[leadbelly]

I found this link:

http://www.tele.pitt.edu/Main/Resour.../laboscope.pdf

As you can see, it makes no mention of a 50 ohm terminator when wiring up your FG, just a straight cable with 2 alligator clips and a BNC on the other end.

Have you perhaps read too much into some phrasing in the manual? 50 ohms is the characteristic impedance of the BNC connector and cabling, and termination can simply mean connecting a cable.

[jackinnj]

for audio it probably isn't necessary when you are just starting out -- for radio frequency (and up) it becomes important -- the generator wants to see a uniform load across the spectrum you are measuring. if the load is mismatched a portion of the signal will be reflected back to the source, when it is fed back it will add and subtract the generator output. the reflection is the bane of our existence for those who design their own antennas and feed systems.

50 ohms is standard for RF work.

on cables -- RG58 has a characteristic impedance of 50 ohms while RG59 is 75 ohms. even the connectors have an effect upon the impedance -- RCA plugs and jacks are pretty awful for RF, but adequate for audio.

as you move up the money chain the impedance match gets better and better, but it isn't a critical issue at audio (I am sure that there are some out there who use gold-plated SMA connectors for audio.)

[markzb]

Hi Guys

Many thanks for your replies. Muchly appreciated! I wont worry too much about it then!

For now......

Cheers

mark

[moamps]

Hi,
LFG1300 has PI 50ohms attenuators 10, 20 and 40dB at output, witch means that 50ohm terminator (or overall load) is needed to get correct (exact) attenuation ratio. I don't use this terminator at output because allways have scope and voltmetar connected and know what I have at output.
If termination is needed, simplest way is connecting BNC T-adapter on output and 50ohm terminator (two 100ohm metalfilm resistor parallel conected in one 50ohm BNC connector) on one output and BNC-2 alligator clips RG58 cable on second output.

Regards
Milan

[timetec]

@ markbz - would it be possible for you to tell me where you obtained your LFG-1300S Function generator manual please ?

I've been looking for a copy for months, but have had no luck in finding one.

Many thanks in advance