What does a 50 ohm output mean for line behind the BNC socket? - Page 1
Forum section: electronics / projects

author: Varal
author_rank: Contributor
timestamp: December 18, 2011, 08:01:58 pm
content: Hi everyone,Long time no typeI've been working on my BSc degree (luckily finished) and stumbled upon a problem - I needed a 15V output from a square wave generator but the maximum I could get was 10V. I decided therefore to try and build a generator of my own (with 20V output). The question I can't get an answer for is how does the situation look with the 50 ohm BNC sockets. I know that a 50 ohm socket requires an RG58 cable (50 ohm characteristic impedance) but what happens "behind" the socket? Can I connect anything to it i.e hook some output opamp straight to the BNC ends or do I need to somehow match the impedance between the socket and the generator circuitry? And if so - how should I do this?Thanks in advance for any reply

author: wkb
author_rank: Super Contributor
timestamp: December 18, 2011, 08:31:36 pm
content: Well, the BNC socket does not magically convert the impedance of your signal generator to match 50Ohm. Your generator must be matched.

author: jahonen
author_rank: Super Contributor
timestamp: December 18, 2011, 08:45:20 pm
content: Easiest matching is to put 50 ohm series resistor between opamp output and BNC socket.Regards,Janne

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 18, 2011, 11:03:31 pm
content: note the 2nd picture...let me tag the link here for my future reference...http://www.ese.upenn.edu/detkin/instruments/HPfuncgen/WaveFormGen/WaveFormGen.html

author: amspire
author_rank: Super Contributor
timestamp: December 19, 2011, 01:15:27 am
content: Quote from: Varal on December 18, 2011, 08:01:58 pmYou can hook an opamp output directly to a BNC socket, but what do you mean by ""BNC Ends"?If you need to use a BNC coax cable, you are better off matching impedance. An opamp will not like driving a length of unterminated coax and most opamps cannot drive a 50 ohm terminated cable without something like an additional emitter follower on the opamp output.If you want to connect your opamp output to a BNC input on an instrument, that is fine as long as the opamp can drive the impedance of the input.If you are asking if you can hook an opamp amplifier input directly to the BNC output of the oscillator, then yes you can. If you are going to use a length of coax to make the connection, you are better off having a 50ohm resistor across the opamp input. If you are not using a length of coax cable, you can probably get away with no resistor, but the output from the generator will probably be double the indicated output voltage.Richard

author: Unknown
author_rank: Unknown
timestamp: December 19, 2011, 01:16:27 am
content: Look up transmission line in your text books. The output impedance of your generator should be matched to the cable and the load, 50 ohms is a common impedance. Note that if the wave length of your signal is much longer than your cable, you don't have to pay as much attention to impedance matching since you're unlikely to see the effects unless your signal has very fast edges (high frequency components). This is the case in audio, where 600 ohm is common, even though there is no such thing as a 600 ohm cable.

author: Varal
author_rank: Contributor
timestamp: December 20, 2011, 03:05:05 pm
content: Thank you everyone for your answers. I have an institute of microwave and antenna engineering at uni so I'll be able to have some more f2f consultations regarding this matter

author: ejeffrey
author_rank: Super Contributor
timestamp: December 20, 2011, 03:25:21 pm
content: Quote from: alm on December 19, 2011, 01:16:27 amSure there is, although it is twin-lead not coax. Twin lead 600 ohm cable is (was) used for long distance analog telephone and telegraph lines. It can also be used as an antenna feedline although 300 ohm is more common. The use of 600 ohm transmission lines in the phone system is what lead to it being a standard impedance for audio circuits even when they were too small to need proper transmission lines.

author: Unknown
author_rank: Unknown
timestamp: December 20, 2011, 03:33:17 pm
content: You're correct, I should have qualified that statement with "there's no such thing as a 600 ohm cable in the audio world". Typical cable impedance are much lower, assuming any attention was paid to characteristic impedance.

author: don.r
author_rank: Frequent Contributor
timestamp: December 20, 2011, 04:25:36 pm
content: Output from a 70's era Wavetek. Note both the attenuation resistors and the 49.9 ohm resistor network on the output.