Need help understanding function generator usage. - Page 1
Forum section: electronics / beginners

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 09, 2014, 03:11:39 pm
content: I recently purchased a used Wavtek 188-S-1257 on ebay. It didn't come with a manual or cables. I managed to find a manual for a slightly different generator, but it isn't very thorough nor does it include any information at all regarding the 3 banana plugs in the lower right of the panel. Before this generator, I've never even actually seen one in use.The bottom jack is labeled "ct." What does that stand for?How are the banana jacks used? What kind of cabling? When to use? I've been told these jacks are 600 ohm impedance. Won't I get a bunch of reflections if I use a cable with a 50 ohm BNC terminations and banana plugs?Am I able to use the 600 ohm output and the 50 ohm simultaneously? The manual I have says that two resistors need to be changed if you want to use the two outputs simultaneously. That model also has a HI and LO 50 ohm output. The "LO" output is in the same location on the panel that the banana plugs are on mine. Does this mean if I want a second output, I have to use the banana plugs?I'm sorry for all the remedial questions. If anyone knows of a decent manual for a similar model online, I'd be most appreciative. I attempted to walk through the test procedure in the outlined in the manual. It shows how to connect the generator to a dual channel scope and dc supply for testing using the HI and LO outputs. I'm not sure how to modify the test to evaluate the unit's functionality because of the lack of a LO and the inclusion of the banana jacks. The manual is locatedhttp://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCwQFjAA&url=http%3A%2F%2Fsites.fas.harvard.edu%2F~phys191r%2FBench_Notes%2FA9%2FWavetek188.pdf&ei=o2JFU_u2IY_KsQT0rYDwCA&usg=AFQjCNEbCv_EbGp6v74fQzVuwTh_3y507Q&sig2=3zmITu9MdfMpCFAl0dtmAw&bvm=bv.64507335,d.cWc. I'd be most appreciative if someone could help me get through the test procedure.

author: linux-works
author_rank: Super Contributor
timestamp: April 09, 2014, 03:29:08 pm
content: if its balanced out, it could be trafo coupled and the ct would be 'center tap'.banana jacks would be for lower freqs and since its balanced, likely for audio.for higher freq/rf use, the bnc shielded is better.my guess is that the 50ohm output is buffer driven and the banana outs are trafo driven.

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 09, 2014, 06:12:20 pm
content: What is meant by "balanced?"

author: linux-works
author_rank: Super Contributor
timestamp: April 09, 2014, 06:18:09 pm
content: google 'balanced vs single-ended'.balanced is also known as 'differential'.sending a signal in phase on one wire and 180 out of phase on the other. you then invert one and sum, this cancels common mode noise and interference. pro audio uses balanced or diff line signaling, for example. trafos are a common and simple way to convert unbal to bal.

author: Holograph
author_rank: Regular Contributor
timestamp: April 09, 2014, 06:19:23 pm
content: Quote from: chipwitch on April 09, 2014, 06:12:20 pmhttp://en.wikipedia.org/wiki/Balanced_lineIt's mainly for common mode noise rejection.I had the same thought on the CT thing, but I can't say I'm confident that it's definitely that.

author: linux-works
author_rank: Super Contributor
timestamp: April 09, 2014, 06:25:38 pm
content: you only need 2 wires to be in balanced mode. 3 wires (ground) is not strictly needed. ground in balanced means 'midpoint' between the 2 other wires and so a center tap would make perfect sense here. a local ground (if the gear wanted it, for some reason) and yet still floating unless the user connects the CT to chassis earth ground.so, I'd bet that ct is the center tap of the output trafo.

author: pickle9000
author_rank: Super Contributor
timestamp: April 09, 2014, 06:49:34 pm
content: Tada! (manual)http://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=9&cad=rja&uact=8&ved=0CGkQFjAI&url=http%3A%2F%2Fw140.com%2Fsbcman%2FTechnical%2520Manuals-Main%2FWavetek%2Fwavetek%2520188-S-1257.pdf&ei=xZRFU42yC4GiyAGEjYGwDA&usg=AFQjCNHQJxiIrm9qWDQv85FYmdh3r18X9g&bvm=bv.64507335,d.aWc

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 09, 2014, 06:55:20 pm
content: So, a black banana plug is tied to the shield of the coax and a red one to the core. By having two signals 180 degrees apart much of the noise is cancelled. The receiver then inverts one signal and sums the two to in effect amplify the signal? Would I be correct in assuming my first sentence in this post incorrectly referenced "coax" and that the balanced method of noise mitigation is typically used in non-coaxial parallel cabling?

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 09, 2014, 06:57:11 pm
content: Quote from: pickle9000 on April 09, 2014, 06:49:34 pmDaaammnnn!!! You're AWESOME! How'd you find that? I was well into the Russian pages in my Google search before I quit searching.

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 09, 2014, 07:03:54 pm
content: I see.. CT must be "center tap." They call it "ground" in the manual. Also, I see the phrase I should have used was "twisted pair" instead of "non-coaxial."Thanks again Pickles. Looks much like the other manual I have, with just a few important changes.

author: pickle9000
author_rank: Super Contributor
timestamp: April 09, 2014, 07:13:42 pm
content: Quote from: chipwitch on April 09, 2014, 06:57:11 pmHere is the google searchWavtek 188-S-1257 pdfAdding pdf for manuals and datasheets will often point to an exact file. If you do the search note the file type is listed (PDF upper left) and go for that one first, usually a direct download.

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 09, 2014, 07:31:03 pm
content: Yeah, Ilookfor "PDF" after I search, but it never occurred to me to actually include itin the search.I learned acouplenew things today!

author: vk6zgo
author_rank: Super Contributor
timestamp: April 10, 2014, 01:24:25 am
content: Quote from: chipwitch on April 09, 2014, 06:55:20 pmBy having two signals 180 degrees apart much of the noise is cancelled. The receiver then inverts one signal and sums the two to in effect amplify the signal?NO --this is a very confused idea!There is only one wanted signal!This signal is just ordinary audio ,but is not referenced to earth,so the red & black conductors are just the two needed to carry any signal-----there is no magic involved!Balancedaudio cable has a pair of conductors carrying the signal,& a shield braid which isonlyfor shielding.The two inner conductors are quite often red & black,hence the banana /binding post colours..Noise & hum,etc is normally "common mode",in that it is induced equally into both conductors of the balanced pair.Cancellation of common mode noise is inherent in the balanced circuitry involved.Broadcast audio uses balanced audio,originallyas a matched 600 Ohm system,using centre tapped transformers at the input & output of devices.(I will describe cancellation in this earlier system,as it is simpler.)The common mode noise signals are unbalanced to earth,so the magnetic fields produced in the transformer cancel.The balanced signals are passed through the transformer.While many more modern systems use low output impedances (around 40 Ohms) "bridged" by higher input impedances of the driven devices,the active circuitry used still allows cancellation of "common mode" noise in a similar manner.Ideal balanced cables wouldn't need a shield,but in the real world,both conductors do not have exactly the same noise signal induced in them,so perfect cancellation is not quite achieved.The shield braid acts as a "belt & braces" method of reducing all pickup to a low level.Although many people will scream at the next comment,most audio cabling I've seen( & I've seen a lot!)has the shield of the cable returned to the "earth"(not always a true earth) of each device.For very low signal levels, such as microphone circuits,the shield is often only connected at one end.

author: linux-works
author_rank: Super Contributor
timestamp: April 10, 2014, 01:34:54 am
content: you are not entirely correct, sorry.balanced can be implemented many ways. a single driving source going into a trafo generates balanced output, with or without a center tap or midpoint. midpoint is not at all needed, of course.balanced can be done with TWO active transmitters, feeding a wire each and 180 out of phase with each other. its common to see that in audio; true active drive using 2 buffer stages. I've seen whole amp boards acting as a 'half driver', requiring 2 to have a balanced drive output.if you do have a midpoint, then the Z between that midpoint and each of the 2 drive wires should be the same. the voltage swing should be the same (magnitude); but if you don't have a 3rd wire, its not a problem as its still just the DIFFERENCE between those 2 wires that is decoded by the balanced line receiver, center-tap ground or not.my hunch is that the CT post is there for easy conversion from bal to single-ended. just take the positive post and the CT and you have a poor man's SE output.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 10, 2014, 02:08:06 am
content: Quote from: linux-works on April 10, 2014, 01:34:54 amThe balanced cable doesn't carehowyou produced the signal --as far as it is concerned,it hasone& only one wanted signal travelling along it.Any of the methods you described can be drawn as a "Black box" with a balanced output.The only time you can say there are two signals 180 degrees out of phase is when you refer it to a "midpoint",& as you said,"midpoint is not at all needed"."as its still just the DIFFERENCE between those 2 wires that is decoded by the balanced line receiver, center-tap ground or not."I think that was what I've been saying!P.S.Perhaps I "over edited" my earlier posting------- there was a lot of "greybeard" stuff I got rid of!The "originally" in the sentence:"Broadcast audio uses balanced audio,originally as a matched 600 Ohm system,using centre tapped transformers at the input & output of devices"made sense in that earlier context,but ,it probably should be"historically"in the edited sentence.I certainly did not mean to imply transformer outputs were the only way to produce balanced signals.It seemed the easiest way to explain to a newb.

author: linux-works
author_rank: Super Contributor
timestamp: April 10, 2014, 03:09:40 am
content: Quote from: vk6zgo on April 10, 2014, 02:08:06 amyes, that's true.I guess what I was trying to say is that you can have 1 generator or 2 and the 2 active wires, like you say, don't know or care how they got their current flowing thru them. but if you DO have a 3 wire 'transport', then looking at ground and the 2 signals should show them being opposite phase. if you have 2 wires, you can't see this but you won't care, reallysome audio guys get picky about how you drive balanced outs. they get fussy if you have 'pseudo-balanced' using 1 generator (signal source) instead of 2 true generators. I don't see any real diff as long as both connections are equidistant from ground and fully floating.

author: codeboy2k
author_rank: Super Contributor
timestamp: April 10, 2014, 10:48:09 am
content: Quote from: pickle9000 on April 09, 2014, 07:13:42 pmOne other trick -- add filetype:pdf not just pdf and you will ONLY see pdf files.It's great for finding books online , nudge nudge , wink wink...Wavtek 188-S-1257 filetype:pdf

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: April 10, 2014, 01:25:07 pm
content: I am not an engineer or into the audio side , much , my interests in the subject are from the RF side of things .Having read the posts and seeing a couple things that seem just wrong and then thinking about some of the other points .A lot has to do with your perspective of using these subjects .I am a ham radio operator and also learning to do repairs and later building .With out grasping the original perspective of the thread , just sig, gen. , and then discussion about balance .From my understanding and from an RF perspective , I'l start with the balanced - 1st off we think of these terms relating to feed-line , a balanced feed-line is most any type of twin-lead 300 ohm TV feed-line , 450 ohm ladder-line and then much higher impedance [ created buy the distance between conductors ] home-brew 4" or 8" .Then unbalanced is coax and what creates unbalance in coax is 3 electrical paths [ remembering skin effect - as the frequency gets higher the current travels closer to the surface ] the center conductor and the inside of the shield making up the wanted signal path called differential-mode , then when there is some impedance mis-match - like change of feed-line , like ladder-line to coax or a mismatch to load - antenna , that mis-match redirects some of the signal at the end of the feed-line - like the coax connector and starts the 3rd path back to the generator - transmitter , and this is called common-mode current .I like to think that if for what ever reason the antenna and feed-line have a mis-match this is where solving the common-mode current issue should be addressed , either by correcting the mis-match [ not always possible because of the limitations of the environment or restrictions of the site ] then a matching system like a gama-match , matching network or one type or other of some of what is mentioned here , baluns or unans - balanced to balanced or unbalanced to whatever , there are many types of baluns - this term becomes generic for ease of discussion .As I understand it baluns come in several variations voltage & current baluns , normally voltage baluns are power-supply and the current baluns are normally related to RF installations and those are further broken down into current baluns that trap common-mode current to stop it traveling into the shack , then next is ratio baluns that are referred to as transformers and modifying / matching the impedance where there maybe a mismatch like 450 ohm ladder-line & connecting to coax , or if the antenna needs matching to whatever feed-line .Hope this helps tie some of this together and any corrections welcome

author: linux-works
author_rank: Super Contributor
timestamp: April 10, 2014, 02:43:32 pm
content: what you are talking about is balanced vs unbalanced *transmission lines*. not the same thing (at all) as balanced or differential mode signalling.skin effect does not enter into this.coax is 'unbalanced' since one conductor IS ground and the other is hot. one of the telltales or rules for balanced is that both conductors are 'of the same' and one is not grounded; they are both 'hot' wires. measuring (even conceptually) the 'distance' or z from each wire to ground should be the same. with coax, one wire acts as a return path and also as ground, so it fails the 'balanced' test.twinlead is a form of balanced (usually you take coax and go thru a trafo and so now you are ground lifted, both wires are equidistant from ground and both wires are 'similar' to each other. but its more common to refer to this as a balanced transmission line than balanced signalling mode.heck, even 2 wire POTS is technically balanced since neither is grounded and both wires are 'hot'.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 12, 2014, 01:35:28 pm
content: One widely used multi-band antenna is the "untuned doublet".The balanced feeder for this antenna operates at extremely high SWR figures,but it does not cause "RF in the shack".Conversely,it is regarded as good practice to use a 1:1 balun with a resonant halfwave dipole to reducecommon mode pickup on a coaxial feeder.There are several other methods to achieve this,such as the "ugly balun",which is not strictly speaking,a balun at all,just the coax feeder would into a coil,acting as a "choke" to reduce common mode RF currents.Any feeder looks like an antenna to any Electromagnetic field, which will induce currents in both conductors of the feeder,to a greater or lesser extent,even in cables which are correctly terminated & have no wanted signal applied.Balanced feeders are one way to minimise the adverse effects of this,as are baluns with coax cables.