How useful is a function generator? - Page 1
Forum section: electronics / testgear

author: Ether
author_rank: Contributor
timestamp: December 20, 2014, 11:23:42 am
content: Hi,I'm interested in purchasing a second hand 2MHz function generator because I have been dabbling around in audio circuitry and I want something to help me troubleshoot tricky stages. Plus, I don't think my 555 timer oscillator can cut it anymore. May I ask specifically what applications are there for function generators and how useful are they. Also, is a second hand 2Mhz function generator for 50 usd shipped worth it or should I just save up for a new one? Side note: What exactly is the difference between a function generator and signal generator?Thank you.

author: poorchava
author_rank: Super Contributor
timestamp: December 20, 2014, 12:14:46 pm
content: It depends on particular model of the second hand generator you want to buy and whether or not it fits your needs.For audio stuff 2Mhz is more than plenty.I don't know if there is any official definition of function generator and signal generators. I think those two can be used interchangeably.Generators are used mainly for checking transfer characteristics of circuits. That is you feed your circuit different waveforms and observe how it behaves, for example if it distorts the signal, how does it react to fast edges, if it doesn't oscillate under some obscure conditions and so on.

author: Retep
author_rank: Regular Contributor
timestamp: December 20, 2014, 12:40:19 pm
content: Quote from: poorchava on December 20, 2014, 12:14:46 pmI don't know of any official definition and doubt there is, but my observation is that function generators can typically can generate multiple wave forms (square, triangle, sine...etc) at relatively low frequencies (< 10 MHz) whereas signal generators are primarily intended for RF applications; they usually only do sine waves and can often reach frequencies of several hundred MHz or even a couple of GHz.For just audio circuits a function generator would be more useful.

author: tggzzz
author_rank: Super Contributor
timestamp: December 20, 2014, 01:28:17 pm
content: Quote from: Ether on December 20, 2014, 11:23:42 amStrange question.If you don't have an application (i.e. problem to solve) then they are completely useless to you. Don't buy one, save your money.If you have a problem to solve, then state it - and we can see if a function generator would help.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: December 20, 2014, 01:40:19 pm
content: Quote from: Ether on December 20, 2014, 11:23:42 amThere isn't one. A Function Generator is a Signal Generator, but not all SGs are FGs (for example, a Pulse Generator is a Signal Generator, an Arbitrary Waveform Generator is a Signal Generator and so on). 'Signal Generator' is an overarching term, while a 'Function Generator' is just a certain type of Signal Generator.A Function Generator is a signal generator that outputs a signal conforming to a fixed set of mathmatical standard functions, i.e. sine wave, triangle, sawtooth etc.

author: GreyWoolfe
author_rank: Unknown
timestamp: December 20, 2014, 02:02:15 pm
content: Quote from: Ether on December 20, 2014, 11:23:42 amI got really good deals on a 2 MHz and 5 MHz function generators. I used the 5 MHz to help a new ham design and build a variable attenuator for 2 meter fox hunting. Put in a 5 MHz signal in and checked that the circuit attenuated the signal both on the breadboard and as a final test after the pcb was made and populated before building it into it's enclosure. He has a lot of fun with it. Also the granddaughter loves to see the 'magic' of Lissajous patterns on the oscilloscopeTggzzz said it- if you need it, it is useful and if you don't, it's not. You can probably find excuses to play with it if you really want to, though

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 20, 2014, 02:24:47 pm
content: I agree with tggzzz. We have no way of knowing whether ANY kind of signal generator would be a good investment for you. What other test equipment do you already have? Do you have an oscilloscope (CRO, DSO, USB, etc.)? Spending money in that area may be more beneficial if you already have a signal generator.Furthermore, you did not explain WHY you think your 555 "timer oscillator" won't cut it anymore? Why buy a signal generator instead of building one? Why do you think you need a "function generator" (implying more complex waveforms) for working with audio circuits?

author: _Wim_
author_rank: Super Contributor
timestamp: December 20, 2014, 02:52:41 pm
content: For audio work I would use a pc and sound card. It has off course less bandwidth, but for better resolution (16 to 20 bit). When using together with free software tools like REW (http://www.roomeqwizard.com/)and ARTA (http://www.artalabs.hr/), you can generate all necesarry test signals, and perform very good measurements at the same time.

author: Muttley Snickers
author_rank: Unknown
timestamp: December 20, 2014, 03:06:00 pm
content: I had to build a function generator as part of a TAFE electronics course many years ago, the frequency range was approximately 20Hz to 200kHz, it still works a treat and I passed the test.Then I got wind that the local university was updating their gear so I went around and picked up two GW Instek GFG-8016G for $40 each. They sat around for a while and then I got into various counting circuits, just CMOS 4510 to 4511 BCD to 7 segment, then later TTL and Atmel.Both generators were useful on these projects, one to simulate the clock pulse (eventually used a 555) and the other for the count input. They have been extremely handy gadgets for some of my projects.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 21, 2014, 12:33:23 am
content: to a carpenter, FG is useless. but for analog circuit designer esp audio, its way very usefull. audio is special case, you dont need 2MHz FG, you should aim for few KHz (just barely enough for audible freq), but super extraordinarily low noise low distortion spec. so they said, i heard stories. but if you not aiming for super high quality audio amp and decide to do other things later, 2-100MHz is good all around FG, your call.

author: jadew
author_rank: Frequent Contributor
timestamp: December 21, 2014, 02:42:51 am
content: I believe a versatile FG is one of the key tools you should have on your bench. If you're doing electronics, you're going to find a use for it, even if it's not apparent at the moment.I'm reaching for it more often than I would have thought, because it's not only useful for designing, but also for debugging and for quick tests.

author: Ecklar
author_rank: Regular Contributor
timestamp: December 21, 2014, 02:44:26 am
content: If you already have other test gear like one or two decent DVM and a scope and power supply, then go ahead and spend the $50 for the FG. For that price, experiment and play and you'll get a better feel and answer for all your questions. Probably better to get the used over a new at first depending on it's capability a new one will set you back quite a bit more than the 50 bucks.

author: free_electron
author_rank: Super Contributor
timestamp: December 21, 2014, 03:25:43 am
content: in practice signal generators are useful for developing or troubleshooting.in your case : when playing with audio equipment and repairing amplifiers for example a simple sinewave oscillator can be helpful. you can check for crossover distortion in the endstage for example ,if the generator has square capabilities you can check things like ringing in the endstage.then again : you can easily make those signals using the soundcard of your computer.. remember to put a series resistors and capacitors to isolate the computers output from your system under test.

author: German_EE
author_rank: Super Contributor
timestamp: December 21, 2014, 09:22:35 am
content: Very useful:SineUsed for signal purity tests and to test for distortion including clipping. If your design can't put out a pure sine wave you might as well stop there.SquareGood for impulse testing and setting tone controls correctly. You can also use it to drive logic.RampUsed to sweep another oscillator up or down, useful for bandwidth testing and, if you sweep an RF oscillator, adjusting things like I.F. stages in receivers.TriangleModulate an FM transmitter with a triangle signal and you get a nice flat topped waveform on the spectrum analyzer, great for deviation measurements.

author: tggzzz
author_rank: Super Contributor
timestamp: December 21, 2014, 09:37:23 am
content: If you are interested in audio, think carefully before getting anything based on an ICL8038. They have interesting glitches easilt visible on a scope. A Wien-bridge based oscillator would be more appropriate.

author: Ether
author_rank: Contributor
timestamp: December 21, 2014, 11:34:04 am
content: Thanks to everyone for the help. I think I'll buy one as I really do find myself in need of a pure sine wave generator.

author: markce
author_rank: Regular Contributor
timestamp: December 21, 2014, 12:00:08 pm
content: If you need sine purity (low distorsion), don't but a function generator, but adedicated sine generator.

author: tggzzz
author_rank: Super Contributor
timestamp: December 21, 2014, 01:31:35 pm
content: Quote from: Ether on December 21, 2014, 11:34:04 amConsider getting one based on an AD9851 DDS. These are remarkably cheap on ebay, and will be a simple implementation of the AD datasheet.Key points: 180MHz clock frequency, 0.1Hz to 50MHz, 70MHz 7th order low-pass filter, need to disconnect the DAC output from the comparator input (thus losing the square wave output), very crude ways of setting the frequency (increment/decrement each digit).

author: atferrari
author_rank: Frequent Contributor
timestamp: December 21, 2014, 03:27:05 pm
content: Quote from: tggzzz on December 21, 2014, 01:31:35 pmI was planning to build one using one of those DDSs but what discouraged me was the eliptic filter required at the output. The rest, besides the PCB, is just judicious programming of a micro.To the OP: I would buy it. When you need any signal to test a new idea, you have it already. Go for it.

author: Ether
author_rank: Contributor
timestamp: December 21, 2014, 03:51:47 pm
content: Update: I checked out another FG and I came across this one for 50 bucks.Its an Elenco gf-8026 that appears to be in decent condition having spent most of its life in a school.Does anybody know any particularly quirks or any information about this FG? And how does it stack up to instek ones that are going for 90 usd?

author: GreyWoolfe
author_rank: Unknown
timestamp: December 21, 2014, 04:00:08 pm
content: If it spent it's life at a school, it may not be in as good of condition as you think. Students are typically hard on test gear since it's not theirs. I saw this attending ITT Technical Institute back in the late 90's. I don't think things have improved since. I would stick with the original plan-get the one for $50 USD shipped if you can be assured it works and you can return if it doesn't.

author: KJDS
author_rank: Super Contributor
timestamp: December 21, 2014, 04:22:06 pm
content: Quote from: atferrari on December 21, 2014, 03:27:05 pmI quite like designing eliptic filters.see here for detailshttps://www.eevblog.com/forum/projects/what-type-of-antialiasing-filter-is-used-in-a-digital-scope/msg389166/#msg389166

author: Yago
author_rank: Frequent Contributor
timestamp: December 21, 2014, 04:26:34 pm
content: Does that Elenco have sawtooth?A handy waveform to have.

author: atferrari
author_rank: Frequent Contributor
timestamp: December 21, 2014, 04:28:01 pm
content: Quote from: KJDS on December 21, 2014, 04:22:06 pmShort and to the point. Gracias.

author: zapta
author_rank: Super Contributor
timestamp: December 21, 2014, 07:03:15 pm
content: For troubleshooting audio circuits you can use a function generator app. Here are the ones for Android. I am sure iOS has similar apps.https://play.google.com/store/search?q=function%20generaotr&c=apps&hl=enAlso useful to ball park the upper limit of your hearing. I tried it with friends and some can hear significantly higher frequencies than others.

author: free_electron
author_rank: Super Contributor
timestamp: December 21, 2014, 07:50:16 pm
content: Quote from: tggzzz on December 21, 2014, 09:37:23 amplus those things are long obsolete. so is the xr2206 , dead , buried , and also has distortion in the sinemake a simple wien bridge oscillator with an opamp. plenty of schematics around , follow it with an opamp in unity gain to get low impedant drive capability. and you are set.

author: tggzzz
author_rank: Super Contributor
timestamp: December 21, 2014, 10:05:22 pm
content: Quote from: atferrari on December 21, 2014, 03:27:05 pmThe design for a AD9851 is in the data sheet! The PCB layout is pretty obvious from the schematic - a long chain of SMD components!Don't connect the comparator input to the filtered DAC output though; transients are visible on the sine wave output.On ebay you can get boards without the display and controls, so you can hookup your own micro to the DDS.

author: cc83
author_rank: Contributor
timestamp: December 22, 2014, 06:46:20 am
content: Quote from: Ether on December 21, 2014, 03:51:47 pmI have a gf-8026 and it works well for most uses. I got mine used on ebay and it was from a school. it had a really bad hum and distortion in certain ranges when generating since waves so I found the schematic online and opened it up. Turns out someone probably blew out a lot of components and then fixed them (almost looks like they were using it for rework practice). The soldering was terrible and the hum was because when the person re-soldered the bridge rectifiers they didn't push the part to the board, they just soldered however they stuck in there at a angle with uneven leads, I also desoldered and resoldered the bypass caps and bypassed a broken trace. After some rework it's a great little signal gen for my uses.So if you are willing to maybe do some work you could take a risk.