Signal Generator Advice Needed

[bob p]

I'm not exactly sure where to post this, so I guess this section of the board is as good as any.

I'm at the point where I need a better test signal source. For the longest time I've been using CDs and test tones, and I'm looking for a better solution. I thought I'd ask if anyone here has recommendations for good less-than-expensive solutions.

At one end of the spectrum, I guess I could go out and buy a function generator, but I think that's really overkill for my situation. (Looking on eBay, simple function generators are selling for as much as I paid for my oscilloscope!) A simple test tone oscillator seems like it would be the best bet for me.

I've looked around for schematics for DIY projects along these lines, and what I've found in the digital realm has been less than satisfying. Maybe I'm just looking in the wrong place.

Can anyone recommend a schematic for a good DIY analog signal generator? For amp work, I guess I could get by with an audio oscillator instead of a function generator, and it would seem simple enough to devise something built upon a simple oscillator. I would be willing to consider both tube-based and solid state options. In some respects, rolling my own little vacuum tube based oscillator would be a fun project, but I'd rather not re-invent the wheel and design this project from scratch. Any ideas on the subject and/or references to schematics or kits would be greatly appreciated!

[Enzo]

Yes, you are looking in the wrong places. For our work, function or audio generator mean the same thing. FGs tend to include stuff like sweeping and DC offsets and stuff, but all of them crank out a sine wave and that is what you mainly need.

Lab quality super function gennys can cost many thousands of $$$, but you can buy one new for under $200.

You need very little. Adjustable output freq and level into a hi-Z load. That covers 99% of them, and it covers 99% of your needs.

Buy new or used, or build from kit or scratch.

Digital schmidgital. ANy basic electronics hobby book will have a schemo for an audio oscillator. Unless you actually need to know the freq - and then I ask WHY? - most of us set the things by ear mostly. An osc circuit is utterly basic. Most electronics supplier like Jameco, Parts Express, MCM sell hobby/educational kits you build, and a simple audio oscillator is always on the menu.

Looking through MCM just now I see Function gens - new with warranty from their house brand for just over $200 and from B&K at just under. They sell a handheld unit for $55 with switchable freq - I prefer continuously variable myself. They sell a house brand audio gen for $185.

But why buy new? Look on ebay for basic stuff, and watch electronic surplus sellers. You can get a prettt nice old HP audio osc for a very few dollars. Search electronic surplus. One place I like is Fair Radio Sales. AN old HP201 or something is more than enough audio generator for you.

AnD get in touch with your local amateur radio operator. They are always haveing gatherings, and there are usually test gear items someone is selling cheap. And those guys as a group are usually more than willing to be helpful to teh inex[perienced.

[Mark Black]

I used one of these for years and it's really not too bad:

http://www.musiciansfriend.com/produ...tor?sku=428700

If you want to try to roll your own the manual (with schematic) for the above unit can be found here:

http://www.rolls.com/main/man.html

I modified mine somewhere along the way so I could switch between sine & square out the sine jack, leaving square output always on for scope trigger.

[Enzo]

There you go, utterly basic, and does everything you need.

[slajeune]

If I ever decide to get a function generator, I might actually go for something like this:

http://cgi.ebay.ca/100kHz-Function-G...QQcmdZViewItem

More than enough for amp needs. It's cheap and in keeping with DIY, you build it!

Cheers,
Stephane.

[Satamax]

If you have a good sound card and a stereo amp conected to it, you could use this!

http://www.david-taylor.pwp.blueyond....html#SweepGen

Done thhis for years, with teh headphones out, and a stereo>mono jack

[Rob Mercure]

Then there's always using any low voltage AC source - such as a wall wart - with a potentiometer across the output - say 50K for a 9 volt wall wart. Then you can vary the output voltage and monitor it with your DMM. Now 60 Hz is a little low but within the bandwidth of every guitar amp I've ever seen. But if you need a higher frequency - say an octave higher - just run the output through a full wave bridge rectifier - no filter caps - and then insert a capacitor, say -0.1 uF, in series with this signal source to block the pulsating "direct current" and you've got a 120 Hz signal source. Unfortunately it's no longer a sine wave but ....

One of the other unfortunate problems with this approach is that both frequencies can mask, or be masked, by either power line or power supply "hum." Now it's really easy to make a simple audio frequency oscillator with an IC and a few parts but IC's were just coming out (round metal cans) when I was in high school in the 1960s so the easiest signal source was a relaxation oscillator made with a neon bulb, a capacitor, and a resistor - and a DC power source.

If you've got a 90V, or so, DC supply you simply put a resistor in series with the supply and the capacitor and neon bulb in parallel across the supply (in series with the resistor). The resistor charges the capacitor until it reaches the ionization voltage of the neon bulb (around 78 volts if I rememeber correctly but this may off on the high side by ten volts or so) then the neon bulb conducts - flashes - discharging the capacitor which starts charging through the resistor again, etc. etc. If you make the resistor variable you can vary the frequency and if you add another capacitor to block DC in series with a potentiometer in parallel with the neon bulb you've got a variable frequency variable voltage "signal generator" which puts out a dirty little "sawtooth" wave (and if the first resistance/capacitance is high enough a flashing light). The frequency is determined by T = RC where T is time in seconds, R is the resistance of the first resistor in ohms and C is the first capacitor in FARADS (divide by 1M for microfarads). Other than the power supply you've it's only 5 parts (90 volt portable radio "B" batteries were also "barely" still available in the late 1960s so you could even have a portable signal generator).

An old fart's approach but easy and functional.

Rob

[Mars Amp Repair]

Elenco generators

The ones I have purchased for my tech is the Elenco SG-9500. It has up to a 10v p-p output sine or square. switchable range & constantly variable freq adust.
I bought both of mine on e-bay one new & the other nearly new in the orig boxes for under $100. One I actually won for $35. I don't see any of the audio ones there right now.
New from CE Dist they are $139...not bad at all. You can go there & view the specs & a pic @ www.cedist.com...if you dont' have an account, you can purchase it for a bit more on AntiqueElectronics.com...the consumer part of CE Distributing.
One really nice feature that I have used is the high output setting 15vp-p. A nice feature for directly driving the driver stage.
Elenco also has kits...I'm not certain if this is available as a kit, tho.

glen

[bob p]

Thanks everyone for their input. I was thinking that there had to be a really easy DIY solution, like a Wein bridge oscillator. As it turns out, the most expedient solution seems to be picking up a used function generator on eBay, so I think I'll go that route and leave the DIY part for amp building.

[Enzo]

A very simple Colpitts osc can be made with very few parts and run off a 9v battery. Look in any hobby electronics book. Or look for a waveform generator chip like the 8038 or equivalent. One chip with a few passive components.

If you are looking ebay, remember it need not be complex, and anything labelled function, audio, or sine wave generator wil work as will audio oscillator.

You can find cheap commercial equipment and also kit built stuff from Eico and Heathkit from the old timers, and stuff like Vellemans from the current crowd.

[bob p]

while we've been discussing things in this thread I had started tinkering with the trem oscillator circuit from a vibrochamp. it does its job well enough, but the sine wave signal isn't symmetrical. its as if the duty cycle is lop-sided. (i have to admit I haven't looked into this to determine why.) while that type of osc would be fine at a higher frequency for circuit tracing, its lack of symmetry made it less than optimal for circuit analysis. Well, the good news is that I picked up a used 2 MHz sweep function generator for all of $30 on eBay. Its Grrrrrrrrrrreat! Now I can even do frequency response plots.

If anyone else decides to go down this route, buying used gear on eBay is a pretty good alternative to creating a DIY project from scratch, or to building a kit. Decent used gear on eBay typically sells for less than what you'd pay for one of the kits described earlier. This makes the DIY option less desirable for test gear -- that is to say, I thought it was a better allocation of my time to buy used test gear and to save my DIY efforts for the amp circuits. Thanks again to Enzo and everyone else for their help!