DIY Signal Generator

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Mike H
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#1 DIY Signal Generator

Post by Mike H »

Due to the enthusiasm re 2nd hand oscilloscopes, it has subsequently also been revealed that some folks are without a usable sig gen. There may be 2nd hand ones about you could pick up, but if not, I have (on and off) been applying my bwain to the pickle of build-yer-own. Cos it can be a bit of a pickle....

Traced the circuit of my SG-2601A the other day but TBH would be very complex to build; all discrete trannies (actually it's basically a low power p-p amp but with a quite unusual FET input arrangement), plus, would be very difficult to match the thermistor used with anything "off the shelf". Plus, it doesn't simulate well in LTspice, which doesn't bode well, unless there's something I've overlooked....

OTOH there is a LTspice "educational" example for a wien null oscillator based around an op-amp. Also, I discovered that Rapid stock tiny weeny little eensy weensy light bulbs that they describe as "grain of rice lamp" (literally, the size of a grain of rice), turns out this might be perfect as the low power, temperature dependent, resistance compensation device.

This simulates very well and would be easy to make, i.e. not a lot of components. It would use the NE5532 audio quality op-amp IC, coupled with a class A emitter follower buffer for low impedance output.

Diagrams later....


 
 
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thomas
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#2

Post by thomas »

Sounds excellent! Looking forward to hearing more...
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shane
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#3

Post by shane »

At the risk of spoiling the fun, you could try something like this:

http://3d2f.com/tags/test/tone/wav/files/
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Mike H
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#4

Post by Mike H »

Oh yeah had a look at some of those before, weren't too impressed TBH and they all seem to what money before you get anything that's really useful.

I can do virtually the same with Cool Edit-Pro, except, nothing above 19 kHz, exceeds computer's audio bandwidth.


Besides spinning a real knob is amazingly quicker and easier :lol:


 
 
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Mike H
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#5

Post by Mike H »

OK well here's something ~ I balls'd up the attenuator bit so have to edit it, and haven't done the squarer section yet, but here's the oscillator section.

Not sure about the 22k pot TBH because another circuit I've got uses a 10k pot with the same capacitor values for 10 Hz to 1.5 MHz, but then again, a reduction drive would be desirable otherwise very awkward getting exact adjustment. Or, Rapid still do 10 turn potentiometers:

http://www.rapidonline.com/Electronic-C ... ters/29588

Otherwise, a vernier dial reduction drive could be used but where rotation is limited to 180°, hence if 22k pot you are at least guaranteed to get >half the total value.

You can get a new vernier dial from Mouser:

http://uk.mouser.com/ProductDetail/Eagl ... GieA%3d%3d


For a description of the principles see here:

http://en.wikipedia.org/wiki/Wien_bridge_oscillator

Including use of a bulb for the stabilising element!

I designed this one to output 15V peak (because my own sig gen does); gain is set slightly greater than X3 by R5 & R6, but at 15V the lamp is taking 1mA and the idea is it heats up until its filament >=500 Ohms. At which point the ratio of R5 to R6 and the bulb is reduced to 2:1 (gain of 3).

If o/p Voltage increases then so does the filament temperature and it's resistance, so gain is reduced to <X3. Thus the o/p is stabilised at 15V peak. (In theory.... :D )

The o/p is class A using an emitter follower with a CCS, and is supposed to maintain full Voltage into a 600 Ohms load. The controlling element is a high quality audio op-amp IC.

To get the full 30V peak-peak Voltage swing without clipping needs a 36V supply, which is max. for the NE5532.

The NE5532 could do it all by itself, but there is likely a bandwidth limitation problem due to the max. Voltage swing (for this device) into a load like 600 Ohms, hence the addition of an o/p stage.

Edit ~ circuit changed!
Last edited by Mike H on Mon Sep 13, 2010 1:01 pm, edited 1 time in total.
 
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Mike H
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#6

Post by Mike H »

Another balls up, my sig gen is not 15V after all, it's 10V :oops:


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Nick
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#7

Post by Nick »

Why not just use the other half of the op amp as an output buffer, I would think it would provide enough drive for most uses?
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#8

Post by Neal »

Why not use a signal generator chip ;)

http://courses.cit.cornell.edu/bionb440 ... r8038a.pdf
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#9

Post by Neal »

...or a ICL8038 chip....
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andrew Ivimey
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#10

Post by andrew Ivimey »

I like 8038s; voltage controllable but they are about £8 each if I remember.
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#11

Post by Mike H »

Thanks, however I particularly wanted to do it the "old fashioned way" as all you can get nowadays (and for some time past) are function generators, and I have an irrational dislike of function generators, if probably unfounded....


Nick wrote:Why not just use the other half of the op amp as an output buffer, I would think it would provide enough drive for most uses?
TBH I think that's probably the simplest option really specially as there's 2 op-amps on the same chip, I was mainly concerned about increased distortion doing that but probably not much in it in the long run :D


Back shortly ....


 
 
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#12

Post by Nick »

Neal wrote:Why not use a signal generator chip ;)

http://courses.cit.cornell.edu/bionb440 ... r8038a.pdf
In the case of that chip, I would say the 1% THD would be a strong reason why not. Depends on what you want to do with the signal I guess.
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#13

Post by pre65 »

So, what is the difference between a "function generator" and a "signal generator" ?
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Nick
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#14

Post by Nick »

I think mainly the complexity of the wave shapes that can be generated.
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#15

Post by Dave the bass »

It's Sine and square that we need mostly though isn't it WRT amp testing?

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