Audio Generator or Function Generator? Which to get? - Page 2

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Re: Audio Generator or Function Generator? Which to get?
On 3/13/2017 4:26 AM, snipped-for-privacy@tubes.com wrote:
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I haven't read all of the posts in this thread, but I see a lot of  
confusion of the terms "function generator" and "audio generator".  
Audio simply refers to the frequency range of the signal generated  
without saying anything about the manner in which it is generated.  But  
most signal generators output a sine wave and perhaps a square and  
triangle wave.  As others have said, they were often analog circuits  
that shaped one waveform into another.  This is different from an  
"arbitrary waveform" generator which some seem to be calling a function  
generator.

An arbitrary waveform generator has a pattern in memory which is played  
through a DAC to produce any function you wish which must repeat only  
when the memory capacity is reached.

A DDS is another type of repeating waveform signal generator that also  
uses digital logic and a DAC.  It uses an incrementer to model the phase  
of a signal cycle which then is translated into a waveform and played  
through a DAC.  If you want a sawtooth wave, you just play the phase  
value through the DAC.  A sine wave may be generated using a lookup  
table or some simple sine generation functions.  Other waveforms can be  
generated using the lookup table or other mathematical functions.  But  
all these signals must repeat at some period as defined by the amount  
the phase register is incremented at each clock.  In fact, that is the  
strength of the DDS over an analog signal generator, the frequency of  
the output signal is very accurately defined.

I'm not sure why you think these devices would not be reliable or are  
inherently inferior.  The Ham community have been using DDS for some  
time now.  Even if they aren't used to directly generate the frequency  
needed, they can be used as a tunable reference frequency with a PLL to  
generate a very accurate frequency of nearly any rational value.

--  

Rick C

Re: Audio Generator or Function Generator? Which to get?
I like my Wavetek 111. It is old, and rated 0.5 % distortion but probably b
ecause of caps that goes up at lower frequencies, like 20 Hz. It is a funct
ion generator and that is what I would recommend.  

For testing of audio equipment, the triangle wave can show distortion quite
 well. The square wave, at about 1 KHz is quite telling about the frequency
 response, though not a direct measurement. It is also fun to play with equ
alization on a 1 KHz square wave. It also has like a half ramp output that  
might be useful for the development of switching power supplies. It goes up
 to 1 MHz. I think that is enough.  

It was made here. Buying one however is a different story. They are old and
 most will need some caps, and they are a PITA to work on. They got all kin
ds of unmarked wires in them between the boards and you really have to watc
h taking them apart. Screwing that up happened to a friend of mine and the  
thing still doesn't work, just don't know which wire goes where.  

Finding a new US made one might be a bit hard, or pricey. The PC based ones
 are alright but if you want to "buy American" I doubt any of it is built h
ere. Even if it is, it is probably only assembled here. Probably built by A
pex in China or Korea, at least the board. The software might have been dev
eloped here.  

What you want has been in existence for a long time. Someone said stick wit
h solid state because tubes drift all over the place, I concur. I don't lik
e waiting for some of my older stuff to just warm up the caps, tubes add a  
whole new dimension to that.  

However tube ones might be more destructionproof. If you are about to take  
a probe from the output and start poking around tube amps you might want to
 consider that. It will drift, but tomorrow it will still drift. There are  
places in tube amps where if you want to stick that probe you'll need like  
a 100K resistor and a cap or it will be stuck at zero Hertz forever. The ad
dition of those components of course will affect the readings as well, so y
ou have to compensate.  

But that's my take on it. Also realize when you connect a PC soundcard to a
 tube amp, it better be well protected. And that is up to you. Resistor, di
odes, whatever it takes. Just dumping -40 volts bias into them might destro
y them. They need to be clamped.  

Bottom line, if you are going to work on old stuff, get old equipment. How  
old ? Depends on what is out there. If you go to eBay and look for wavetek,
 look at the ones that have the big knob on the front. And you might as wel
l get a freq counter because those things are not that accurate when it com
es to frequency. If you have a scope in good calibration though you can rea
d it on that.  

And that's another thing, you might want an old scope. They used to go to 2
0 V/div which gave you 200 V/div with a 10X probe. Newer ones do not go tha
t high and you would have to get a 100X probe. Using those in AC mode is a  
PITA because it takes so long to settle.

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