My ancient Heathkit audio signal generator, which uses 3 tubes and a light bulb in an oscillator circuit, claims to produce sine waves with a total harmonic distortion less than 0.1%.
My semi-ancient Hewlett-Packard HP202B oscillator only claims to produce THD less than 1%.
I would have expected the HP instrument to have considerably better performance specifications because of its much more elaborate circuit.
Are we simply looking at pessimistic vs. optimistic ratings?
Sounds like the HP202B is the same old RC lamp-controlled oscillator, same as the Heathkit you describe (and an Eico 377 that I have, that has a terrible 20-200Hz band!), so there shouldn't be any fundamental difference between them. Maybe the ratings are as you say.
Thing about the HK, I bet, is it may not be tuned to low distortion (however you might do that), while the HP I would imagine came set from the factory.
Tim
-- Deep Fryer: a very philosophical monk. Website:
if I were buying a low-distortion oscillator, neither would be amongst my consideration.
What you're seeing is that a THD spec, all by itself, does not the performance of an oscillator describe.
Compare the other specifications of the two oscillators, e.g.:
Response flatness: I would expect the HP to be substantially better in spec and actuality
Settling time after aburpt frequency change, I would expect the HP to be FAR superior.
Short-term amplitude stability vs time: I would, again, expect the HP to be much better
Long-term amplitude stability vs time: ditto for the HP.
Phase noise/jitter: guess what? I'd expect the HP to be much better.
If you need to build a simple, cheap, easy-for-someone- of-unknown-skill-to-build oscillator, you make one set of design choices. If you need to build a rugged, highly stable, dependable unit which will has better short- and long-term stability, can be swept rapidly in frequency while maintaining that stability, and so on, you make a different set of choices. Both choices have costs and consequences.
If I wanted cheap (at original prices only), I'd consider the Heath, but I'd have to be willing to live with the fact that I can't depend upon it as a source of stable, dependable sine waves. If I needed a lab instrument that I could depend upon to not have the amplitude bounce substantially when I swept frequency, that I could implicitly trust for amplitude flatness over the full range, that would maintain its calibration for a long time, I wouldn't buy the Heath.
But if I wanted dependably low distortion, I'd have something else entirely.
THD as speced by the manufacturer is not the only measure of equipment quality. For example, a lot of the HP equipment would meet spec near their end of life or the end of life of their tubes. while Heathkit equipment might meet spec only if routinely maintained.
HP's audio signal generators were renouned for their relatively high power output, on the order of 1 watt for some models.
When you look at THD specs, you have to consider the whole operational environment, including the power level, the impedance of the rated load, and the frequency range.
mc> My ancient Heathkit audio signal generator, which uses 3 tubes and a mc> light bulb in an oscillator circuit, claims to produce sine waves with mc> a total harmonic distortion less than 0.1%.
mc> My semi-ancient Hewlett-Packard HP202B oscillator only claims to mc> produce THD less than 1%.
mc> I would have expected the HP instrument to have considerably better mc> performance specifications because of its much more elaborate circuit.
mc> Are we simply looking at pessimistic vs. optimistic ratings?
Some designs take the output at a different location and/or extra filtering to achieve better distortion numbers. It also depends on the circuit design, for example using a Wein Bridge configuration instead of multiple RC phase shifts, etc...
Yes, I had a HP 220CD, if I remember the model number correctly. And it was wonderful to look at inside. It got left behind on one of my moves.
Today, for practically any purpose that I can imagine in my home shop, the audio output on my PC sound card rules. A decent voltmeter tells me the output level.
I had a version that used a pair of 6V6's for about 20 watts output. It was built to test telephone lines and carrier current loops. Add the right transformer to the output, and it made a great 120 VAC variable frequency power supply. I had some of those large 24 VAC school clocks and used one with the carrier loop generator to vary the speed f the clock. Would would tell a pesky salesman he could have 15 minutes, then I would turn up the frequency to only give him four or five minutes, then slow it down and tell him his time was up, then point to the clock over his head. They never caught on. ;-)
--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
As an aside Jim Williams of Linear Technology has written very intelligent pieces on HP Wien-bridge oscillators and building an improved solid state version. I doubt it is on the web but you may be better at finding it than me.
Most PC sound cards vastly outperform legacy audio signal generators, both for low distortion and flatness. They also have excellent settling times.
The trick is finding an inexpensive one with good frequency response. My best meters are Flukes (not cheap) or the ProTek 506 (flat enough but not wonderful and still not exactly cheap).
If you're talking about transconductance, *any* two small-signal silicon transistors, even of different part numbers, will be better matched than 99% of tube "matched pairs."
And after three months of use, the tubes will have drifted all over the place, but the transistors won't.
And if you're talking differential offset voltage or drift of same, the transistors beat the tubes by volts.
And you can't compare beta, bacause tubes don't have it.
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