THD claims of audio signal generators

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?

I think that's right.

Would the THD be appreciably lower at the mid audio frequencies?

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.

The HP generators derived from the Model 200 will run forever, or at least more than one human lifetime. There are no proprietary parts that are likely to wear out with normal use. If needed the transformers and the tuning capacitor could be repro'd for far less setup costs than having a replacement IC taped out, masked and a run made such as would be needed on many solid state units using proprietary ICs.

Everyone should own and care for one of these things. They were manufactured until 1986 and represent the antithesis of all that is wrong, immoral, indecent, and generally f***ed up in the electronics business today. (And most will run on 220 balanced power with only a flick of the voltage switch...)

A fucktard and spaznerolli like always.

"mc"

Betcha that is true only above 50 Hz and below 20kHz.

If it manages < 1% THD at that frequency, using bulbs to stabilize the level - it is tops.

.......... Phil

"mc"

BTW

If you cannot tell what the THD is, then it don't matter to you.

....... Phil

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. ;-)

"Brat Ludwig" = PSYCHOPATH

........ Phil

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.

If you use them a lot, the tubes wear out.

Tubes have always been proprietary parts. OK, you HP 200C has say a 6SN7 in it. Which manufacturer's 6SN7 is the right one to use?

The tube-bigot lie here is that most ICs in good audio test equipment are standard parts, or can be readily replaced with standard parts.

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).

It usually takes several weeks into a new year before we see things so remarkably clueless.

That's totally backwards.

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.

John

What a pile of nonsense!

...Jim Thompson

No, transistor Gm is to tube Gm as...

Good transistor design is beta independent, as good tube design is tolerant of variations in transfer curves, grid current, and transconductance.

John

Uh..!? Tubes from different manufacturers are _massively_ better matched than any two transistors or ICs from the _same batch_!

Tim

-- Deep Fryer: a very philosophical monk. Website: