Signal Generator: Which one to buy ? What to look for in a signal generator ?

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The Hewlett-Packard is a commercial instrument made in America. GAG is South East Asia. The 209A has a low distortion mode switch on the rear panel for low frequency sine waves (audio) that's down near .05% measured, and will provide sine and square waves concurrently at the standard 600 ohm impedance. Flatness is ±0.5% over the audio range.
 

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Just to make sure I understand right.

The computer option has very low distortion, much lower than the function generator.
Is this true only in sine wave ?

The function generator generates square waves much better than the computer option.
 
haha, that's not square at all.

Ringing (not that much as in #33) is inherent in digital synthesis. The DDS instrumentation ones do filter the ringing but they lack in rise time as you push them higher to their limit VS analogue ones due to LPF. Talking under 500$. Full rise time is still available on their TTL output which is not filtered and you will be able to see the inherent ringing. Very good stability in PPM over the years and great functionality is the other side of the coin. Very good signal quality is offered by TTI-Thandar made in UK in medium to low prices (relatively, as generators go), they do good analogue still.
If using soundcard FFT for THD which is great for that, there are many cheap enough good solutions for nicely shaped square waves up to 1MHz from instruments that go to 3 or 5MHz. Their THD won't be better than -50dB though.
 
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