Waveforms from a Function Generator, What is Good Enough?
As the manual shows, the HP3311A first generates a triangle wave, and then "shapes" it into a sine wave, using the four-segment resistor-diode network shown in Figure 4-2 (on page 4-2). The ~130 Hz waveform in your upper waveform has the classic shape of such a shaper.

The ~1.4 MHz (?) signal in your second trace shows that this shaper has frequency response issues.

Whether this is "good enough" depends on your application. I once needed to generate audio test tones with &lt;70 dB distortion as built-in test for some telecom equipment. I had to go to a great deal of trouble writing DSP code to synthesize waveforms that could meet this specification.

Other applications will have much less stringent requirements. The typical use of this sort of signal generator is to test amplifiers, and measure their overall frequency response. The distortion hardly matters at all in that case.

Your unit may require some tweaking to get down to the 3% THD specified performance. See section 5-26, starting on page 5-11.