Yamaha @PET RP-U100 personal receiver Measurements part 2

The bottom traces in fig.4 are a similar spectral analysis taken of the receiver's output while it decoded data representing a 1LSB DC offset. Other than the presence of identical power-supply components, this noise floor is suspiciously lower than that in fig.3. Repeating the analysis with data representing a dithered 16-bit tone at -110dBFS resulted in the upper traces in fig.4. Now the noise floor correlates with that in fig.3. The difference between the two sets of traces in this graph indicates that the DAC actually turns off in the presence of a signal that its designers have determined will indicate a reviewer's attempt to measure dynamic range!

Fig.4 Yamaha RP-U100, spectrum of dithered 1kHz tone at -120dBFS (top) and -1LSB (bottom), with noise and spuriae, 16-bit data (right channel dashed).

Note also that extending the measurement bandwidth to 200kHz in this graph reveals the presence of a high level of ultrasonic noise, due to the D/A converter's noise-shaping. The RP-U100's noise floor is actually a little higher in level than what would correspond with true 16-bit performance, but I'm surprised that Yamaha didn't low-pass-filter the analog output to get rid of the noise peak between 100kHz and 200kHz.

The receiver's D/A linearity error (fig.5) is respectably low for what must be a low-cost part. But with the highish level of ultrasonic noise in its output, the waveform of an undithered 1kHz tone at -90.31dBFS (fig.6) is obscured.

Fig.5 Yamaha RP-U100, departure from linearity, 16-bit data. (Right channel dashed, 2dB/vertical div.)

Fig.6 Yamaha RP-U100, waveform of undithered 1kHz sinewave at -90.31dBFS, 16-bit data.

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