SM 5 BSZ - Linux pc radio, linearity of A/D boards.
(Mar 28 2001)

Use of low pass filter to get a pure test signal

Using a LC low pass link, feeding the test signal across a 0.5 ohm resistor to get a very low feed impedance one gets a very pure test signal. The low pass link is also a series resonator and it gives a substantial up transformation of the voltage. The fundamental at 6.5kHz is amplified by 31dB while the second harmonic at 13kHz is attenuated by 10dB. To suppress the second harmonic by further 20dB a capacitor was added across the inductor.

Without any filter, feeding the same level into the 0.5 ohm resistor the audio generator produces second harmonic at 42 dB below the fundamental. The filter is made from a iron free high Q coil made with multi strand wire so the filter itself will not produce any second order harmonic. The signal generator plus filter combination should have the second harmonic at least 100dB below the fundamental.

Fig.1. shows the screen when the distorsion free 6.5kHz signal is fed into a Soundblaster PCI board, "Audio PCI 97 (CS4297A)" according to the ossmix printout. The signal level is set just below clipping as indicated by rapid growth of many spurs. The peak amplitude is 18750, nearly 5dB below 32767 where clipping should start for a 16 bit converter. The second harmonic is 90dB below the fundamental.

Fig.2. shows the screen when the distorsion free 6.5kHz signal is fed into a Delta 44 board. The signal level is set just below clipping as indicated by rapid growth of many spurs. The peak amplitude is 6130000, 2.7dB below 8388607 where clipping should start for a 24 bit converter. The second harmonic is 96dB below the fundamental.

Conclusions

It is obvious that the Delta 44 board is better than the Soundblaster PCI. The difference is very small, the max S/N ratios differ by only about 8dB. In fact the SB PCI board performs as a 15bit device while the Delta 44 is like a 16bit device. With maximum input signal the second harmonic is suppressed about 6dB better in the Delta 44 as one would expect (hope for) having one more bit.

Already 15 bit is enough for most situations, but in densly populated areas the dynamic range limitations will cause problems.


Fig. 1. SB PCI spectrum of pure sine wave. The level of the third harmonic is not reliable, the filter in use has a notch at 13kHz making the level of the second harmonic reliable.

Fig. 2. Delta 44 spectrum of pure sine wave. The level of the third harmonic is not reliable, the filter in use has a notch at 13kHz making the level of the second harmonic reliable.