2004-04-10 by Jouni Verronen
These AF spectrums tell of system performance.
Voltage differentials between device grounds easily show on AF spectrum screen. In figure 1. output spectrum of a function generator is shown, sine output 121 mVrms. It can be seen that noise is high at the low end, and it consists of 100 Hz harmonics, which most obviously has something to do with rectification in a power supply.
In figure 2 there is an isolation transformer in between, which cuts the ground connection.
Figure 3 shows 1 kHz 0.3 Vrms signal from HP8903B with an isolation transformer. As can be seen, second harmonic is -69 dB and distortion 0.04 %.
HP8903B is a lab grade audio analyzer with very low distortion consisting of an AF generator and a level/distortion measurement section. The manufacturer specifies that generator distortion is -80 dB or better. Measuring the own generator with the inbuilt distortion meter gave 89.6 dB or 0.0033 % at 1 kHz. So it can be taken that practically all the harmonics shown in the picture are generated in the soundcard.
However, 70 dB dynamic range is enough for many measurements. And third harmonic, which relates to intermodulation distortion, is down more, 81 dB.
In a sampling process one has also to ascertain that higher alias frequencies do not fold on the wanted band. For example 26 kHz folds on 19 kHz when the sampling frequency is 45 kHz. (26-45/2 = 45/2 -19)
There is a low-pass filter in the soundcard to prevent that. I checked my card for a couple of frequencies.
freq. Attenuation shows at 26 kHz 58 dB 19 kHz 27 kHz >90 dB 18 kHz
Back to SB_FFTC main page
On RF measurements using a down converter
Back to the FrontPage