Soundcard Spurious Signals
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I have been evaluating the Virtins spectrum analyser using a laptop PC connected to a 'Creative Soundblaster x-fi USB'.
Initially I was using it to look at the frequency spectrum from a home constructed Wien Bridge Oscillator. As well as the expected oscillator harmonics, I was puzzled to see some frequency components about 60-70dB below the fundamental that were not simply harmonically related. I was initially unsure if these were originating from the oscillator or from the soundcard.
To investigate, I removed the Wien Oscillator and looped the soundblaster ‘line out’ phono connectors directly back into the ‘line in’ sockets. I used the built in Virtins signal generator to generate a 1 kHz sinewave. The spurious signals were still present.
The spurious signals are not fixed in frequency – they shift depending upon the frequency of the fundamental. Here are some examples:-
The first column is the sig gen frequency. The second column is the lowest frequency spurious signal. The third column is the next higher frequency spurious signal. These are some 60-70dB below the fundamental. There are other similar spurious signals at higher frequencies, but they are lower in amplitude.
800 Hz 3.1 kHz 4.7 kHz
1kHz 2.9 kHz 4.9 kHz
1.2 kHz 2.7 kHz 5.1 kHz
1.5 kHz 5.4 kHz 6.3 kHz
Can anyone suggest where these spurious signals may be originating?
The soundblaster x-fi USB noise and distortion specifications suggest they should not be there.
I can see nothing obvious using the Virtins or an external oscilloscope.
I have also tried connecting a 3rd order 1 kHz low pass filter (18dB / octave) between the line out and line in connectors to ensure the spurious signals were not coming from the signal generator.
Initially I was using it to look at the frequency spectrum from a home constructed Wien Bridge Oscillator. As well as the expected oscillator harmonics, I was puzzled to see some frequency components about 60-70dB below the fundamental that were not simply harmonically related. I was initially unsure if these were originating from the oscillator or from the soundcard.
To investigate, I removed the Wien Oscillator and looped the soundblaster ‘line out’ phono connectors directly back into the ‘line in’ sockets. I used the built in Virtins signal generator to generate a 1 kHz sinewave. The spurious signals were still present.
The spurious signals are not fixed in frequency – they shift depending upon the frequency of the fundamental. Here are some examples:-
The first column is the sig gen frequency. The second column is the lowest frequency spurious signal. The third column is the next higher frequency spurious signal. These are some 60-70dB below the fundamental. There are other similar spurious signals at higher frequencies, but they are lower in amplitude.
800 Hz 3.1 kHz 4.7 kHz
1kHz 2.9 kHz 4.9 kHz
1.2 kHz 2.7 kHz 5.1 kHz
1.5 kHz 5.4 kHz 6.3 kHz
Can anyone suggest where these spurious signals may be originating?
The soundblaster x-fi USB noise and distortion specifications suggest they should not be there.
I can see nothing obvious using the Virtins or an external oscilloscope.
I have also tried connecting a 3rd order 1 kHz low pass filter (18dB / octave) between the line out and line in connectors to ensure the spurious signals were not coming from the signal generator.