Tone Generation: Sound Card vs. Wien-bridge Oscillator
This article compares sine-wave tone generation using a digitally synthesized approach with a 24 bit/96 kHz quality sound card
and the classic analog Wien-bridge oscillator circuit. RMAA 6.1.2
was used to perform an audio spectrum analysis in order to show noise and harmonic distortion components.
A Creative Technologies Audigy 2ZS Platinum Pro sound card was used.
The Wien-bridge circuit, with component values suitable for 2 kHz oscillation is shown below in breadboard layout:
A high-performance LM4562 dual op amp, with GBW=55MHz SLR=20V/us THD=0.00003% has sufficient
bandwidth and slew-rate to be useful in the Wien-bridge circuit up to frequencies of several hundred kHz. In addition, the very low THD of this IC ensures minimal
distortion contribution from the op amp.
The schematic diagram for the Wien-oscillator is shown below:
A standard 1869 incandescent lamp (10V @ 14 mA) was used as the conventional stabilization component. The table below shows the measured
peak output oscillation amplitude for different negative feedback resistors (Rf). The amplitude of oscillation is determined by the oscillation condition
that the lamp-resistance at the oscillation voltage is exactly half the feedback resistance Rf, providing a negative feedback fraction of 1/3, identical to the positive feedback
fraction of the Wien RC network at the resonant peak, and ensuring the "bridge" is in balance. The oscillation build-up quickly converges onto this stable oscillation condition
|Rf (ohms) ||Vp (volts)||Vrms (volts)|
The two curves below show the measured DC current versus voltage (I-V) and corresponding 1869 lamp resistance versus lamp voltage. The oscillation RMS voltages in the table
above are consistent with Rlamp = 1/2*Rf:
The oscillation frequency of the Wien-bridge oscillator is 1/(2*Pi*Rwb*Cwb). The table below lists measured Wien-bridge oscillation frequencies
as a function of resistance values (Rwb) for Cwb = 0.001 uF:
|510 ohm||320 kHz|
|1.0 kohm||162 kHz|
|7.87 kohm||20.8 kHz|
|78.7 kohm||2.05 kHz|
To compare the quality of a sine-wave tone generated with the Wien-bridge circuit above with a PC audio sound card generated tone, RMAA was configured in RECORD
mode and a 24 bit/96 kHz WAV file was played and looped back from the sound card LINE OUT to the LINE IN connector. The spectrum was displayed. Then, the output
of the Wien-bridge circuit was connected to the sound-card LINE IN connector and adjusted (using the Ro 10 kohm level adjustment potentiometer) to an identical
signal level as the WAV file (~ -1 dB in the RMAA display window).
Spectra for the digitally-synthesized 2 kHz wav file at 24 bit/96 kHz sampling rate and the Wien-bridge circuit are shown below:
As expected the 2 kHz Wien-bridge oscillator circuit shows very low harmonic distortion and very low noise, at least as good as the 24 bit/96 kHz synthesized
tone. In particular the first harmonic (at 4 kHz) is considerably higher for the synthesized case. On the other hand, due to the inherent limitation of the thermal stabilization
process in this simple Wien-bridge circuit, there is apparent "line broadening" of the fundamental tone as compared to the digitally-synthesized 2 kHz tone which
is exceptionally stable and narrow of course due to the CPU and D/A converter high-frequency clock control inherent in the digital synthesis approach.
In summary, the classic Wien-bridge circuit with a simple incandescent lamp stabilization element can be a useful circuit for creating a very low distortion
and low noise test signal source. The harmonic distortion level is extremely low, comparable to high-quality synthesized 24 bit/96 kHz digital audio signals.