Total harmonic distortion (THD) and noise are the most common parameters quoted when evaluating a quality of a sound system as they reveal much about the overall performance of an amplifier or any other equipment. There are many approaches to measure these parameters and none of them is perfect.
Nowadays, using easily available and cheap computer audio interfaces allows measuring THD+N down to -100dB (0.001%). Unfortunately this is not enough when evaluating so-called ultra-low audio devices. Well designed DACs are quite capable of converting digital signal to analog and reaching -125dB is not uncommon. The weaker part of the audio interfaces is the conversion of analog signals to digital. Here the lower limit is not as good as digital to analog conversion and is usually limited to -105dB. There are of course other limitations as well.
The first link in the chain of the equipment necessary for THD+N measurement is the signal or tone generator. Its performance should succeed by at least 10dB the performance of the device under measurement (DUT) otherwise the measurement won’t be precise enough.
Building such an oscillator capable of covering the whole audio spectrum is very difficult and expensive. Fortunately measuring the THD+N at 1kHz has become a kind of a standard nowadays and brings enough information to compare different amplifiers for example.
The current project describes a simple and easy to build ultra low distortion 1kHz oscillator. As far as I know this approach has not been implemented before. The oscillator is very stable and the frequency does not float and the amplitude is stable after the first 3 seconds.
The whole circuit consists of a couple of multi feedback (MFB) 1kHz low pass filters build around U1 and U2 (OPA2211), a gyrator based 1kHz pass-band filter build around U3 and U4 (OPA2211), a voltage amplifier U5 (OPA1611), a buffer amplifier capable of running low impedance load U6,7,8,9 (2xLM4562) and positive feedback part (a lamp and a couple of resistors).
As the lamp is the main source of distortions (mainly 3rd and 5th harmonics) the choice of the lamp is very important. Using a 12V 0.5-1.5W lamp is preferable as the volt/resistance relation is very linear when the voltage across the lamp is between 1-2V and the generated harmonics are quite limited.
Here are the specs of the oscillator:
Oscillating frequency: 1kHz, easily adjustable +/-5%
Output voltage: up to 3.0Vrms
THD <0.00001% (-140dB)
Noise – 0,31uVrms (10kHz BW)
