Homodyne detection

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The homodyne detection is a method to the modulation of a vibration through mixture to detect with an almost same reference frequency. In radio technology one speaks of a direct mixing receiver , in measurement technology of a lock-in amplifier .

The laser light comes from the left and is influenced differently after the beam splitter. The mixer follows at the top right to form the difference frequency.

If the reference frequency has a strongly deviating value (this is the rule), one speaks of heterodyne detection and the device is then called a heterodyne receiver .

principle

The term homodyne in optical interferometry expresses that the reference radiation for the mixer is obtained from the same source as the signal, but before it is modulated. In the case of a scattering experiment with a laser ( laser Doppler anemometry ), the laser beam is divided into two parts. One is fed directly to the mixer ( photodiode ), while the other is first directed to the system to be examined. The light scattered there then also reaches the mixer, which forms the difference frequency. This arrangement has the advantage that it is insensitive to fluctuations in the frequency of the laser. Usually the scattered beam is weak, so the near constant component of the detector signal can be used as an indicator of the intensity of the local oscillator. Fluctuations in the intensity of the laser can thus be compensated for.

In radio technology, homodyne detection is understood to be a synchronous demodulator , which is also referred to as a direct mixer receiver and the method is also referred to as coherent demodulation . In contrast to the application in optics, the phase position required for the mixing process is not transmitted as a reference. This often leads to considerable technical effort, because the phase position - the time alignment - of the carrier frequency must be reconstructed in the receiver. One possibility for analog color television is the cyclical transmission of the phase information with the aid of a burst signal . In the case of digital transmission methods, the phase position can be reconstructed or adjusted using known and cyclically repeated patterns in the data stream.

In measurement technology, homodyne detection allows extremely weak signals of known frequencies to be filtered out of an interfering background.

Reduction of the 1 / f noise by shifting to a higher frequency range

The disadvantage is the limited sensitivity due to the strong 1 / f noise . In the figure, the "amount of noise" corresponds to the area of ​​the left blue band, assuming a highest signal frequency of 100 Hz.

If, on the other hand, the input voltage is mixed with the frequency f = 10 kHz, this corresponds to an amplitude modulation of a 10 kHz carrier with the measurement voltage. The Fourier analysis of the output signal shows that the input signal is contained in the sidebands of the carrier. These extend over the range f - 100 Hz to f + 100 Hz. If only this high range is subsequently amplified, the 1 / f noise is considerably lower than in the base range 0 to 100 Hz, indicated by the smaller blue area on the right in Image. In the final synchronous rectification for demodulation, only this small amount of noise appears in the output signal.

With higher requirements and signal frequencies below about 10 ¹⁰  Hz, heterodyne receivers are always preferred in order to keep the 1 / f noise low. At higher frequencies the phase noise of the mixer oscillator causes problems.