Homodyne detection

Homodyne detection is a method for detecting the modulation of an oscillation by mixing with an almost same reference frequency. In the wireless technology is called a direct conversion receiver.

If the reference frequency has an outlying value (which is usually the case ), one speaks of a heterodyne detection and the device is then called superheterodyne receiver.

Principle

The term homodyne detection in the optical interferometry expresses that the reference radiation to the mixer from the same source as the signal before the modulation, however, is obtained. In the case of a scattering experiment with a laser (Laser Doppler Anemometry ), the laser beam is divided into two parts. One is the mixer (photodiode ) is fed directly, while the other is first drawn to the system under investigation. Where the scattered light then passes also to the mixer that forms the frequency difference. This arrangement has the advantage that it is insensitive to fluctuations in the frequency of the laser. Usually, the scattered beam is weak, so that the substantially constant component of the detector signal can be used as an indication of the intensity of the local oscillator. Intensity fluctuations of the laser can thus be compensated.

In the wireless technology is understood to mean a synchronous homodyne detection, also referred to as a direct conversion receiver, and the method as a coherent demodulation. In contrast to the application of the optics required for the mixing phase is not transmitted as a reference. This often leads to considerable technical difficulties because of the phase position of - the time alignment - of the carrier frequency has to be reconstructed in the receiver. One possibility is the analog color television the cyclic transmission of the phase information using a burst signal. In digital transmission methods can be reconstructed or tracked by known and cyclic repeated pattern in the data stream, the phase position.

In the measurement technique is referred to as a lock-in amplifier, which allows extremely small signals from an interfering background to filter when the frequency is known.

A disadvantage is the limited sensitivity due to the strong 1/f-noise. The image corresponds to the " amount of noise " of the surface of the left blue ribbon when starting from a highest signal frequency of 100 Hz.

Is the input voltage, however, with the frequency f = 10 kHz are mixed, this corresponds to a modulation amplitude of a 10 KHz carrier to the measurement voltage. Fourier analysis of the output signal indicates that the input signal is contained in the sidebands of the carrier. These extend over the range f - 100 Hz to 100 Hz f If only this high range is amplified below, the 1/f-noise is considerably lower than in the baseline range 0 to 100 Hz, indicated by the smaller blue area to 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 1010 Hz superheterodyne receiver are always preferable to maintain the 1/f-noise low. At higher frequencies, the phase noise of the mixer oscillator causes problems.

• Electrical Measurement
• Modulation (technology)
• Color television technology
• Optical Metrology