DE102020213283A1 - Electro-optical mixer - Google Patents

Abstract

The invention relates to an electro-optical mixer (1) having at least one photodiode (PD1, PD2) for receiving an optical input signal, wherein the at least one photodiode (PD1, PD2) converts the optical input signal received into an electrical input signal, and further having a electrical mixer (MX), wherein the electrical mixer (MX) has at least one input for a local oscillator signal to be mixed first signal (LO), wherein the electrical mixer (MX) further has a further input for a second signal to be mixed, wherein in operation at least one of the first input and the further input receives input signals via at least one matching network (M1; M2), a mixed signal being made available at the output of the mixer during operation.

Description

The invention relates to an electro-optical mixer.

background

In the field of data transmission, it is known to transmit data over long distances using glass fibers. In other areas, e.g. radar technology, glass fibers are used to distribute signals, e.g. local oscillator signals, to several stations for further use.

In order to send data or a radar signal wirelessly, the optical signal is converted into an electrical signal before it is upconverted. Optical signals are typically converted into electrical signals using a transimpedance amplifier (abbreviated to TIA).

The up-conversion to the desired RF band is then carried out using an electric mixer. The two components, transimpedance amplifiers and mixers, increase the complexity of the overall system. Furthermore, the components generate power loss, which impairs the efficiency of the system.

Ultimately, each individual component has a certain bandwidth. By interconnecting the two components, the bandwidth of the overall system is determined by the bandwidth of the individual components, i.e. the available bandwidth decreases.

From the publication "Analysis and Simulation of a Wireless Phased Array System with Optical Carrier Distribution and an Optical IQ Return Path" by the authors S. Kruse, C. Kress, H. G. Kurz, T. Schneider, and J. C. Scheytt, in 2020 13th German Microwave Conference (GeMiC), Mar. 2020, pp. 140-143. it is known to convert optical signals into electrical signals by means of a photodiode and a transimpedance amplifier. The output signal of the transimpedance amplifier can then be pre-processed using a DC-balanced buffer and a VGA before this signal is multiplied with another signal using a mixer.

A disadvantage of the previously mentioned solution is that, in order to detect high-frequency optical signals, a separate transimpedance amplifier must be used in order to minimize the influence of the parasitic capacitance. However, this transimpedance amplifier generates additional noise. Additional noise degrades the signal-to-noise ratio.

Furthermore, the transimpedance amplifier consumes power, making this solution unsuitable for use in low-power applications or battery-powered systems.

Furthermore, a transimpedance amplifier consumes additional chip area on the order of magnitude (typically about 0.25 mm ² ). This additional chip area is expensive.

Furthermore, the probability of failure of the overall system increases with the number of components.

In another approach, Mach Zehnder modulators (abbreviated MZM) are used as mixers. However, the output signal from the Mach Zehnder modulator is again optical and must first be converted into an electrical signal using a photodiode. However, the electrical signal obtained in this way is a current signal. For further processing, this current signal must be converted into a voltage signal using a transimpedance amplifier.

The disadvantage of this solution is that voltages in the range of 2-3V are required to drive the Mach Zehnder modulator. Since transistors with sizes in the nanometer range are typically used, there is already a risk for the transistors here, so that long-term stability is not given or can only be increased by using other means. However, this increases the complexity as well as the costs of such a solution.

Furthermore, these driver components in turn increase the noise of the mixer. Additional noise degrades the signal-to-noise ratio.

Furthermore, the Mach Zehnder modulator requires additional chip area of the order of magnitude (typically several mm ² ). This additional chip area is expensive.

Another disadvantage is the limited bandwidth of Mach Zehnder modulators, which is in the range of less than 30 GHz. However, this means that it can no longer be used for numerous high-frequency applications.

The invention has set itself the task of avoiding one or more problems from the prior art, in particular of offering a cost-effective solution.

The object is solved by an electro-optical mixer according to the independent claim. Advantageous configurations are against stood the dependent claims, the description and the figures.

• 1 ein Blockschaltbild eines erfindungsgemäßen elektrooptischen Mischers gemäß Ausführungsformen der Erfindung,
• 2 ein Blockschaltbild eines erfindungsgemäßen elektrooptischen Mischers gemäß einer weiteren Ausführungsformen der Erfindung
• 3 eine erste Ausführungsform einer elektrooptischen Lokal-Oszillator Konversion gemäß Ausführungsformen der Erfindung; und
• 4 eine zweite elektrooptische Lokal-Oszillator Konversion gemäß Ausführungsformen der Erfindung.

The invention is explained in more detail below with reference to the figures. In these shows:

• 1 a block diagram of an inventive electro-optical mixer according to embodiments of the invention,
• 2 a block diagram of an inventive electro-optical mixer according to a further embodiment of the invention
• 3 a first embodiment of an electro-optical local oscillator conversion according to embodiments of the invention; and
• 4 a second electro-optical local oscillator conversion according to embodiments of the invention.

In the following the invention will be illustrated in more detail with reference to the figures. It should be noted that different aspects are described, which can be used individually or in combination. That is, each aspect can be used with different embodiments of the invention unless explicitly presented as purely alternative.

Furthermore, for the sake of simplicity, only one entity will generally be referred to below. Unless explicitly noted, however, the invention can also have several of the entities concerned. In this respect, the use of the words “a”, “an” and “an” is only to be understood as an indication that at least one entity is used in a simple embodiment.

Insofar as methods are described below, the individual steps of a method can be arranged and/or combined in any order, unless something different is explicitly stated in the context. Furthermore, the procedures can be combined with one another unless otherwise expressly indicated.

Specifications with numerical values are generally not to be understood as exact values, but also include a tolerance of +/- 1% to +/- 10%.

References to standards or specifications are to be understood as references to standards or specifications that are/were in force at the time of filing and/or - if priority is claimed - at the time of priority filing. However, this is not to be understood as a general exclusion of applicability to subsequent or superseding standards or specifications.

In 1 1 shows a block diagram of an inventive electro-optical mixer according to embodiments of the invention. In this case, as in the following figures, a simplified scheme is shown in order to motivate the mode of operation. In other words, additional components can be provided in a real implementation. Furthermore, the selected components can be replaced by functionally similar ones, eg bipolar transistors by FET transistors. Finally, it should be mentioned that the earth symbols are always to be understood as small-signal earth.

Very generally, the electro-optical mixer 1 has at least one photodiode for receiving an optical input signal.

In 1 an electro-optical mixer 1 is shown, which has a photodiode PD1 for receiving an optical input signal. In 2 an electro-optical mixer 1 is also shown, but having two photodiodes PD1, PD2 for receiving an optical input signal.

The at least one photodiode (PD1, PD2) converts the received optical input signal into an electrical input signal.

In 1 the photodiode PD1 converts the received optical input signal into an electrical input signal. In 2 the photodiodes PD1, PD2 convert the received optical input signal into an electrical input signal.

Very generally, the electro-optical mixer 1 also has an electrical mixer MX. This mixer can, for example, be made up of several (integrated) transistors (shown as (npn) bipolar transistors) Q1...Q4. The invention is not limited to a specific technology.

The electrical mixer MX has at least one input for a first signal LO to be mixed.

The first signal LO to be mixed can be a local oscillator signal, for example.

The electrical mixer MX also has a further input for a second signal to be mixed.

The exact assignment of which signal is present at the first input and which signal is present at the second input is not relevant for understanding. Rather, this can after use and / or electrical properties of the inputs are suitably chosen by a person skilled in the art.

During operation, at least one of the first input and the further input receives input signals via at least one matching network.

In the 1 and 2 receives at least the first (differential) input E1, E2 via the matching network M1; M2 input signals. The second signal to be mixed is made available at the second (differential) input LO+, LO-.

At the output of the mixer MX, also referred to as a switching quad, a mixed signal IF is made available during operation.

in the in 1 and 2 In the architecture presented, the optical signal RF can be mixed directly with an electrical signal LO+, LO- without a transimpedance amplifier or a Mach Zehnder modulator. In contrast to the prior art, an electrical signal is generated directly at the output.

In doing so, 1 a two quadrant mixer and in 2 presented a four quadrant mixer.

This in 1 The block diagram shown shows a two-quadrant electro-optical mixer, which does not require an additional transimpedance amplifier or a Mach-Zehnder modulator. The charges that are generated in the photodiode PD1 can be multiplied by the electrical signal LO in a switching quad MX after matching M1, M2 of the impedances between the photodiode PD1 and the exemplary linear time invariant system (matching network). The switching quad here is the linear time variant system. In this case, the matching networks M1 and M2 can be different, since the impedance of the anode and cathode of the photodiode PD1 can be different.

It should be mentioned at this point that non-linear time-variant mixers, so-called NLTI mixers, can also be used with the invention.

In this case, the electrical signal L0 can also be the signal from the photodiode PD1 or derived from it. Then the circuit can be used as a frequency doubler.

This in 2 The block diagram shown shows a four-quadrant mixer. The four-quadrant mixer does not require an external transimpedance amplifier or Mach-Zehnder modulator either. The functionality is similar to that 1 . In order to map the four quadrants, however, the optical signal must be applied differentially.

Differential charge carriers can be generated in the photodiodes PD1, PD2 by the differential optical signal. These differential charges, after matching the impedances M1, M2 in a quad MX switch, are multiplied by the electrical signal LO in a quad MX switch. The switching quad MX is the linear time variant system. The matching networks M1 and M2 can be the same. As a rule, this leads to a more symmetrical structure, so that the signal routing can also be symmetrical. Again, an NLTI system can be used here.

In this case, the electrical signal L0 can also be the signal from the photodiode PD1 or PD2 or derived therefrom. Then the circuit can be used as a frequency doubler.

In the 1 and 2 Inductances L1 and L2 are shown, respectively. These can be used to bias the photodiode(s) PD1, PD2. The output impedance of the anode and cathode connections can be matched to the input impedance of the inputs E1, E2 of the mixer MX (ie to the emitter connections shown) by means of the matching networks M1 and M2. The mixer MX is here an example of the switching quad of a Gilbert cell. In principle, however, other mixer types can also be used, such as a single balanced mixer.

The matching networks M1, M2 preferably provide a DC path to small-signal ground, so that DC components do not reach the mixer MX.

As already indicated, the electrical signal LO can also be generated from an optical signal. Exemplary controls are in 3 and 4 outlined, in which case the considerations relating to the optical signal RF and the photodiode(s) PD1 and PD2 can be applied in the same way.

In embodiments of the invention, the matching networks M5, M6 can in turn match the connection impedance to the impedance of the LO+ or LO- connections—here the base impedance.

The respective output currents—here collector currents—of the transistors Q1 . . . Q4 can be summed in the nodes N1 and N2.

The output of the nodes N1 and N2 can now in turn be adapted to the requirements of the output IF via matching networks M3 and M4.

In one embodiment of the invention, the first signal to be mixed receives its input signal via first matching networks M1 M2 during operation.

In one embodiment of the invention, the second signal to be mixed receives its input signal via a second matching network M5, M6 during operation.

The presented electro-optical mixer 1 allows to multiply an optical signal RF by an electrical signal IF directly, i.e. without the presence of a transimpedance amplifier.

The presented embodiments therefore consume less power. Consequently, they are particularly well suited for use in energy-limited systems.

Furthermore, the presented electro-optical mixer 1 has a lower complexity. This reduces the development and manufacturing costs.

In addition, the presented electro-optical mixer 1 has less noise. As a result, the bandwidth can be used over a large area. As a result, there are also possibilities to use less demanding components elsewhere, so that comparable results can be achieved more cost-effectively with less effort.

In addition, the chip area on which an electro-optical mixer 1 according to the invention can be integrated can be reduced by at least the size of a transimpedance amplifier or a Mach Zehnder modulator.

As a rule, the expected bandwidth of the electro-optical mixer 1 according to the invention will essentially depend on the matching networks M1 . . . M4, M5, M6 and/or on the semiconductor technology used.

For example, the LO voltage can be small, typically in the range of less than 100 mV, particularly about 75 mV. Compared to the typical voltages of a Mach Zehnder modulator of 2-3 V, the problems described at the beginning no longer arise.

Furthermore, no extra DC-balanced buffer needs to be used with the presented four-quadrant mixer in order to generate a fully differential signal.

Claims (7)

Electro-optical mixer (1) having at least one photodiode (PD ₁ , PD ₂ ) for receiving an optical input signal, wherein the at least one photodiode (PD ₁ , PD ₂ ) converts the received optical input signal into an electrical input signal, and further having an electrical mixer (MX), wherein the electrical mixer (MX) has at least one input for a first signal to be mixed (LO), wherein the electrical mixer (MX) further has a further input for a second signal to be mixed, wherein in operation at least one of receives input signals from the first input and the further input via at least one matching network (M1; M2), a mixed signal being made available at the output of the mixer during operation. Electro-optical mixer (1) after claim 1 , characterized in that the electrical mixer is a two-quadrant mixer or a four-quadrant mixer. Electro-optical mixer (1) after claim 1 or 2 , characterized in that the local oscillator signal is derived from the optical signal, a frequency-doubled signal being made available at the output of the electrical mixer (MX). Electro-optical mixer (1) according to one of the preceding claims, characterized in that the electrical mixer (MX) is a four-quadrant mixer, the optical input signal being routed via an optical balun (OBAL), the output signals being routed via two photodiodes (PD ₁ , PD ₂ ) and two matching networks (M1; M2) are led differentially to the electrical mixer (MX). Electro-optical mixer (1) according to one of the preceding claims, characterized in that the electrical mixer (MX) is a four-quadrant mixer, the optical input signal already being present differentially, the output signals being transmitted via two photodiodes (PD ₁ , PD ₂ ) and two matching networks (M1; M2) are routed differentially to the electrical mixer (MX). Electro-optical mixer (1) according to one of the preceding claims, characterized in that the first signal to be mixed im Operation receives its input signals via a first matching network (M1; M2). Electro-optical mixer (1) according to one of the preceding claims, characterized in that the second signal to be mixed receives its input signals via a second matching network (M5, M6) during operation.

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