EP1461858A2

EP1461858A2 - Opto-microwave transducer with suppression of local oscillator signal, and system using same

Info

Publication number: EP1461858A2
Application number: EP02806326A
Authority: EP
Inventors: Christian Rumelhard; Laurent Paszkiewicz
Original assignee: CNAM Conservatoire National des Arts et Metiers
Current assignee: CNAM Conservatoire National des Arts et Metiers
Priority date: 2001-12-28
Filing date: 2002-12-10
Publication date: 2004-09-29
Also published as: FR2834393B1; WO2003061113A2; WO2003061113A3; FR2834393A1; AU2002364633A1

Abstract

The invention concerns a transducer for mixing a data signal at an intermediate frequency (IF) modulating an optical carrier with a microwave or millimetric wave from a local oscillator (LO). The microwave signal is then used as carrier for transmission to mobile terminals for example. Advantageously, said transducer performs a function for photodetection of the data signal, a function for mixing said data signal with the microwave signal and a function for suppressing in the output signal the LO frequency. Therefor, the transducer comprises two phototransistors in common transmitters, a current source connected to the two transmitters and a coupler for coupling the signals exiting from the collectors of the two phototransistors.

Description

"Opto-microwave transducer with suppression of the local oscillator signal, and system implementing such a transducer."

The present invention relates to an opto-microwave transducer for suppressing the local oscillator signal, and system implementing such a transducer. It finds a particularly interesting application in the field of mixed telecommunications networks. These are, for example, digital data transmission networks comprising a part by optical fiber and then a hertzian part. The radio link concerns the last hundred meters for an outdoor link or the inside of a building for an intra-building link (indoor). Digital information is therefore first transmitted on an optical carrier and then on a microwave or millimeter carrier, the current frequencies for the radio part being in particular 24 or 60 GHz. These links therefore include a transducer in which the data signal is transferred from an optical carrier to a microwave carrier at 24 or 60 GHz for example.

Generally, this transfer involves an optoelectronic mixer mixing the frequency data signal IF called intermediate frequency with the microwave frequency carrier OL. The result of this mixing is an output signal comprising different intermodulation products. More precisely, this output signal comprises four components respectively at frequencies RF (RF = OL + FI), OL, FI and LB (LB = OL - FI). The component that we want to isolate for a transmission is called high frequency RF sideband. It is therefore essential to eliminate the other components. The component at the IF frequency is of very low power and can very easily be deleted. Conventional techniques are known which make it possible to suppress the frequency component LB, called the low sideband. However, it is difficult to suppress the frequency component OL because it generally has a power greater than that of the high sideband, in addition the frequencies RF and OL are very close.

In current transducer circuits, a mixer is used followed by a filter produced in hybrid technology so as to eliminate the frequency component OL downstream. Other current circuits, in order to eliminate the frequency component OL, require the sending of two radiofrequency signals phase shifted with respect to one another by means of two optical fibers followed by an optical phase shift difficult to implement , these two fibers illuminating two separate mixers. The disadvantage of the circuits of the prior art is the complexity of their implementation and the use of a large number of electronic components. However, by way of example, in telecommunications systems using a transducer, the cellular networks include cells of small diameter, of the order of a few hundred meters. This therefore requires a large number of transducers, hence the importance of minimizing the number of electronic components in order to reduce the cost price of each transducer.

The present invention aims to remedy the aforementioned drawbacks by proposing an integrated transducer circuit which does not require an external hybrid circuit for the suppression of the frequency component OL.

Another object of the invention is to limit the number of components in the transducer in order to reduce the cost price. The above objectives are achieved with an opto-microwave transducer for transferring a data signal modulating an optical carrier to an electric carrier, this transducer comprising two phototransistors arranged as a common emitter or as a common source in the case of MESFET. According to one embodiment of the invention, the phototransistors are two identical bipolar transistors with heterojunction. The first phototransistor is able to detect the data signal and to mix this data signal with the electric carrier introduced at the base of this first phototransistor. This first phototransistor therefore performs a photodetection function by detecting the data signal carried by the optical signal, and a mixing function by mixing this data signal with the electric carrier which is a microwave or millimeter signal. Generally, this microwave signal is obtained from a local oscillator at the frequency OL. The second phototransistor is able to mix on the one hand the electric carrier phase shifted by substantially 180 ° and introduced at the base of this second phototransistor with the data signal phase shifted by substantially 180 ° on the other hand. The second phototransistor performs a mixing function. The electric carrier phase shifted by 180 ° can come from a phase shift operated on the microwave signal leaving the local oscillator. However, the 180 ° phase shifted data signal comes from the first phototransistor. The transducer further comprises a bias current source connected to the emitters of the two transistors, this current source being able to transfer the data signal detected by the first phototransistor to the second phototransistor with a phase shift of substantially 180 °. According to one advantageous characteristic of the invention, this current source is a current mirror.

The results of the mixing operations carried out by the two phototransistors are signals coming out of the collectors of the two phototransistors. These signals each include a frequency component OL. The invention is remarkable by the fact that the frequency component OL exiting through the collector of the first phototransistor is 180 ° out of phase with respect to the frequency component OL exiting through the collector of the second phototransistor and the RF signals are not out of phase d 'one collector to another. Thus, the transducer further comprises a coupler for combining the signals from the collectors of the two phototransistors so as to suppress the frequency components OL.

With such a transducer according to the invention, three functions are therefore integrated in the same integrated circuit, namely a photodetection function, a mixer function, and a function for removing the frequency component OL. This avoids the addition of an external hybrid circuit to this integrated transducer circuit for the suppression of the frequency component OL, hence a significant cost saving. This transducer therefore makes it possible to mix a data signal arriving on an optical carrier with a microwave or millimeter signal so as to obtain at output a microwave or millimeter signal modulated by the data signal. According to an embodiment of the invention, the two phototransistors can be polarized by means of a DC collector voltage and a basic DC voltage. Furthermore, the DC bias voltage on the basis of the second phototransistor can be adjustable so as to correct the polarization offset due to the detection of the data signal on the first phototransistor.

Preferably, the two phototransistors are polarized in a non-linear region. According to a preferred embodiment of the invention, the coupler is adapted for an operating frequency centered around the RF frequency of the high sideband.

According to another advantageous characteristic of the invention, the point of polarization of the phototransistors is such that, on the collectors of the phototransistors, the signal power in the high sideband is maximum. Indeed, the signal in high sideband (RF) being generally less powerful than the signal OL, one seeks to minimize the difference in power between the two, either by decreasing the signal OL, or by increasing the signal RF, or both actions, by adaptation, by the point of polarization, etc. According to another aspect of the invention, an opto-microwave communication system is proposed comprising a central site emitting an optical signal modulated by a signal IF intermediate frequency, and a local receiver to receive this modulated optical signal and transmit a microwave electrical signal modulated by the IF intermediate frequency signal. Advantageously, the local receiver comprises an opto-microwave transducer according to the invention, this transducer being able to detect the intermediate frequency signal and then to mix this intermediate frequency signal with the microwave electrical signal while removing a component whose frequency is identical to the frequency of the microwave electrical signal. Other features and advantages of the invention will become apparent in the description below. In the accompanying drawings given by way of nonlimiting examples: - Figure 1 is a simplified diagram of the transducer integrated circuit according to the invention, in particular illustrating the configuration of the two phototransistors;

- Figure 2 is an electronic diagram of a current mirror arranged in the transducer according to

The invention;

- Figure 3 is a simplified electronic diagram of a coupler covering the two phototransistors of the transducer according to the invention; and - Figure 4 is an exploded view of the transducer according to the invention as shown in Figure 1.

Referring more particularly to FIG. 1, the transducer according to the invention will allow the mixing of a signal at an intermediate frequency IF of the order of 2 GHz modulating an optical carrier 12 of wavelength 1.55 μm, 1.3 μm or 0.8 μm with a microwave signal 10 in millimeter wave coming from a local oscillator of frequency OL. As can be seen in FIG. 1, the transducer according to the invention is composed of two identical bipolar phototransistors Q1 and Q2 arranged in common emitters 2 and 4. These two transmitters 2 and 4 are connected to a current mirror 7 imposing a bias current Ipol. The bases 1 and 5 of the phototransistors Q1 and Q2 are respectively connected to adaptation circuits 9 and 12. The collectors 3 and 6 of the two phototransistors supply a coupler 8.

The polarization of the two phototransistors is done by means of a DC collector voltage, a base voltage and current Ipol imposed by the current mirror 7 on the two transmitters. The phototransistors are polarized at the same operating point by means of the polarization voltage on the bases 1 and 5. An adjustment of the voltage on the base 5 of the phototransistor Q2 makes it possible to compensate for the difference in polarization due to the detection of the optical signal 12 Figure 2 illustrates a conventional internal diagram of the current mirror. This is mainly composed of two transistors Q3 and Q4, the transistor Q4 having its base and its collector at the same potential. The value of the bias current Ipol passing through the transistor Q3 is determined by the voltage Vpol and the resistor R connected to the collector of the transistor Q4.

A local oscillator not shown provides the microwave signal 10 (electric carrier) of frequency OL. This microwave signal 10 feeds the base 1 of the phototransistor Ql through the adaptation circuit 9. A phase shift of the microwave signal 10 gives the microwave signal 11 which is injected into the base of the phototransistor Q2 through the adaptation circuit 12. The frequency data signal FI modulating the optical carrier 12 is detected in the base-collector junction of the phototransistor Ql, then mixed with the microwave signal 10. A polarization of the phototransistors Ql and Q2 in an area non-linear provides a mixture on the collectors. Thus, on the collector 3 of the phototransistor Q1, a signal is obtained resulting from the mixing between the microwave signal 10 and the frequency data signal FI. This signal on the collector 3 mainly comprises four components. A first component called the high sideband at the RF frequency (with RF = FI + OL), it is this component which it is desired to isolate particularly with respect to a second component of frequency OL. In particular, we can advantageously adjust Ipol so as to polarize the phototransistors Ql and Q2 such that the two frequency components RF and OL have close powers. There is also a third component of frequency IF but of very low power and a fourth component called low sideband of frequency LB (with LB = OL - FI).

According to an advantageous characteristic of the invention, each phototransistor is polarized so as to have the lowest possible power difference between the frequency component RF and the frequency component OL. One can for example modify the collector-emitter voltage Vce of each phototransistor. For example, for simulations on a phototransistor with OL equal to 38 GHz, IF equal to 2 GHz, RF equal to 40 GHz, an injected optical power of 165 μW, and a modulation rate of 0.5, the bias voltage Vce of transistor is 0.4 V; this corresponds to a point of polarization situated at the bend of the characteristic le (collector current) as a function of Vce of the phototransistor considered.

The current source represents an open circuit for the IF microwave signal. It is this particularity which is used in order to phase-shift the data signal by 180 ° between the collectors of each of the transistors.

The detection of the data signal modulating the optical wave 12 being done on only one of the two phototransistors, this data signal is phase shifted between the collectors 3 and 6 of the two phototransistors by means of the current mirror 7. The current source represents an open circuit for the frequency data signal FI, for example 2 GHZ. However, the current mirror no longer fulfills this role of open circuit for the microwave signal 10 of frequency OL, since this microwave signal is at a frequency that is too high. A big part of this microwave signal 10 is lost in the current source. It is for this reason that it is necessary to apply this microwave signal to the two bases separately, by shifting them by 180 °. Consequently, one also finds on the collector 6 of the phototransistor Q2 four components but with frequencies respectively equal to RF (-OL-FI = RF) for the first component, -OL (the sign - representing the phase shift of 180 °) for the second component, -FI for the third component, and FI-OL for the fourth component.

The signals on the collectors 3 and 6 are then coupled in the coupler 8 such as that illustrated for example in FIG. 3. This coupler 8 comprises two transistors Q5 and Q6 arranged in common collectors. Each base of these two transistors Q5 and Q6 is preceded by an adaptation circuit 13 or 14 such as circuits 9 and 12, before being connected to collector 3 or 6. The collectors of transistors Q5 and Q6 are connected to the base of a third output transistor Q7. These three transistors Q5, Q6, and Q7 can be bipolar heterojunction transistors whose reverse transmission coefficient S12 is low, in our case of -20 dB. One can also use for example other coupler such as the ilkinson coupler.

During the coupling of the signals from the collectors 3 and 6 in the coupler 8, the frequency components OL cancel each other out since in phase opposition. On the other hand, the RF frequency components are added since in phase. There is thus obtained at the output of the transducer (transistor Q7) a mixing spectrum freed from the frequency OL of the local oscillator. The other components, namely the low sideband and the IF frequency component, can be suppressed in a conventional manner. Figure 4 is an exploded view of the various elements described. We see that the adaptation circuit can be a circuit composed of a capacitor and an inductance. This transducer therefore uses a minimum number of electronic components integrated into a single integrated circuit, which makes it possible to reduce the cost price compared to the circuits of the prior art using an external hybrid circuit.

A simulation was performed by taking OL = 38 GHz, FI = 2 GHz, and RF = 40 GHz. The 38 GHz microwave signal (OL) is sent on the basis of the phototransistor Q1 and is phase shifted by 180 ° before being also sent on the basis of the phototransistor Q2. Each of the bases is adapted in accordance with FIG. 4. Thus, the RF frequency component (the upper side band on the side Ql) on the collector of the phototransistor Ql comes from the mixture produced between a frequency signal FI and the microwave frequency signal OL both not phase-shifted, while the RF frequency component (the high side band on the side Q2) on the collector of the phototransistor Q2 comes from the mixture produced between the frequency signal FI and the microwave frequency signal OL both phase shifted 180 ° with respect to the signals on the phototransistor Ql. Frequency collectors OL are also obtained on the collectors. The signals of the two collectors are then coupled by means of a coupler centered around 40 GHz. The result is that the frequency components OL disappear and the frequency components RF are added: component [RF (Ql)] + component [RF (Q2)] with RF (Q1) = (FI (+ 0 °) + OL (+ 0 °) and

RF (Q2) = (FI (+ 180 °) + OL (+ 180 °). Of course, the invention is not limited to the examples which have just been described and numerous arrangements can be made to these examples without departing from the scope of the invention.

Claims

1. Opto-microwave transducer for transferring a data signal modulating an optical carrier to an electrical carrier, this electrical carrier modulated by the data signal forming a signal in high sideband, characterized in that it comprises:

- two phototransistors arranged as a common emitter or as a common source, the first phototransistor being able to detect the data signal and to mix this data signal with the electric carrier introduced at the base of this first phototransistor, the second phototransistor being able to mix the electric carrier phase shifted by substantially 180 ° and introduced at the base of this second phototransistor with the data signal phase shifted by substantially 180 °;

- a bias current source connected to the emitters of the two transistors, this current source being able to transfer the data signal detected by the first phototransistor to the second phototransistor with a phase shift of substantially 180 °; and

- a coupler combining the signals from the collectors of the two phototransistors so as to remove components whose frequency is identical to the frequency of the electric carrier.

2. Transducer according to claim 1, characterized in that the current source is a current mirror.

3. Transducer according to one of claims 1 or 2, characterized in that the two phototransistors are polarized by means of a DC collector voltage and a basic DC voltage.

4. Transducer according to claim 3, characterized in that the DC bias voltage on the basis of the second phototransistor is adjustable so as to correct the bias offset due to the detection of the data signal.

5 Transducer according to any one of the preceding claims, characterized in that the two phototransistors are polarized in a non-linear region.

6. Transducer according to any one of the preceding claims, characterized in that the coupler is suitable for an operating frequency centered around a frequency being the sum of the frequencies of the electric carrier and the data signal.

7. Transducer according to any one of the preceding claims, characterized in that the phototransistors are bipolar transistors with identical heterojunction.

8. Transducer according to any one of the preceding claims, characterized in that the electric carrier comes from a local oscillator.

9. Transducer according to any one of the preceding claims, characterized in that the coupler comprises three bipolar heterojunction transistors whose reverse transmission coefficient is low.

10. Transducer according to any one of the preceding claims, characterized in that the point of polarization of the phototransistors is such that, on the phototransistors collectors, the signal strength in the high sideband is maximum.

11. Opto-microwave communication system comprising a central site transmitting an optical signal modulated by an intermediate frequency signal containing useful information, and a local receiver for receiving the modulated optical signal and transmitting a microwave electric signal modulated by the intermediate frequency signal, characterized in that the local receiver comprises an opto-microwave transducer according to any one of the preceding claims, this transducer being capable of detecting the intermediate frequency signal and then mixing this intermediate frequency signal with the microwave electrical signal while removing a component whose frequency is identical to the frequency of the microwave electrical signal.