JPS6351613B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a multiplex optical transmission system in which the transmitting side and transmitting side of a local station are coupled with the receiving side and transmitting side of an opposite station using optical fibers.

In recent years, in-vehicle audio equipment has become increasingly multi-functional, resulting in increasingly complex device configurations. Therefore, we left only the radio receiver's operation part and the cassette deck near the driver's seat, such as in the center console, and installed the power amplifier and
A system configuration is emerging in which the main body of the Chuna is stored in the trunk. In such a case, it is effective to connect the console section and the trunk room with an optical fiber for a noisy vehicle-mounted system. Even in this case, direct brightness modulation of the transmitted light with the transmitted signal tends to cause distortion because the forward current vs. luminous intensity characteristic of the light emitting diode has nonlinearity. Therefore, in order to reduce distortion and expand the dynamic range, a carrier wave is frequency-modulated using a signal to be transmitted, and the transmitted light is intensity-modulated using the modulated carrier wave. However, in stereo, even if there is only an AC component, it is necessary to transmit two L and R signals from the cassette deck to the power amplifier during playback, and when recording, it is necessary to transmit the L and R signals from the tuner to the cassette deck. If this were to be achieved using a transmission system of one signal per optical fiber, four optical fibers would have to be installed.

FIGS. 1 and 2 are block diagrams showing the configuration of the transmitting side and receiving side of one general station on the premise of bidirectional multiplexed optical transmission.

That is, the light-emitting diode and optical fiber 8 shown in FIG. 1 and the light-receiving diode 10 and optical fiber shown in FIG. 2 are placed close to each other in the transmitting/receiving device of one station, and perform data transmission with the other station. The transmitting side TX has the same number of carrier wave oscillators 1 and 2 as the signal to be transmitted, that is, the modulation signal (in this example, L and R signals of stereo broadcasting are assumed, but three or more may be used), and the L and R signals, respectively. Frequency modulation circuits 3 and 4 that frequency modulate the carrier waves f ₁ and f ₂ , a mixing circuit 5 that multiplexes the outputs of these modulation circuits on the frequency axis, an amplifier (driver) 6 that amplifies the output, and an amplifier 6 The light-emitting diode 7 is driven by the output of the light emitting diode 7, and the light whose brightness is modulated by the diode 7 is transmitted to the other station (terminal) through the optical fiber 8.

The receiving side RX includes a light receiving diode 10 that receives brightness-modulated light from the other station through an optical fiber 9, a broadband bandpass filter 11, a preamplifier 12 that amplifies its output, and a narrow bandpass filter that frequency-separates the output. band band pass filter 1
3 and 14, and demodulation circuits 15 and 16 that individually amplify and demodulate these outputs. Note that the center frequency of filters 13 and 14 is the receiving carrier frequency.
It is set to f ₃ and f ₄ .

In the multiplex optical transmission system described above, each of the optical fibers 8 and 9 can transmit a plurality of signals, so even if the number of signals to be transmitted increases, there is no need to increase the number of optical fibers.

By the way, in reality, in order to avoid creating imbalance in the transmitter/receiver circuit design, the frequency f ₁ mentioned above is
It is common to set ~f ₄ to a close value. In this case, the transmitting frequencies f ₁ , f ₂ at the transmitting side TX and the receiving side
The receiving frequencies f ₃ and f ₄ at RX are f ₄ < f ₂ as shown in Figure 3.
If there is a relationship of <f ₃ < f ₁ , the band of the bandpass filter 11 on the receiving side RX must pass the reception frequencies f ₃ and f ₄ , so the center frequency will be near the transmission frequency f ₂ , The leakage component of the transmission frequency _f2 from the light emitting diode 7 of the transmitting side TX is transmitted to the receiving side.
When the RX light receiving diode receives the signal and passes through the filter and preamplifier 12, so-called wrap-around occurs, the received frequency is
It becomes difficult to extract only the f ₃ and f ₄ components. Therefore, as shown in FIG. 4, the reception carrier frequency f ₃ ,
By dividing f ₄ and the transmission carrier frequencies f ₁ and f ₂ into low and high groups, it is possible to avoid placing the transmission frequencies f ₁ and f ₂ within the band of the bandpass filter 11 of the receiving side RX. However, in this case, if the frequency interval fs is selected incorrectly, the transmission frequency
Due to f ₁ and f ₂ , third-order distortion spurious 2f ₂ −f ₁ is superimposed on reception frequency f ₃ and causes interference. i.e. the sender
Third-order distortion spurious 2f ₂ − due to transmission frequencies f ₁ and f ₂ generated by TX and output from light emitting diode 7
If a part of f ₁ is received by the light receiving diode 10 of the receiving side RX as described above and goes around to the receiving side RX, there is a problem that interference will occur with the original received data received at the receiving frequency f ₃ . . This thing is
The same goes for the other station whose reception frequencies are f ₁ and f ₂ and its transmission frequencies are f ₃ and f ₄ , and the spurious 2f ₃ −f ₄ interferes with the reception frequency f ₂ .

Therefore, in the present invention, among the frequencies f ₁ ' to f ₄ ' that are close to each other as shown in FIG. 5, the lower frequencies f ₃ ' and f ₄ ' are compared to f ₃ and f ₄ in FIG. Set by shifting the predetermined frequency Δf to the lower side, and also set the frequency of the high group.
On the contrary, f ₁ ′ and f ₂ ′ are set to be shifted higher by a predetermined frequency Δf.

That is, FIG. 5 shows the transmission frequencies f ₁ ′, f ₂ ′ and the reception frequencies f ₃ ′, f _{4 ′} shown in FIG.
For f ₂ and receiving frequencies f ₃ and f ₄ , the following relationships are established: f ₁ ′=f ₁ +Δf, f ₂ ′=f ₂ +Δf, f ₃ ′=f ₃ −Δf, f ₄ ′=f ₄ −Δf, This shows that they have been shifted and set to have the following values. In this way, a frequency difference of 2.DELTA.f will occur between the receiving frequency f ₃ ' and the spurious signal 2f ₂ '-f ₁ ', which can be removed by the filter 11. The value of Δf is approximately Δf=1/3 fs to 2/3 fs. For example, in Figure 4, f ₄ = 8MHz, f ₃ = 9MHz,
When f ₂ = 10MHz, f ₁ = 11MHz, the fifth
In the figure, Δf is set to about 0.3MHz, f ₄ ′=7.7MHz,
Let f ₃ ′=8.7MHz, f ₂ ′=10.3MHz, and f ₁ ′=11.3MHz. The same goes for the other station, and the third-order distortion spurious 2f ₄ ′−
f ₃ ′ occurs at a location 2Δf lower than f ₁ ′. Incidentally, since the frequency shift is relative, it is not necessarily necessary to do as shown in FIG. For example, the high group f ₁ ′,
It is also possible to leave f ₂ ′ as it is (f ₁ ′=f ₁ , f ₂ ′=f ₂ ) and shift the low groups f ₃ ′ and f ₄ ′ toward the lower side by 2Δf.

As described above, according to the present invention, only the reception carrier frequency can be easily extracted, and the third-order distortion spurious 2f ₂ −f ₁ due to the transmission carrier frequency does not overlap the reception frequency and cause interference. .

[Brief explanation of drawings]

1 and 2 are block diagrams showing the general configuration of bidirectional multiplexed optical transmission, FIGS. 3 and 4 are explanatory diagrams showing an example of frequency allocation of carrier waves, and FIG.
The figure is an explanatory diagram showing the frequency arrangement of carrier waves according to the present invention. In the figure, TX is the transmitting side, RX is the receiving side, 1 and 2 are carrier wave oscillators, 3 and 4 are frequency modulation circuits, 5 is a mixing circuit, 7 is a light emitting diode, 8 and 9 are optical fibers, 10 is a light receiving diode, 13 and 14 are filters, and 15 and 16 are demodulation circuits.

Claims (1)

[Scope of Claims] 1. The transmitting side and receiving side of the local station and the receiving side and transmitting side of the opposite station are coupled by an optical fiber, and each transmitting side transmits a frequency-modulated signal using a carrier wave of a different frequency. By multiplexing the modulated carrier waves,
It is a multiplex optical transmission method in which an optical signal whose brightness is modulated by a light emitting diode is transmitted to the receiving side of the other station via an optical fiber, and each receiving side receives and demodulates the modulated carrier wave from the other station received by a light receiving diode. Then, the transmitting carrier frequency of the own station and the transmitting carrier frequency of the opposite station are grouped into low and high frequencies, and the third-order distortion spurious of the transmitting carrier of the own station is transmitted to the transmitting carrier wave from the opposite station. A multiplex optical transmission system characterized in that the frequency of each of the carrier waves is set so as not to affect the carrier waves.