JP3407620B2 - Homodyne transmission / reception circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a homodyne transmission / reception circuit using a common signal source for a signal generator and a receiver. This invention is particularly, for detecting the reflected signal by injecting a high frequency signal to the radar or transmission line, to measure the distance and relative velocity of the object by receiving the reflected wave by sending a high-frequency signal to the space Thus, the present invention is suitable for use in a reflectometer for inspecting an operation or a failure inside a transmission line. 2. Description of the Related Art FIG. 4 shows a configuration of a conventional FM-CW (frequency modulation-continuous wave) type homodyne transmission / reception circuit. This homodyne transmission / reception circuit includes a modulation signal generation circuit 1, a voltage controlled oscillator (VCO) 2, a power amplification circuit 3, a power distributor 4, a circulator 5, an antenna 6, and a frequency mixer 7. The modulation signal generating circuit 1 generates a triangular wave, and the voltage controlled oscillator 2 uses this triangular wave as a modulation input to generate a frequency-modulated signal. This signal is amplified by the power amplifier circuit 3, and the power distributor 4
Is divided into a transmission signal and a local oscillation signal to the frequency mixer 7. The divided transmission signal passes through a circulator 5 for sharing the antenna 6 for transmission and reception, and is transmitted from the antenna 6 to the air. The transmitted signal propagates in the air, hits the object 8, and a part of the signal returns to the antenna 6 again. The signal received by the antenna 6 passes through the circulator 5 and is guided to a frequency mixer 7. The frequency mixer 7 performs frequency conversion by mixing this signal with the local oscillation signal branched by the power divider 4. [0003] frequency-converted output obtained by the frequency mixer 7, the distance by which includes two components of the beat component f _bv by the beat component f _bR and the relative speed. The beat component f _bR due to the distance is generated when the frequency of the local oscillation signal subjected to the triangular modulation changes while the signal reciprocates between the antenna 6 and the object 8. That is, the distance to the object is R [m], and the frequency deviation (total swing width) is Δf.
[Hz], the modulation frequency f _m (Hz), the speed of light c [m
/ S] When (= 3 × 10 ^8), the beat component represented by _{f bR = 4R · Δf · f} m / c is generated. On the other hand, the beat component f _bv due to the relative speed is generated when the frequency of the received signal changes due to Doppler shift. That is, the carrier frequency f _c (Hz), the relative velocity v [m
/ S] (a positive value when the object approaches), a beat component represented by f _bv = 4R · Δf · (2v / c) is generated. [0004] Thus, the frequency converted output, if the transmission frequency of the rise, f _u = f _bR -f _bv transmission frequency in the case of lowering is observed as _{f d = f bR + f bv} , f bR = _{(f d -f u) / 2} f bv = (f d + f u) / 2 by a component due to the distance R by component and the relative velocity V can be separated. FIG. 5 schematically shows noise generated in a frequency conversion output in a conventional FM-CW system homodyne transmission / reception circuit. In the homodyne receiving circuit, the transmission frequency and the local oscillation signal frequency of the frequency mixer are obtained from the same signal source. Therefore, the frequency conversion output is obtained in the baseband starting from DC. The noise generated in this band is 1) thermal noise generated by the resistance component of the frequency mixer and other circuits, and 2) the active element when the passive mixer is used as the frequency mixer and the active mixer is used. phase noise of the DC vicinity generated by the signal flows, 3) modulation component f _m and its high-frequency component of the FM modulated local oscillation signal has three interdigital noise generated frequency conversion output. When a commercially available low noise frequency mixer is used,
Thermal noise is about -150 dBm / Hz, phase noise is 1 kHz
In a frequency band equal to or lower than z, the level increases from the signal level of the thermal noise toward the DC side at a rate of 10 dB / dec. However, comb-like noise as high as -60dBm at the fundamental frequency band of the modulation frequency f _m, but reduced levels of harmonics as the frequency becomes higher, the number of drops until the equivalent level of thermal noise It is higher than the tenth harmonic and larger than the other two. Since generally the received signal input from the antenna is very small (-100 dBm or less), the beat frequency component f _b by the reception signal, reception performance is degraded significantly by comb-like noise. As a conventional technique for solving this problem, Japanese Patent Application Laid-Open No. H5-40169 discloses that the influence of sawtooth noise is eliminated by a heterodyne configuration. FIG. 6 shows an example of the configuration. In this conventional example, in addition to the configuration shown in FIG. 4, a new signal source 21 that generates a frequency f _L sufficient to separate a beat signal component from a comb-like noise component, and is driven by the output of this signal source 21 And a frequency mixer 23 for further converting the output of the frequency mixer 7 into a frequency. FIG. 7 shows an output of the frequency mixer 7 obtained from the transmission / reception circuit having the configuration of FIG. By switching the high-frequency signal supplied from the circulator 5 to the frequency mixer 7 at the frequency f _L , a sideband is generated at a frequency of ± f _{L in} the received signal, and f _After leaving by _L , the baseband signal is obtained again by the frequency mixer 23. According to the above-mentioned publication, it is described that such a heterodyne configuration can eliminate a demodulated wave of an AM component caused by a slope of a frequency-output voltage characteristic of an oscillator near a direct current. It has been suggested that it can be removed. However, the technique disclosed in the above publication has a problem that the received signal level is degraded by 7 to 10 dB by the switching circuit and the structure is complicated. The present invention, this problem was solved a relatively simple circuit configuration, the interdigital noise modulation component f _m and its high-frequency component of the FM modulated local oscillator signal is generated in the frequency conversion output It is an object of the present invention to provide a homodyne transmission / reception circuit capable of eliminating the influence of the above. According to the present invention, there is provided a homodyne transmission / reception circuit comprising: a signal generator for generating a continuous frequency-modulated high-frequency signal; and a high-frequency signal generated by the signal generator for an object. Transmitting and receiving means for transmitting and receiving the reflected signal from the object, a part of the high-frequency signal generated by the signal generating means is branched and supplied,
A homodyne transmission / reception circuit having a first frequency mixing means for performing frequency conversion by mixing it as a local oscillation signal with a reflection signal received by the transmission / reception means, and is substantially the same as the first frequency mixing means. The local oscillation signal is input to one input, and the other
To the other input, mixed into the reflected signal from the transmitting and receiving means
And the high frequency inputted to the first frequency mixing means
The reflection received by the transmitting / receiving means corresponding to the signal component
A signal obtained by attenuating the local oscillation signal is input instead of the signal.
And a means for subtracting the output of the second frequency mixing means from the output of the first frequency mixing means. According to the above publication, the main cause of the noise deterioration is caused by the slope of the frequency versus output power characteristic of the oscillator near DC. In other words, the local oscillation signal contains an AM modulation component, and when this is mixed with the main signal (the received reflection signal), the AM modulation component (and its harmonics) appear at the output. However, according to the experiments performed by the inventors of the present invention, even when a frequency modulation signal is input to the local oscillation input in a state where the reception signal input of the frequency mixer used as the frequency mixing means is terminated, it is ideal. Is an analog signal similar to the frequency modulation waveform of the local oscillation input, although one of the inputs of the frequency mixer has no signal, so its output should also be a signal.
Output was found to appear. From this, the inventors of the present application believe that the comb-tooth noise appears even when the main signal does not exist, and that the cause is due to imperfections of the frequency mixer. It is thought that such imperfections can be solved by improving the circuit configuration of the frequency mixer and its constituent elements, but this increases the cost of the frequency mixer. Therefore, in the present invention, two equivalent frequency mixers are used, one of which receives the main signal but the other does not receive the main signal, and subtracts the obtained two outputs to obtain the two frequency mixers. Offsets imperfections in the mixer. FIG. 1 is a diagram showing a first embodiment of the present invention, and shows a circuit configuration of a homodyne transmission / reception circuit. This homodyne transmission / reception circuit includes a modulation signal generation circuit 1 for generating a continuous frequency-modulated high-frequency signal.
A voltage controlled oscillator (VCO) 2, a power amplifier circuit 3 for transmitting a high-frequency signal generated by the voltage controlled oscillator 2 to an object, and receiving a reflected signal from the object, a power distributor 4, The circulator 5, the antenna 6, and a part of the high-frequency signal generated by the voltage-controlled oscillator 2 are branched and supplied, and mixed with a reflected signal received by the antenna 6 as a local oscillation signal and supplied through the circulator 5. And a frequency mixer 7 for performing frequency conversion. The frequency mixer 7 further has substantially the same configuration as the frequency mixer 7, and receives the reflected signal received by the antenna 6 without inputting the reflected signal. A frequency mixer 9 to which a part of the output high-frequency signal is input as a local oscillation signal, and an output of the frequency mixer 9 to an output of the frequency mixer 7 And a differential circuit 10 to subtract al. Strictly speaking, a bandpass filter is provided at each output of the frequency mixers 7 and 9, and the output of the bandpass filter is subtracted by the differential circuit 10, but the description is omitted here. The modulation signal generating circuit 1 generates a triangular wave, and the voltage controlled oscillator 2 uses this triangular wave as a modulation input to generate a frequency-modulated signal. This signal is amplified by the power amplifier circuit 3, and the power distributor 4
Is divided into a transmission signal and a local oscillation signal to the frequency mixer 7. The divided transmission signal passes through a circulator 5 for sharing the antenna 6 for transmission and reception, and is transmitted from the antenna 6 to the air. The transmitted signal propagates in the air, hits an object, and a part of the signal returns to the antenna 6 again. The signal received by the antenna 6 (hereinafter referred to as “main signal”) passes through the circulator 5 and passes through the frequency mixer 7,
It is led to 9. The frequency mixer 7 performs frequency conversion by mixing this signal with the local oscillation signal branched by the power divider 4. The input of the main signal is input to the frequency mixer 9 by the terminating resistor, and the other input is supplied with the local oscillation signal branched by the power divider 4. The differential circuit 10 subtracts the output of the frequency mixer 9 from the output of the frequency mixer 7. The differential circuit 10 can be easily realized by using an element such as an operational amplifier. The basic operation of this embodiment is the same as that of the prior art. However, since the two frequency mixers 7 and 9 are driven by the local oscillation signal branched by the power divider 4,
Comb-like noise is generated in each of these two outputs similarly to the conventional output. On the other hand, although the main signal is input to the frequency mixer 7, the main signal is not input to the frequency mixer 9. Therefore, taking the difference between the outputs of the two frequency mixer 7,9, only the signal component f _b associated with the main signal is obtained, comb-like noise is canceled out. The comb-shaped noise generated in the output of the frequency mixer 7 and 9, the modulation signal f _m to similar waveform, that is, a change in the dc level of the waveform with the harmonic distortion, frequency mixer 7,9 In the output of A than a comb-shaped spectrum shape, as the signal waveform is analogous to the modulation waveform f _m. In such noise, the DC imbalance corresponding to the positive and negative of the local oscillation signals input to the frequency mixers 7 and 9 is modulated by the local oscillation signal whose frequency continuously changes, and the local oscillation signals and This is considered to be caused by generating a DC component having a similar waveform. For this reason, the noise characteristics are mainly determined by the characteristics of the frequency mixer and the modulation waveform of the local oscillation input, and are not affected by the main signal input having a relatively small signal level. Therefore, by taking the difference between the outputs of the two frequency mixers 7 and 9 having different input conditions of the main signal, a signal free of the above-described noise can be obtained. This is particularly remarkable when the two frequency mixers 7 and 9 have similar characteristics made of an integrated circuit or the like. FIG. 2 is a diagram showing a second embodiment of the present invention. This embodiment is the same as the first embodiment in that two frequency mixers 7 and 9 are used. Instead of terminating the main signal input of the attenuator 9,
This is different from the first embodiment in that a local oscillation signal is input via the interface. In this embodiment, the characteristics of the circulator 5 are not ideal, the isolation from the input of the transmission signal to the output of the reception signal is finite, and the transmission signal is transmitted through the path of the power divider 4, the circulator 5, and the frequency mixer 7. Suitable when is coming back. That is, the local oscillation signal branched by the power divider 4 is converted by the attenuator 11 into the circulator 5.
The level is adjusted to a level corresponding to the amount of the leakage signal (so that the phase difference may be equal in some cases) and supplied to the main signal input of the frequency mixer 9. With this configuration, the circulator 5
Does not operate ideally, the frequency mixers 7 and 9 operate almost the same, and only the baseband signal component of the received signal obtained by the antenna 6 can be obtained at the output of the differential circuit 10. FIG. 3 is a diagram showing an example of measurement using a homodyne transmission / reception circuit according to the present invention. In the circuit shown in the second embodiment, a state of a line which may have a failure is inspected. An example is shown below. For this inspection, the device under test 12 is connected to the antenna 6 in the above-described embodiment, and the distance to the failure point is determined. This measurement can be similarly performed using the circuit shown in the first embodiment. According to the present invention, the detectable range and the distance accuracy of the fault point can be improved. As described above, according to the present invention,
It is possible to reduce the noise (f _m Noise) having an FM modulation frequency generated in the output of the homodyne transceiver circuit of FM-CW mode in the fundamental wave, it is possible to improve the reception performance.
The effect of this improvement is more remarkable as the frequency is closer to the fundamental wave of the _fm noise, which means that the performance is improved more when the distance is short or the relative speed is small. Further, the undetectable area (distance, speed) due to the comb-shaped spectrum is also reduced. Further, since the modulation frequency can be increased by an amount corresponding to the noise reduction for an object from which the same received signal can be obtained, the time required for detection can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a first embodiment of the present invention. FIG. 2 is a diagram showing a second embodiment of the present invention. FIG. 3 is a diagram showing an example of measurement using the homodyne transmission / reception circuit of the present invention. FIG. 4 is a diagram showing a configuration of a conventional FM-CW system homodyne transmission / reception circuit. FIG. 5 is a diagram schematically showing noise generated in a frequency conversion output in a conventional FM-CW system homodyne transmission / reception circuit. FIG. 6 is a diagram showing a configuration of a conventional example in which the influence of sawtooth noise is eliminated. FIG. 7 is a diagram illustrating an output of a frequency mixer obtained from the transmission / reception circuit having the configuration of FIG. 6; [Description of Signs] 1 Modulation signal generation circuit 2 Voltage controlled oscillator 3 Power amplification circuit 4 Power distribution circuit 5 Circulator 6 Antennas 7, 9, 23 Frequency mixer 8 Object 10 Differential circuit 11 Attenuator 12 DUT 21 Signal Source 22 high frequency switch

Continuation of front page (56) References JP-A-4-265882 (JP, A) JP-A-2-157679 (JP, A) JP-A-62-104226 (JP, A) JP-A-5-40169 (JP) , A) JP-A-11-118915 (JP, A) JP-B-50-27718 (JP, B1) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01S ^7/ 00-7/64 G01S 13/00-13/95

Claims (1)

(57) Claims 1. A signal generating means for generating a continuous frequency-modulated high-frequency signal, and transmitting the high-frequency signal generated by the signal generating means to an object and Transmitting and receiving means for receiving a reflected signal from an object; and a part of a high-frequency signal generated by the signal generating means is branched and supplied, and mixed as a local oscillation signal with the reflected signal received by the transmitting and receiving means. In a homodyne transmission / reception circuit comprising a first frequency mixing means for performing frequency conversion by, having substantially the same configuration as the first frequency mixing means, the local oscillation signal is input to one input, The other
Input to the reflected signal from the transmitting / receiving means
The high frequency signal input to the first frequency mixing means.
Corresponding to the reflected signal received by the transmitting / receiving means.
The signal obtained by attenuating the local oscillation signal is input instead of the signal
And a means for subtracting an output of the second frequency mixing means from an output of the first frequency mixing means.