JPH0231354B2 - - Google Patents

Description

Detailed Description of the Invention (Technical Field) The present invention relates to a sonar that performs early homodyne detection of echo signals and converts them into baseband signals for reception, and particularly relates to a method for handling this baseband signal.

(Prior Art) High frequency type (also called RF type) sonar that directly handles a received signal or superheterodyne type sonar that temporarily converts the received signal into an intermediate frequency and handles it has been known. In contrast to this type of system, baseband type sonar is also being considered. The baseband method differs from the high frequency method and superheterodyne method in that it uses receiving beamforming (day map).
This is a method in which the received echo signal is synchronously detected with the output of a local oscillator that approximately matches the nominal center frequency before being applied, and is converted into both quadrature and in-phase (commonly known as q and i, respectively) signals that include a DC component. be. The advantage is that the resolution of the beamformer only needs to match (in the Nyquist sense) exactly the distance resolution unit of the system. It also has the advantage that it is very convenient for extracting Doppler components, and that it is relatively resistant to interference and unnecessary noise because unnecessary band components can be blocked early by a baseband filter.

By the way, this method handles weak signals ranging from DC to low frequencies close to the frame repetition frequency (furthermore, this signal is a linear signal with a wide dynamic range before being subjected to logarithmic compression, etc.). Must be. Therefore, as with television video and audio amplifiers, hum, DC offset, flicker noise, 1/2
There is a disadvantage that f (f is frequency) noise and the like must be considered. This problem did not arise with the high frequency method or the superheterodyne method.

Also, the echo signal containing DC is finally processed by |i|+|q| as √ ² + ² or its approximation, and then shaped into a video signal for brightness modulation, but if you try to perform logarithmic compression, this √ ² + ² or |i
|+|q| is compressed, and a logarithmic amplifier (hereinafter referred to as a log amplifier) is required as a DC amplifier, and the so-called bipolar long amplifier has a slightly different purpose. As an alternative, a result with no practical difference can be obtained by logarithmically compressing i and q separately and then detecting (taking the absolute value) and adding them. However, in this case, conversely, i and q themselves are bipolar signals Therefore, although a bipolar log amplifier is required, the bipolar log amplifier is required to have direct current stability and reliability.

Therefore, in practice, baseline clamping (DC regeneration) must be performed at key points to prevent DC offset. In other words, there is a problem in that it is necessary to provide a moment of no input signal immediately before each transmission/reception, and then perform direct current restoration of the entire system using a clamp pulse.

(Object of the Invention) The present invention has been made in view of the above-mentioned points, and its purpose is to somewhat ease the troublesome handling of baseband signals. An object of the present invention is to provide a baseband sonar that can capture echo signals without having to do so.

(Structure of the Invention) The present invention achieves the above object by providing a first modulated local oscillation signal which is obtained by modulating a local oscillation frequency signal with a rectangular wave signal having a period twice that of the transmission trigger, and a first modulated local oscillation signal that is 90%゜A phase shifter means for obtaining a second modulated station oscillation signal with a phase shift, first and second balanced modulators that perform balanced modulation on the echo signal to obtain an i signal and a q signal, respectively; and the i signal, the first and second bipolar log amplifiers for logarithmically compressing the q signal, respectively, and a synthesis circuit for obtaining the square of the sum of the squares of the respective outputs of the two log amplifiers; The present invention is characterized in that the signals from the first and second modulation stations are applied to the balanced modulators of the above, respectively, and the DC level is automatically obtained without performing DC reproduction of the echo signal.

(Embodiments) Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is an explanatory diagram showing one embodiment of the present invention, and only the configuration for one channel is shown to simplify the explanation. In the figure, 1 is a probe for transmitting and receiving ultrasonic waves, and 2 is a pulser that generates pulses to drive the probe 1 in synchronization with a wave transmission trigger. The received ultrasonic waves are converted into electrical signals by the probe 1 and applied to the first stage amplifier 3. The output of the amplifier 3 is guided in parallel to first and second balanced modulators 4a and 4b. The output of the third balanced modulator 5 is given to the first balanced modulator 4a as a first modulation station oscillation signal. Further, a second modulating station oscillation signal having a phase shift of 90° from the first modulating station oscillating signal is applied to the second balanced modulator 4b via a phase shifter 6 having a phase shift of 90°. There is.
The third balanced modulator 5 phase-modulates the oscillation output of the local oscillator 7 to 0° and 180° based on the result of frequency-dividing the transmission trigger by two using the flip-flop 8. The balanced modulator 5, phase shifter 6, local oscillator 7 and flip-flop 8 constitute phase shift means for obtaining first and second modulated local oscillation signals.

The i signal obtained from the first balanced modulator 4a is input to a first log amplifier (bipolar log amplifier) 10a through a first low-pass filter 9a. On the other hand, q obtained from the second balanced modulator 4b
Similarly, the signal passes through the second low-pass filter 9b and is then sent to the second log amplifier (bipolar log amplifier) 10.
b.

The outputs of these log amplifiers 10a and 10b are
It is added to the synthesis circuit 11 that synthesizes √ ² + ² ,
The obtained output is generated into a video signal via the video amplifier 12 and sent to the CRT display tube 13 as a luminance signal.
added to.

In such a configuration, although it is essentially a baseband system, it is possible to create a system in which essentially no direct current component appears.

That is, as shown in FIG. 2, each time the transmission trigger shown in A is given, the phase of the output of the third balanced modulator 5 changes by 180 degrees as shown in B, and the baseband obtained in response to this changes in phase. The polarity of the signal is reversed for each transmission sequence. Therefore, even without performing DC regeneration, by cutting DC through a circuit such as CR coupling, the DC level will automatically settle down or reach the DC zero level over a long period of time for repeated wave transmission. becomes obvious.

In this method, there is no need to change the processing of √ ² + ² to |i|+|q| in any way from the previous method, at least as long as the system only needs to obtain the luminance signal. Furthermore, similar bipolar log amplifiers used in the heterodyne system or high frequency system can be used without problems with some modifications. That is, it is only necessary to consider the interstage coupling so that the usable limit low frequency can sufficiently cover the frequency that is the reciprocal of twice the repetition period, and there is no need to consider the DC response.

In addition, when using the echo signal as a Doppler signal, the outputs of the low-pass filters 9a and 9b are used, but the Doppler module (not shown) side that receives this receives the output signal of the flip-flop 8 at the same time, and the output signal of the flip-flop 8 is taken in at the same time. Shift the phase by 180 degrees each time and return to the original state before use.

FIG. 3 shows an example of signals i and q when configured in this way. In the figure, the horizontal axis is a time axis (equivalent to distance) and shows the state of each signal received from the time of wave transmission. As seen in the figure, the even numbered i and q signals ie, qe and the odd numbered i and q signals
i ₀ and q ₀ are vertically symmetrical with respect to each other, and as long as this is viewed in units of twice the repetition period T, it can be seen that DC regeneration is unnecessary and there is no problem in using a bipolar log amplifier.

(Effects of the Invention) As described above, according to the present invention, in a baseband sonar, it is possible to realize a signal handling method in which substantially no DC component appears in the baseband region.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram showing one embodiment of the present invention, and FIG.
The figure is a time chart for explaining the operation, and FIG. 3 is a waveform diagram showing an example of i and q signals of received echo signals. 1... Probe, 2... Pulser, 3... First stage amplifier, 4a, 4b, 5... Balanced modulator, 6... 90° phase shifter, 7... Local oscillator, 8... Flip-flop, 9a, 9b...Low pass filter, 10
a, 10b...Log amplifier, 11...Synthesizer.

Claims (1)

[Claims] 1. Obtain a first modulated local oscillator signal by modulating a local oscillation frequency signal with a rectangular wave signal having a period twice that of the transmission trigger, and a second modulated local oscillator signal whose phase is shifted by 90 degrees from the first modulated local oscillator signal. phase shifter means, first and second for balanced modulating the echo signal to obtain an i signal and a q signal, respectively;
a balanced modulator, first and second bipolar log amplifiers that logarithmically compress the i signal and the q signal, respectively, and a synthesis circuit that obtains the square of the sum of the squares of the respective outputs of the two log amplifiers. The first,
The baseband method is characterized in that the first and second modulation station oscillation signals are added to the second balanced modulator, respectively, and a DC level is automatically obtained without performing DC reproduction of the echo signal. Sonar.