JP2674711B2 - Spectrum display circuit - Google Patents

Description

TECHNICAL FIELD The present invention relates to a spectrum display circuit, and more particularly to a spectrum display circuit for a received signal of a receiver.

(Prior art)

As a device that can display the status of a received signal in a specific frequency band as a spectrum, there is, for example, a device called a bandscope or a spectrum analyzer. As shown in FIG. 4, the basic configuration thereof controls the sweep signal display means 45 oscillated from the sweep signal oscillating means 1 and the sweep signal is inputted to the local oscillator 3 where the tuning local oscillation signal is supplied. It is converted to sl and input to the mixer 41. A signal to be displayed as a spectrum, for example, a received input signal is input to the mixer 41, and is tuned by the local oscillator signal sl. The signal thus swept-tuned is input to the display means 45 via the amplifier 42, the filter 43, the wave detector 44, etc., and displayed as a spectrum.

According to this device, since the state of the input signal in the vicinity of the reception frequency can be displayed on the display, for example, the tuning index is displayed on the display so that the frequency corresponding to the index position becomes the tuning frequency. However, there is an advantage that the signal of the target frequency can be tuned visually quickly and accurately.

[Problems to be solved by the invention]

In the above configuration, the VCO is used as the local oscillator 3.
(VCO) is used, and the VCO has the drawback that the oscillation frequency tends to fluctuate due to external factors such as changes with time and ambient temperature changes. If the frequency fluctuates, it becomes difficult to perform the tuning operation accurately.

The present invention has been proposed in view of the above conventional circumstances, and an object thereof is to provide a spectrum display circuit in which, when a VCO is used as a local oscillator, the center frequency of the VCO does not change. is there.

[Means for Solving the Invention]

The present invention employs the following means to achieve the above object.

That is, the sweep signal generating means, a local oscillator whose oscillation frequency is controlled by the output of the sweep signal generating means, and an intermediate frequency signal generating means for forming an intermediate frequency signal by the local oscillator output and the input signal are provided. In a spectrum display circuit capable of detecting a frequency signal and displaying the level thereof, a frequency discriminator for converting the frequency into a corresponding voltage near the center oscillation frequency of the output of the local oscillator controlled by the sweep signal. A center detecting means for detecting a specific voltage range near the center voltage of the sawtooth wave obtained from the sweep signal generating means, a gate means for turning on the output of the center detecting means, and passing the frequency discriminator output, And a control means for correcting the oscillation frequency of the local oscillator based on the output of the gate means.

The control means may include an integrating circuit that integrates the output of the frequency discriminator and directly control the local oscillator by the output of the integrating circuit, or an integrating circuit that integrates the output of the frequency discriminator. It may be configured by a control oscillator whose oscillation frequency is controlled according to the output of the integration circuit and a mixer for mixing the control oscillator output and the local oscillator output.

(Operation)

In the above configuration, when the center frequency of the output of the local oscillator controlled by the sweep signal shifts, a control voltage corresponding to the shift is obtained from the integrating circuit that constitutes the control means.
Therefore, the local oscillation frequency is corrected based on this control voltage to return the center frequency to a normal value.

〔Example〕

FIG. 1 shows an embodiment of the present invention. A digital signal from a digital signal generator (not shown) is converted into an analog signal by the D / A converter 11 constituting the sweep signal generating means 1 to obtain a sawtooth sweep signal Ss. This sweep signal Ss is input to a voltage controlled oscillator (hereinafter referred to as VCO) 3, which is a local oscillator, and a local oscillation signal Sl whose frequency changes according to the voltage change of the sweep signal Ss is obtained from this VCO 3. This local oscillation signal Sl is input to the mixer 41, and after being mixed with the received input signal, only a specific intermediate frequency is extracted via the amplifier 42 and the bandpass filter 43. The output of the VCO3 is also input to the mixer 13, where it is mixed with the output from the crystal oscillator 20 that oscillates a predetermined reference frequency, and VCO3
The output near the center oscillation frequency of is converted into a frequency within the range permitted by the frequency discriminator 15 below, and the frequency discriminator 15 corresponds to the frequency as shown in FIG. Converted to Sd.

If the frequency discriminator 15 operates near the center oscillation frequency of the VCO 3, the crystal oscillator 20 and the mixer 13 are omitted, and the output of the VCO 3 is directly input to the frequency discriminator 15 to output the voltage. Can be converted to Sd.

The output Sd of this frequency discriminator 15 is the gate control signal Sg below.
It is input to the gate 17 which is ON / OFF controlled by.

On the other hand, the digital signal is also input to the center detection circuit 16, and when the gray level of the digital signal reaches the gray level near the center, the gate control signal is turned on to open the gate.

For example, when the digital signal has 8 bits and 256 gradations, the gate 17 is turned on between 124 to 132 gradations, the output Sd of the frequency discriminator 15 is passed, and the signal is input to the control means 2 in the next stage. In this example, the control means 2 comprises a DC amplifier 21 and an integrating circuit 22.
An output of 21 is averaged to obtain a control voltage Sc, and the control voltage controls the VCO3.

With this configuration, when the center oscillation frequency of VCO3 is not shifted, as shown in FIG. 3 (a), the gate output Si in which zero is symmetrically changed to the positive and negative by the same voltage width is obtained, and the gate output Si is integrated. Even if the circuit 22 is integrated, the value of the control voltage Sc becomes zero. When the center frequency is deviated, as shown in FIG. 3 (b), the gate 17 outputs a voltage biased in either the positive or negative direction with respect to the zero point as the gate output Si. From this, the voltage of the value corresponding to the deviation is averaged and obtained as the control voltage, and the VCO3 can be controlled based on this.

The time constant of the integrating circuit 22 must be set to a large value so that the frequency change of the VCO 3 due to the control voltage Sc does not affect the frequency change due to the sweep signal Ss.

FIG. 2 shows another embodiment of the present invention. As the control means 2, in addition to the DC amplifier 21 and the integration circuit 22, a VC whose output frequency is controlled by the output voltage of the integration circuit 22.
An oscillator 23 such as O or VXO (variable crystal oscillator) and a mixer 24 that corrects the output frequency of VCO3 by the output of the oscillator 23 are added.

Furthermore, the output of the mixer 24 is a mixer of the received signal system.
It is input to 41 as well as to the frequency discriminator 15.

With this configuration, the control voltage output from the integrator circuit 22 is input to the oscillator 23 such as VCO or VXO, and the signal of the frequency controlled by the output voltage of the integrator circuit 22 is obtained. With this signal, the mixer 24 outputs VCO3. I am trying to correct the frequency. Even in this configuration, the relationship between the output of the integrating circuit 22 and the shift is the same as that in the example shown in FIG. However, unlike the example of FIG. 1, in the example of FIG. 2, the sweep local oscillator VCO3 and the oscillator 23 that corrects the deviation of the center oscillation frequency of the VCO3 are provided separately, so that the control voltage Si is conditioned to oscillate VCO3 It has no effect.

〔The invention's effect〕

As described above, according to the present invention, when the center frequency of the swept VCO deviates from a predetermined value, the deviation can be automatically corrected, and an accurate center frequency can always be maintained. become.

[Brief description of the drawings]

1 and 2 are circuit diagrams showing an embodiment of the present invention, FIG. 3 is an explanatory view showing the relationship between the shift of the center frequency and the gate output, and FIG. 4 is an outline of the spectrum display circuit. circuit diagram.

Claims (2)

(57) [Claims] 1. A sweep signal generating means, a local oscillator whose oscillation frequency is controlled by an output of the sweep signal generating means, and an intermediate frequency signal generating means for forming an intermediate frequency signal by the local oscillator output and an input signal. In a spectrum display circuit capable of detecting the intermediate frequency signal and displaying the level thereof, frequency discrimination for converting the frequency into a corresponding voltage near the center frequency of the output of the local oscillator controlled by the sweep signal. And a center detecting means for detecting a specific voltage range near the center voltage of the sawtooth wave obtained from the sweep signal generating means, and a gate means for turning on the output of the center detecting means and passing the output of the frequency discriminator. And an integrating circuit for integrating the output of the frequency discriminator output from the gate means. Spectrum display circuit, characterized in that a control means for controlling the parts oscillator. 2. A sweep signal generating means, a local oscillator whose oscillation frequency is controlled by the output of the sweep signal generating means, and an intermediate frequency signal generating means for forming an intermediate frequency signal by the output of the local oscillator and an input signal. In a spectrum display circuit capable of detecting the intermediate frequency signal and displaying the level thereof, frequency discrimination for converting the frequency into a corresponding voltage near the center frequency of the output of the local oscillator controlled by the sweep signal. And a center detecting means for detecting a specific voltage range near the center voltage of the sawtooth wave obtained from the sweep signal generating means, and a gate means for turning on the output of the center detecting means and passing the output of the frequency discriminator. An integrating circuit for integrating the output of the frequency discriminator output from the gate means, and an oscillation frequency according to the output of the integrating circuit. Spectrum display circuit but which is characterized by comprising a control oscillator controlled, and a control means having a 該制 patronized oscillator output and mixer for mixing the local oscillator output.