JPH0580986B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION A. Object of the Invention [Field of Industrial Application] The present invention relates to an improvement of a superheterodyne spectrum analyzer.

[Prior Art] FIG. 3 shows the configuration of a conventional superheterodyne spectrum analyzer. In FIG. 3, the sweep oscillator 7 outputs a sawtooth wave as shown in FIG.
oscillator) 5. Also, this sawtooth wave
Used for frequency sweep in addition to the horizontal axis of CRT20.
Since a sawtooth wave is applied to the VCO 5, it outputs a frequency _V that changes depending on the applied voltage, and adds this to the mixer 3.

On the other hand, an input signal of frequency _i (usually _i includes many frequency components) is applied to the low-pass filter 1. The low-pass filter 1 is used to pass frequencies within the range that the spectrum analyzer is intended to measure, and to cut out other frequency components. For example, if the spectrum analyzer is for observing the band from 0 to ₀ , it is a filter that has the characteristic of cutting frequency components above ₀ . Let _L be the frequency of the output signal of this low-pass filter 1.

The mixer 3 performs mixing by calculating equation (1), and sends the output signal (frequency _M ) to the next stage BPF amplifier (band pass filter amplifier 9).
Add to.

_M = _V − _L (1) The BPF amplifier 9 selects and amplifies only frequencies around a certain frequency ₁ .

In Figure 3, in order to further increase frequency selectivity and gain gain, mixer 11, oscillator 13 and BPF are used.
It is constructed of a so-called double superheterodyne type in which the signal is amplified by an amplifier 15. This mixer 11
The apparatus shown in FIG. 3 can operate even without the oscillator 13 and BPF amplifier 15.

The amplitude of the output of the BPF amplifier 15 is detected by a detector 17, noise components are removed by a video filter 19, and the output is added to the vertical axis of the CRT 20.

In the apparatus shown in FIG. 3 as described above, a frequency spectrum waveform as shown in FIG. 4 is displayed on the CRT 20. Its operation is as follows.

The frequency _M applied to the BPF amplifier 9 is the frequency _L whose frequency value is shifted by the frequency _V from the VCO 5, as shown in equation (1). In other words, the frequency _M is a value obtained by shifting the frequency _L , which is obtained by removing unnecessary components (components not to be measured) of the input frequency _i , by the frequency _V.

To give a specific example, if the spectrum analyzer has a measurement range from 0 to ₀ , each frequency shown in FIG. 3 is selected as follows, for example. The variable frequency range _V of VCO5 is _V = ₁ ~
( ₁ + ₀ ), output frequency _L of low-pass filter 1 =
0 to ₀ , the center frequency of the BPF amplifier is ₁ .

Therefore, the frequency _M is a mixture of a certain instantaneous frequency value, for example, _V = _V1 , and a frequency _L that includes frequency components from 0 to ₀ , so from equation (1), _M is ( _V1 − ₀ ) ~ This is a frequency that includes the frequency component of _V1 .

Among the frequencies _M with such a band, the BPF
Only the frequency values corresponding to the center frequency ₁ of the bandpass filter in the amplifier 9 can be selected and passed to the next stage. Therefore, this passable frequency is shifted by sweeping the output frequency _V of the VCO 5.

Therefore, a frequency spectrum waveform as shown in FIG. 4 is obtained.

[Problem that the invention seeks to solve]

However, if the measures shown in Fig. 3 are implemented, the circuit operation differs from the ideal state, so the BPF amplifier 9
Since the input frequency _M includes frequencies other than those expected in equation (1), there is a problem that this may cause noise.

Note that the noise referred to in this specification is not the noise generally included in an input signal or the like. That is, even if the input signal does not generally contain noise, it is a waveform that appears as "noise" by being superimposed on the true signal portion of the spectrum waveform.

To be more specific, the cut-off characteristics of the low-pass filter 1 are poor, and the isolation of the mixer 3 is poor.
There is a problem in that the input frequency _i is added to the output _M of the mixer 3 in a transparent manner, and _one component included in the input _i is detected at the same time as the signal that has been properly mixed using equation (1). Therefore, a signal whose frequency has been shifted by mixing and a signal which has passed through and has not been frequency shifted are mixed, resulting in noise as shown in FIG. 2.

Due to the nonlinearity of mixer 3, harmonic components of the output frequency _V of VCO 5 and input frequency _i are added to the output _M of mixer 3, which is then transferred to BPF amplifier 9.
may pass through. That is, ₁ = q· _V −p· _i ′ (2) q, p: integer When _i ′ other than the input frequency _i is detected in this way, it also becomes noise as shown in FIG.

When noise is detected in this way, the dynamic range of the spectrum analyzer decreases. Therefore, in order to obtain the necessary dynamic cleanse,
It is necessary to use very expensive circuit elements, such as the mixer 3.

SUMMARY OF THE INVENTION An object of the present invention is to provide a superheterodyne spectrum analyzer that eliminates the above noise and has a wide dynamic range without using particularly expensive circuit elements.

B "Structure of the Invention" [Means for Solving the Problems] In order to solve the above problems, the present invention provides a VCO 5 to which a sawtooth wave is added, an input signal passed through a low-pass filter, and an output signal of the VCO 5. A device for displaying a frequency spectrum waveform of an input signal on a CRT, comprising a mixer 3 for mixing, and a first BPF amplifier ₉ into which the output of the mixer 3 is introduced and whose bandpass filter has a center frequency of 1. A second BPF amplifier 51 whose bandpass filter has a center frequency different from the first _one ( ₁ + Δ), a first BPF amplifier 9, and a second BPF amplifier 51.
When the second BPF amplifier 51 is selected, the means 57, 58 for switching the
means 54 for shifting the output frequency of the VCO 5 by (Δ); a first BPF amplifier 9 and a second BPF amplifier 51;
The present invention includes means 55 and 56 for selecting a low signal level from among the frequency spectrum signals obtained through the CRT and adding it to the CRT.

〔Example〕

Hereinafter, the present invention will be explained in detail using the drawings.

FIG. 1 is a diagram showing an embodiment of a spectrum analyzer according to the present invention. The differences between FIG. 1 and the conventional FIG. 3 are as follows.

Switch 5 is connected between mixer 3 and BPF amplifier 9.
A switch 58 is also provided between the mixer 11 and the BPF amplifier 15, and a new BPF amplifier 51 is provided between the contacts of these two switches 57 and 58.
and a mixer 52 and an oscillator 53 are provided. The center frequency ₂ of the bandpass filter of this newly installed BPF amplifier 51 is
It is set so that ₂ = ₁ + Δ. Note that ₁ is the center frequency of the BPF amplifier 9. Further, the mixer 52 and the oscillator 53 have the same function as the mixer 11 and the oscillator 13.

An adder 54 is provided before the VCO 5. This adder 54 adds the output of the sweep oscillator 7 and the signal _S1 and outputs the result to the VCO 5. This signal _S1 has a value of 0 and an offset voltage.
Two types are selected as appropriate: V〓.

A waveform memory 55 and a comparison circuit 56 are provided between the video filter 19 and the CRT 20. The waveform memory 55 temporarily stores the signal waveform for one sweep from the video filter 19. The storage method may be analog or digital. In addition, the comparison circuit 56
compares the signal waveform from the video filter 19 and the signal waveform from the waveform memory 55, and sends the signal with the smaller level to the next CRT 2.
It is applied to the vertical axis of 0.

The configurations other than those described above are the same in FIG. 3 and FIG. 1. Therefore, the same constituent element numbers as in FIG. 3 are given, and a redundant explanation thereof will be omitted.

The operation of the present invention will be explained below, but first an overview of the present invention will be described. FIG. 2 is a diagram showing a spectrum waveform displayed on the CRT 20. In the present invention, first, a spectrum waveform as shown in FIG. 2 is obtained using a conventional circuit as shown in FIG. 3, and this waveform data is stored in the waveform memory 55.

Next, switch 57 and 58 are changed so that the center frequency of the bandpass filter of the BPF amplifier becomes ( ₁ +
Δ) circuit and the mixing frequency
Correspondingly, the frequency of _V is also multiplied by an offset amount as ( _V + Δ). In this way, the frequency of the signal that has been properly mixed through the mixer 3 and that passes through the BPF amplifier 51 does not change from that in FIG. 2, as shown by equation (3).

From equation (1), _L = _V − _M If we focus on the pass frequency of the BPF amplifier, we can set _M = ₁ , so _L = ( _V + Δ) − ( ₁ + Δ) = _V − ₁ (3) However, The unmixed, ie, unmixed signal (noise) deviates from FIG. 2 by (Δ) in the bandpass filter. That is, the position of the frequency shifts by the amount of noise as shown in FIG.

Then, the second
The comparison circuit 56 compares the waveform shown in the figure with the new waveform shown in Fig. 2, and the signal level of the smaller one is determined.
When displayed on the CRT 20, it becomes a spectrum waveform with noise removed, as shown in FIG.

The operation will be explained in detail below.

(A) When the switch contact is on the a side When the switches 57 and 58 are on the contact a side, the signal S ₁ applied to the adder 54 is set to 0. In this case, the same operation as in Fig. 3 is performed, and for example, spectrum waveform data as shown in Fig. 2 is stored in the waveform memory 5.
It is stored in 5.

(B) When the switch contact is on the b side Next, when the switches 57 and 58 are switched to the contact b side, the center frequency of the bandpass filter becomes ( ₁ +
Δ) is switched to the circuit on the BPF amplifier 51 side.
At the same time, the signal S ₁ applied to the adder 54 also becomes the offset voltage V, so the output frequency of the VCO 5 is
During the period when switches 57 and 58 are on the contact b side,
The frequency is shifted by (Δ). That is, VCO
The output of 5 is ( _V + Δ). When a spectrum waveform is obtained in this state, a waveform as shown in FIG. 2 is obtained, as explained in relation to equation (3) above.

That is, it is mixed normally and the BPF amplifier 5
The frequency of the signal that has passed through mixer 3 does not change, and the signal (noise) that has passed through mixer 3 deviates from its position on the waveform.

Then, the comparison circuit 56 compares the spectrum waveform for one sweep stored in the waveform memory ₅₅ [for example, FIG. , FIG. 2], and the smaller level signal is displayed on the CRT 20. In other words, when comparing at the same frequency value,
The noise component appears on either side of FIG. 2, but the original signal appears at the same level. Therefore, the small level signal of is the true input spectrum.

Note that in the above explanation, the BPF amplifier 9 with a center frequency of ₁ and the BPF amplifier 51 with a center frequency of ( ₁ + Δ) are used.
Although an example has been described in which a pair (two frequency relationships) is provided, the configuration is not limited to this, and for example, it may be configured with three or more. That is, the center frequency ( ₁ +
Δ′) BPF amplifier, mixer, and VCO circuit as the first
Two switches 57 and 58 in the figure (in this case, the switch has increased switching contacts a, b, and c)
It may be added in between. in this case,
Although the configuration is more complicated, it reduces the chance that separate noises will coincidentally overlap, so that it is possible to achieve a further reduction in noise than in FIG. 1. In this case, the waveform memory 55 and the comparison circuit 56 operate to display the minimum level up to that point on the CRT 20.

In addition, although the double superheterodyne configuration is shown in FIG. 1, the configuration is not limited to this.
A single superheterodyne type, a triple superheterodyne type, etc. may also be used.

C. "Effects of the Present Invention" As described above, according to the present invention, since there is a plurality of frequency relationships, there is no possibility of erroneously displaying noise etc. that are not originally present in the input. Therefore, the dynamic range can be widened. Further, the above-described high-performance spectrum analyzer can be realized with a simple configuration without using particularly expensive circuit components.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a configuration example of a spectrum analyzer according to the present invention, FIG. 2 is a diagram of frequency spectrum waveforms for explaining the operation of the present invention, and FIG.
The figure shows an example of the configuration of a conventional spectrum analyzer, FIG. 4 shows an example of a CRT display, and FIG. 5 shows a waveform applied to the VCO 5. 1...Low pass filter, 3...Mixer, 5...
...VCO, 9,51...BPF amplifier, 20...
CRT, 54...adder, 55...waveform memory,
56... Comparison circuit.

Claims (1)

[Claims] 1. A VCO 5 to which a sawtooth wave is added, a mixer 3 that mixes the input signal passed through the low-pass filter and the output signal of the VCO 5, and the output of this mixer 3 is introduced to set the center frequency of the band-pass filter. ₁
A device for displaying the frequency spectrum waveform of an input signal on a CRT, comprising: a first BPF amplifier 9 whose bandpass filter has a center frequency different from the first _one ( ₁ + Δ); 51, a first BPF amplifier 9 and a second BPF amplifier 51
When the second BPF amplifier 51 is selected, the means 57, 58 for switching the
means 54 for shifting the output frequency of the VCO 5 by (Δ); a first BPF amplifier 9 and a second BPF amplifier 51;
A spectrum analyzer comprising means 55 and 56 for selecting a small signal level from among the frequency spectrum signals obtained through the CRT and adding it to a CRT.