JPH052950B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a superheterodyne spectrum analyzer, and more particularly to improving the reliability of swept frequencies.

[Prior Art] FIG. 3 shows a circuit diagram of a conventional spectrum analyzer. In the figure, a ramp waveform is output from a sweep oscillator 7 and applied to a voltage controlled oscillator (hereinafter referred to as VCO) 5. Since a ramp waveform is applied to the VCO 5, it outputs a frequency _fV that changes depending on the applied voltage, and adds this to the mixer 3.

On the other hand, an input signal S _i (usually S _i includes a large number of frequency components) is applied to a 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. Let the frequency of the output signal of this low-pass filter 1 be _fL .

The mixer 3 performs mixing represented by f _M =f _V ±f _L , and applies the output signal (frequency _M ) to a band pass filter (hereinafter referred to as BPF) 4 .
The BPF 4 only passes frequencies around a certain frequency _f1 . Note that normally, this f ₁ is a frequency value included in the lower frequency (f _V −f _L ) among the frequencies output from the mixer 3. Amplifier 8 amplifies the signal from BPF 4.

Furthermore, in order to increase frequency selectivity and gain gain, the mixer 11, oscillator 13, BPF 15, and amplifier 16 are configured as a so-called double superheterodyne system.

The frequency resolution of the spectrum analyzer configured in this manner is determined by the filter bandwidth of the BPF 15. Therefore, the BPF 15 is configured such that the passband width can be changed so that the frequency resolution of the spectrum analyzer can be freely set.

The amplitude of the output of the amplifier 16 is detected by a detector 17, noise components are removed by a video filter 19, and the output is applied to the vertical axis of the CRT 20. CRT
A signal from the sweep oscillator 7 is applied to the horizontal axis 20, and waveform data as shown in FIG. 3 is displayed.

[Problems to be solved by the invention] By the way, the accuracy of the frequency swept by the spectrum analyzer configured as described above is
5 depends on the accuracy of the oscillation frequency.

However, in general, the relationship between the control voltage of the VCO 5 and the output frequency changes depending on temperature and the like, so it is difficult to obtain high frequency accuracy.

Therefore, if you want to obtain the desired frequency accuracy,
Although it is necessary to control the voltage value of the sawtooth liquid generated by the sweep oscillator 7 depending on conditions such as temperature, such control is complicated and not practical.

The present invention has focused on these points, and its purpose is to provide a spectrum analyzer that has a relatively simple configuration and can display frequencies with high accuracy.

[Means for Solving the Problems] In order to solve the above problems, the present invention provides a spectrum analyzer that performs frequency sweep using the output signal of a voltage controlled oscillator and displays the frequency spectrum of the input signal on the display unit. An oscillator that outputs a reference frequency f _p , a harmonic generator that outputs multiple harmonics m・f _p (m=1, 2,...) of this oscillator, and an output m・f of this harmonic generator.
a first mixer that mixes f _p and the output f of the voltage controlled oscillator; a pass band is narrower than the width of the reference frequency f _p from 0; m·f _p + f (= a bandpass filter that selectively passes the frequency component of the constant); a second mixer that mixes the output m·f _p +f of the bandpass filter with the output f of the voltage controlled oscillator; A local signal generator includes a low-pass filter added to the mixer that mixes the input signal by selectively passing only the harmonic components m and f _p of the output, and detects level changes in the output signal of the local signal generator. , a frequency axis signal generating section that generates a signal to be added to the frequency axis of the display section based on the number of changes thereof.

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

FIG. 1 is a block diagram showing an embodiment of a spectrum analyzer according to the present invention, and the same parts as in FIG. 3 are given the same reference numerals.

In FIG. 1, 21 is an oscillator that outputs a reference frequency f _p , and the output signal of this oscillator 21 is sent to a harmonic generator 22 that outputs multiple harmonics m·f _p (m=1, 2,...). has been added. As such an oscillator 21, for example, a crystal oscillator is used, and as the harmonic generator 22, for example, a step recovery diode is used. 23 is this harmonic generator 2
2's output m・f _p and the voltage controlled oscillator 5's output f
The first mixer mixes the signals, and its output signal is applied to the bandpass filter 24. The passband of this bandpass filter 24 is 0
is narrower than the width of the reference frequency f _p , and selectively passes a frequency component of m·f _p +f (=constant) out of the output of the first mixer 23 . 25 is a second mixer that mixes the output m· _p + of this band pass filter 24 and the output f of the voltage controlled oscillator 5, and the output signal of this second mixer 25 is applied to a low pass filter 26. . This low-pass filter 26 has a pass characteristic of selectively passing only the harmonic components m and _p of the output, and its output signal is sent to the mixer 3 that mixes the input signal Si.
has been added to. That is, the system from the sweep oscillator 7, the VCO 5, and the oscillator 21 to the low-pass filter 26 constitutes a local signal generating section A that adds the local oscillation frequency _fV to the mixer 3 that mixes the input signal Si.

27 is a detector that detects level changes in the output signal of the pandobus filter 24; for example, when the level of the output signal is maintained at zero level, it outputs L level, and changes as it crosses the zero level. In this state, it outputs H level. The output signal of the detector 27 is added to a counter 28, and the number of times the signal is at H level is counted. The counted value of this counter 28 is added to a DA converter 29 and converted into an analog signal, and the converted analog signal is added as a frequency axis signal to the horizontal axis of a CRT 20 used as a display section. That is, the system from the wave detector 27 to the DA converter 29 detects level changes in the output signal of the local signal generator A, and based on the number of changes, the CRT 2
A frequency axis signal generation unit B is configured to generate a signal to be added to the zero frequency axis.

The operation of the device configured as described above will be explained using the timing chart shown in FIG.

In FIG. 2, a indicates the output of the sweep oscillator 7, b indicates the output of the band pass filter 24, c indicates the output of the low pass filter 26, and d
indicates the output of the wave detector 27, and e indicates the output of the DA converter 29.
shows the output of

The sweep oscillator 7 generates a sawtooth wave as shown in a.
Output to VCO5. The output frequency f of the VCO 5 applied to one input terminal of the first mixer 23 and one input terminal of the second mixer 25 changes from, for example, 995 MHz to 1305 MHz in accordance with this sawtooth wave. The oscillator 21 outputs, for example, 10 MHz as the reference frequency f _p to the harmonic wave generator 22, and the harmonic generator 22 outputs 10 MHz, 20 MHz, ..., 510 MHz,
A large number of harmonics represented by 520MHz, . . . , 790MHz, 800MHz, 810MHz, . . . are applied to the other input terminal of the first mixer 23. Here, the passband of the bandpass filter 24 is set, for example, from 1809MHz to 1811MHz.
Then, the output of the bandpass filter 24 shown in b is from 510MHz+1299MHz to 510MHz+1301M of the frequency components of the sum of the outputs of the first mixer 23.
Hz, 800MHz + 1009MHz to 800MHz + 1010MHz,
It is a frequency component that is swept in the range from 1809MHz to 1811MHz, such as 810MHz + 999MHz to 810MHz + 1001MHz. Note that the first mixer 23
The frequency component of the difference between the outputs of is filtered by bandpass filter 2.
It is sufficiently lower than the passband of No. 4. The outputs of these bandpass filters 24 are applied to the other input terminal of a second mixer 25. low pass filter 2
For example, if the cutoff frequency of 6 is 810MHz, the frequency component shown in c output from this low-pass filter 26 to one input terminal of mixer 3 as the local oscillation frequency fv is 10M from 510MHz to 810MHz.
Each Hz.

Then, as the input signal Si, for example, 0 to 300MHz
is added, the passband of the bandpass filter 4 is set from 509MHz to 511MHz, for example,
The output frequency of the oscillator 13 is set to 500MHz, and the passband of the bandpass filter 15 is set, for example, from 9.9MHz to 10.1MHz.

In this way, the harmonic m·f _p of the output frequency f _p of the oscillator 21 with high frequency accuracy is selected while sweeping the output frequency f of the VCO 5 according to the sawtooth wave shown in a, so that the local The accuracy of the oscillation frequency _fV becomes as high as the accuracy of the output frequency _fp of the oscillator 21.

On the other hand, the output level of the bandpass filter 24 shown in b is detected by the detector 27 as shown in d, and is binarized at a constant threshold level.
The counter 28 counts up at the falling edge of the binary detection output. Note that the count value of the counter 28 is cleared prior to starting the sweep. The count value of this counter 28 is D/A
The signal is applied to the converter 29, converted into an analog signal as shown in e, and applied to the horizontal axis of the CRT 20 as a frequency axis signal.

With this configuration, the accuracy of the horizontal axis (frequency axis) of the ORT 20 can be increased.

Furthermore, if the variation in the output frequency f of the VCO 5 is within the passband width of the bandpass filter 24, the variation can be compensated for, and the configuration of the VCO 5 can be simplified.

Note that the output frequency f _p of the oscillator 21 sets the display resolution, and even if the display resolution can be changed by arbitrarily changing it to multiple frequencies using a frequency divider or phase-locked loop, etc. good. However, in this case, it is necessary to adjust the passband width of the bandpass filter 24 or the output frequency f _p of the oscillator 21 so that the passband width of the bandpass filter 24 is narrower than the output frequency f _p of the oscillator 21. be.

Further, the detector 27 may be connected to the output terminal of the mixer 23 or the output terminal of the low-pass filter 26.

[Effects of the Invention] As described above, according to the present invention, a spectrum analyzer capable of highly accurate frequency display can be realized with a relatively simple configuration.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of the configuration of a spectrum analyzer according to the present invention, FIG. 2 is a timing chart for explaining the operation of FIG. 1, and FIG. 3 is a diagram showing an example of the configuration of a conventional spectrum analyzer. It is. 5... VCO (voltage controlled oscillator), 7... sweep oscillator, 20... CRT, 21... oscillator, 22...
...High frequency generator, 23, 25...Mixer, 24...
...Band pass filter, 26...Low pass filter, 27...Detector, 28...Counter, 29...
...DA converter.

Claims (1)

[Claims] 1. In a spectrum analyzer that performs frequency sweep using an output signal of a voltage controlled oscillator and displays the frequency spectrum of an input signal on a display unit, the oscillator outputs a reference frequency f _p and the multiple harmonics of this oscillator. A harmonic generator that outputs m・f _p (m=1, 2,...) and an output m・f of this harmonic generator.
a first mixer that mixes f _p and the output f of the voltage controlled oscillator; a pass band is narrower than the width of the reference frequency f _p from 0; m·f _p + f (= a bandpass filter that selectively passes the frequency component of the constant); a second mixer that mixes the output m·f _p +f of the bandpass filter with the output f of the voltage controlled oscillator; A local signal generator includes a low-pass filter added to the mixer that mixes the input signal by selectively passing only the harmonic components m and f _p of the output, and detects level changes in the output signal of the local signal generator. , a frequency axis signal generation section that generates a signal to be added to the frequency axis of the display section based on the number of changes thereof.