JPH0727797A - Automatic correction circuit of tracking error of spectrum analyzer - Google Patents

Abstract

PURPOSE:To easily perform a correction operation, to eliminate a tracking error clue to a change in ambient temperature and to perform a stable measurement by a method wherein an oscillator is swept, a calibrating operation is performed and the center frequency of an IF filter is found by a control part. CONSTITUTION:The automatic correction circuit is constituted of a synthesizer 13, a variable oscillator 21, mixers 12, 15, 20, a fixed oscillator 17, a switch 14, an attenuator 16, a BPF 16 and a control part 23. Then, in a calibrating operation to find a center frequency, the switch 14 is changed over to the side of a tracking generator (TG) in advance, the oscillator 21 is swept, and the level of an output 34 at every sweep frequency is read out. As a result, the frequency of a maximum output level point is found. The frequency is found as an IF center frequency at a present ambient temperature and in a present apparatus state. The center frequency which has been found is set in the oscillator 21, the switch 14 is returned to the side of a measurement, and the calibrating operation is completed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention, in a spectrum analyzer having a built-in tracking generator (hereinafter referred to as TG), monitors the deviation between the TG frequency on the transmitting side and the frequency on the receiving side to analyze a predetermined range. If it does not, the oscillation frequency on the TG side is finely adjusted. This relates to a circuit that prevents a measurement error due to a tracking error.

[0002]

2. Description of the Related Art FIG. 3 is a block diagram showing the configuration of an embodiment of a conventional spectrum analyzer. The tracking error means that the center frequency (f8) of the BPF 16 that is a narrow band pass filter and the frequency (f7) that is input to the BPF by mixing the actually received signal are matched due to changes in ambient temperature and the like. This is a phenomenon that disappears and causes a decrease in the received signal level. This tracking error becomes more remarkable as the selection of the measurement bandwidth becomes sharper. Occurrence of a measurement error due to this causes a deterioration in the performance of the measuring instrument, which is an undesirable phenomenon.

Generally, in a broad band range (for example, 10 KHz), the output level does not decrease and it does not matter. On the contrary, in a sharp band width range (for example, a band width of 200 Hz), the center frequency of the BPF falls outside the band width range due to a change in ambient temperature. As a result, the output level decreases. To eliminate this, a means of adjusting it as needed is needed.

A conventional method of correcting a tracking error will be described. First, the user connects the TG output 31 and the reception input 32 with the external cable 33 (semi-rigid cable or the like). Next, press the switch on the spectrum analyzer to execute the calibration function. Then, the spectrum analyzer starts calibration for tracking error correction.

The frequency of the synthesizer 17 (variable voltage oscillator and signal distribution circuit) has no relation to the calibration and is set to an arbitrary frequency. The f2 oscillation frequency of the oscillator 21 is, for example, 4 GHz, and a minute frequency (for example, ± 5 KHz) can be varied. On the other hand, the oscillator 17 is a fixed frequency oscillator.

The output frequency f3 on the TG side applies the signal of the synthesizer 17 common to the receiving side to one input of the mixer 20, and inputs the signal of the oscillator 21 to the other input of the mixer 20 to mix (f3 = F1-f2) and supplies the output to the next output unit 19. Here, the input signal is given a predetermined output level by a low-pass filter, a buffer amplifier and an attenuator in the output section, and then output as a TG output 31. Then, this output signal is given to the input 32 on the receiving side through the external cable 33.

Next, on the receiving side, the received input signal passes through the attenuator of the attenuator section 11 and then the mixer 12
Supply to one input. To the other input of the mixer 12, the signal of the synthesizer 17 common to the TG side is input, and after being mixed by the mixer 12, the output frequency f5
Is passed through a low-pass filter and then supplied to one input of the next mixer 15.

The output frequency signal f6 of the fixed oscillator 17 is input to the other input of the mixer 15, and after mixing by the mixer 15, the output frequency f7 is supplied to the input of the BPF 16 of the bandpass filter. This BPF has second and third mixers inside and lowers to a required frequency. Then, after passing with a predetermined bandwidth by a narrow band BPF (for example, a crystal filter) or an LC tuning filter, a signal output 34 is output. Then, the signal level is digitally processed by an AD converter to obtain spectrum data of each frequency.

Since the oscillators 21 and 17 are PLL type oscillators, the frequencies do not shift. The synthesizer 17 is a variable voltage oscillator,
Since it is supplied to both the G side and the receiving side, the output frequency after mixing by the mixer 12 is canceled out and is always a constant frequency (f5 = f2) and has no effect. As a result, the output frequency f7 of the mixer 15 is always a constant frequency regardless of the sweep frequency of the synthesizer.

However, the center frequency of the narrow band BPF used in the BPF 16 has a temperature characteristic. The value varies about ± 3 to 10 PPM / ° C. For example, at f8 = 3.58MHz, the ambient temperature is 5
When the temperature changes by 0 ° C., it also changes by ± 1700 Hz. The influence of this frequency fluctuation value is to set the bandwidth setting of the filter narrow (for example, 200 Hz bandwidth setting).
Then, it easily goes out of the band width range, and the output level is remarkably reduced. As a result, a large error occurs, which is a measurement problem.

Due to the above-mentioned problems, in the past,
I am doing the following. First, when the user needs the calibration, the calibration function is manually executed after connecting with the external cable 33 as shown in FIG. This means controls the oscillation frequency of the oscillator 21 by the CPU so as to match it with the center frequency of the BPF 16. That is, the TG side output frequency is finely adjusted.

The second is a means for correcting the deviation of the center frequency of the BPF 16. This means is disclosed in JP-A-57-166.
It is a “tracking error correction circuit” filed in 566. This means is realized by providing a varicap diode in series with the crystal filter and changing the center frequency of the crystal filter. That is, the IF frequency on the receiving side is finely adjusted.

[0013]

As described above, when the band width setting of the filter of the BPF is set to be narrow and used for measurement, the T
A frequency shift occurs between the G output frequency and the IF side tuning frequency (IF frequency). Because of so-called tracking error, there is a problem that the output level is lowered and a measurement error occurs.

Therefore, the problem to be solved by the present invention is to provide a calibration circuit, find the center frequency on the IF side during calibration, and set the oscillator on the TG side to match the found center frequency. And This aims to provide a stable measurement at all times.

[0015]

In order to solve the above problems, the present invention provides a synthesizer 13 and a variable oscillator 21.
And mixers 12, 15 and 20, a fixed oscillator 17, a switch 14, an attenuator 18, a BPF 16, and a controller 2.
It is composed of 3 and. Then, the IF center frequency is obtained by performing calibration at any time or by a change over a certain temperature. By setting this value in the oscillator 21, the tracking error is eliminated.

In a spectrum analyzer having a built-in tracking generator, the TG section 22 is provided with an oscillator 21 having the same frequency as the IF frequency. Then, a switch 14 for switching between measurement and calibration is provided. This switch is connected to the external reception signal and the oscillator 2 concerned.
This is a switch for selecting either one of 1.

An attenuator 18 that attenuates and supplies the signal level is provided between the output of the oscillator 21 and the switch. The reason for this is that the oscillator 2 caused by insufficient switch isolation when measuring the original received signal.
This is to prevent the influence of residual spurious due to the signal leakage of 1.

First, when performing the calibration, the switch is switched to the TG side. And the oscillator 21
Output level 3 of BPF16 for each frequency
Read 4. As a result, the frequency value at the maximum output level position is known. This is obtained as the IF center frequency. Then, this frequency value is set in the oscillator 21. Finally, the switch is switched to the measurement side and used for the actual measurement. The CPU of the control unit 23 controls these series of operations.

Regarding the connection relationship, the oscillator 2 that can be finely adjusted
The output signal of 1 is supplied to one input of the mixer 20, and is also supplied to the input terminal of the attenuator 18. Synthesizer 1
The output signal of No. 3 is applied to the other of the mixers 20 and one of the mixers 12. The output signal of the mixer 20 is supplied to the input of the output section 19, and the output signal of the output section 19 is supplied to an external device under test (DUT) as a TG output signal.

The received input signal input from the DUT is supplied to the other input of the mixer 12 via the attenuator 11. The output signal of the mixer 12 is connected to one input of the switch 14, and the output signal of the attenuator 18 is connected to the other input of the switch. And
The output signal of the switch is supplied to one input of the mixer 15. The output signal of the oscillator 17 is
5 to the other input. Then, the output signal of the mixer 15 is supplied to the input of the BPF 16. Then, the output of the BPF is output for measuring the output level.

It is a tracking error automatic correction means for a spectrum analyzer with a built-in tracking generator, which has the above components.

[0022]

In the oscillator 21, the center frequency of the crystal filter in the BPF 16 may be deviated due to component variations,
Alternatively, it has a function of finely adjusting the oscillation frequency in order to be able to correct the deviation due to the change in ambient temperature. Further, by sweeping around the IF frequency at the time of executing the calibration, it also has a function to be used when obtaining the center frequency.

The attenuator 18 suppresses the generation of residual spurious. That is, since the frequency of the oscillator 21 is the same as the IF frequency, the switch 14 has insufficient isolation due to the stray capacitance. As a result, the signal of the oscillator 21 leaks to the mixer 15, and it is measured as if it were a received signal.
The attenuator 18 is inserted to eliminate this. In other words, by suppressing the signal level to the minimum necessary level, there is an effect of preventing erroneous measurement due to leakage.

The switch 14 has a function of switching to the external reception signal side during normal measurement, and switches to the signal of the oscillator 21 during calibration to switch to the mixer 1.
It has the function of supplying 5.

As a whole, the calibration for measuring the deviation of the tracking error and the presence or absence thereof can be performed at any time to easily perform the correction. As a result, it is possible to always measure the received signal level stably, and it has a function of eliminating the occurrence of a measurement error.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to the configuration block diagram of one embodiment of the spectrum analyzer of the present invention shown in FIG.

In summary, during calibration, the external reception signal is cut off and the oscillator 2 is used instead.
The output signal of No. 1 is directly applied to the IF side via the switch 14. After that, the oscillator 21 is swept to obtain the frequency at which the output signal 34 becomes maximum. The frequency of the maximum output level obtained as described above is the obtained center frequency. After that, the oscillator 21 is set to this frequency and used for the subsequent measurement. Here, as the TG output frequency, a frequency obtained by subtracting the sweep frequency of the synthesizer 13 and the oscillator 21 is the TG output frequency. For this reason, the frequency value of the difference of the oscillator 21 from the previous time is offset-added to the synthesizer. As a result, the same sweep range as before calibration can be swept, and the original measurement is not hindered.

The conditions for executing the calibration include automatic execution and manual execution. The first is a method of providing an in-machine temperature sensor and executing it when there is a change above a certain temperature. The second is a method of executing it at regular intervals. This interval time can be set arbitrarily. Third
Is a method of manually executing the calibration when the user gives an instruction to execute the calibration. The fourth is a method of executing calibration by utilizing the free time after sampling the received signal data. The fifth method is selection of a combination of the above execution methods. That is, they may be executed alone or in combination of a plurality of them.

The execution condition may be increased as the bandwidth selectivity of the spectrum analyzer is sharply selected. This means that, for example, when the bandwidth setting is 5 KHz or more, calibration is not necessary, and when the bandwidth setting is 50 Hz, the calibration is performed every ± 1 ° C. This combination condition may be provided in advance in a non-volatile memory or the like so that the user can arbitrarily change it.

For example, in the case of the temperature condition, a change (for example, ±
3 ° C), the execution is started. Here, an example of the temperature sensor is shown as the calibration condition, but as described above, the temperature sensor may be executed at regular time intervals, or may be executed by using the idle time of measurement. good.

An example of calibration will be described.
First, when the calibration is started, the switch 14 is switched. The external reception input 41 is cut off, and the output signal 42 of the oscillator 21 is switched to the switch 14 via the attenuator 18.
Of the switch switching output 43 to the input of the mixer 15.

Here, the attenuator is, for example, 80d.
Provide an attenuation amount of b or more. The reason is that the frequency of the oscillator 21 is the same as the IF frequency, so that the measurement of the original externally received signal (particularly a weak signal) is an obstacle to spurious effects. For this,
The signal of the oscillator 21 needs to have no influence at the time of measurement, so the attenuator 18 is placed here.

At a high frequency of about 4 GHz, the switch has a problem that isolation cannot be sufficiently obtained. For example, as shown in FIG. 2, even when the changeover switch is configured with two stages of 14A and 14B, only isolation of about 30 to 40 db can be obtained. This is because the stray capacitance between the contacts of the switch 14 prevents isolation. For this reason, the signal level at the input 42 of the switch is supplied as low as possible. In short, at the time of calibration, only the center frequency needs to be obtained.

Next, a calibration procedure for obtaining the center frequency will be shown. The switch is switched to the TG side in advance. Then, the oscillator 21 is swept to read the level of the output 34 for each sweep frequency. As a result, the frequency at the maximum output level point is obtained. Then, it is determined that this frequency is the IF center frequency under the current ambient temperature and device state. After setting the obtained center frequency in the oscillator 21, the switch is returned to the measurement side to complete the calibration.

As described above, it goes without saying that the difference value between the previous time and the present time of the oscillator 21 is offset-added to the synthesizer 13. The reason for this is TG
This is because the output frequency sweep range needs to be swept in the same range as before calibration. And
The above series of calibration operations is performed by the control unit C
The PU controls the execution.

Here, the oscillator 21 (PLL oscillator)
Has a sweep frequency width of, for example, 4 GHz ± 5 KHz. The sweep width is set so that the sweep range can be swept in consideration of the temperature characteristic of the crystal filter that mainly determines the IF center frequency and the variation range of the natural frequency of the crystal filter component.

When a temperature sensor is provided, the following method can be adopted. That is, the variation data of the center frequency obtained for each temperature is stored in advance in the non-volatile memory. Then, at the time of use, setting data of the center frequency corresponding to each temperature is read from the memory and set in the oscillator 21. According to this method, T
It becomes unnecessary for the G output frequency to cause a frequency shift each time calibration is performed or to suspend output. As a result, it is possible to eliminate the condition that hinders the spectrum measurement of the user.

Another method is the following calibration method. That is, the TG output frequency is f3 =
Since it is f1−f2, the frequency Δf changed when the oscillator 21 is calibrated is added to the sweep ramp voltage on the synthesizer side. As a result, the calibration can be performed at the same time without changing the TG output frequency. In the case of this method, the spectrum of the received signal as a spectrum analyzer cannot be measured, but the calibration can be performed at any time without affecting the TG output frequency to the outside.

The switch 14 includes the mixer 12 and the mixer 15.
I put it in between. As another method, a method in which a part of the output of the TG output 31 is provided to the receiving side input 32 by providing the switch is conceivable, but this frequency requires a switch having a wide frequency range and a wide band. Not a good idea.

[0040]

Since the present invention is configured as described above, it has the following effects.

It has become possible to easily perform correction by performing calibration for measuring the deviation of the tracking error and the presence or absence thereof at any time. As a result, even if there is a change in ambient temperature (or a change over time in the components), the tracking error can be eliminated by executing the calibration. Therefore, stable spectrum measurement of the received signal level can be performed at all times, and it is possible to provide good measurement with the effect of eliminating the occurrence of measurement error.

Further, the warm-up time after power-on was conventionally required to be about 30 minutes, but the measurement accuracy of the spectrum measurement does not deteriorate even if the measurement is started after shortening the warm-up time. Also, since the IF center frequency can be corrected by calibration, B
It is not necessary to use an expensive component having good temperature characteristics as the crystal filter component used in the PF, and an inexpensive component can be used, which enables cost reduction.

[0043]

[Brief description of drawings]

FIG. 1 is a configuration block diagram of an embodiment of a spectrum analyzer of the present invention.

FIG. 2 is a diagram showing an example of an internal circuit configuration of a switch of the present invention.

FIG. 3 is a configuration block diagram of an embodiment of a conventional spectrum analyzer.

[Explanation of symbols]

 11 Attenuator section 12, 15, 20 Mixer 13 Synthesizer 14 Switch 16 BPF 17, 21 Oscillator 18 Attenuator 19 Output section 22 TG section 23 Control section

Claims (1)

[Claims] 1. A spectrum analyzer with a built-in tracking generator, wherein a variable oscillator (21) capable of sweeping an IF frequency is provided in a TG section, and an external reception signal and a corresponding signal are used as a switch for switching between measurement and calibration. It has a switch (14) for selecting one of the oscillators (21), an attenuator (18) for attenuating the output of the oscillator (21) and supplying it to the switch, and sweeping the oscillator (21). A tracking error automatic correction circuit for a spectrum analyzer, which has a control unit (23) for performing the calibration to obtain the center frequency of the IF filter and is equipped with the above.