JPH10160773A - Fm signal measuring apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an FM signal measuring apparatus which can be built easily in a digital modulation system measuring apparatus and exhibiting highly accurate demodulation characteristics without requiring any troublesome adjusting work. SOLUTION: An FM modulated signal S(t) to be measured is converted through an A/D converter 23 into a digital data which is then fed to a quadrature demodulator 24 generating base band components I(t), Q(t) having orthogonal phase through frequency conversion. Subsequently, an FM demodulator 26 calculates the phase ϕ of the signal to be measured by performing arctangent conversion according to a formula ϕ(t)=tan<-1> [Q(t)/I(t)] and demodulates the modulated signal by performing differentiation for that phase ϕ. A second frequency measuring means 32 determines the difference Δf between the carrier frequency of the signal to be measured and the frequency fc of local oscillation signal of the quadrature demodulator from the DC component of demodulate output. The carrier frequency of the signal to be measured is determined by adding the frequency f0 of a signal from the quadrature demodulator 24 and the frequency fc of a local oscillation signal from a local signal generating section 21.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM signal measuring apparatus for performing various measurements on a signal whose frequency or phase is modulated by an analog modulation signal.

[0002]

2. Description of the Related Art In recent years, mobile radios that perform communication using frequency modulation or phase modulation (hereinafter, referred to as FM modulation) have shifted to digital FM modulation.
In some countries and regions, analog FM modulation, which directly modulates a carrier signal with an analog modulation signal, is used in a large proportion, and it is expected that both modulation methods will be used together in the future.

On the other hand, as an FM signal measuring device for measuring this type of radio, a device dedicated for the modulation system is used, and it is necessary to measure both types of radio. In some cases, there is the inconvenience of preparing two types of measuring devices in advance and replacing the measuring devices according to the modulation method of the wireless device.

For this purpose, it is conceivable to add a measuring function of an analog modulation type radio to a measuring device for measuring a digital modulation type radio which is expected to increase in the future.

[0005]

However, in a measuring device for measuring a radio device of the analog modulation system, an analog type radio device is required to obtain a very high linearity capable of detecting a slight distortion of the modulation characteristic of the radio device. At present, a great deal of time is spent adjusting demodulators, and simply incorporating such a demodulator in a digital modulation type measurement device would complicate the configuration of the measurement device and increase its size. , The cost is high.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an FM signal measuring apparatus which solves this problem and which can obtain a highly accurate demodulation characteristic without troublesome adjustment work and which can be easily incorporated into a digital modulation type measuring apparatus. And

[0007]

To achieve the above object, an FM signal measuring apparatus according to a first aspect of the present invention converts a signal under test whose frequency or phase is modulated by an analog modulated signal into a digital signal. Output A / D converter (2
3) and the signal output from the A / D converter is
A quadrature which is frequency-converted into baseband components I (t) and Q (t) whose phases are orthogonal to each other by a local oscillation signal having a frequency f _C substantially equal to the carrier frequency of the signal under test input to the / D converter, and output. Demodulator (24) and baseband components I (t) and Q (t) output from the quadrature demodulator
The following equation φ (t) = tan ^-1 [Q (t) / I (t)] is subjected to inverse tangent conversion to calculate the phase φ of the signal under measurement, and the differential with respect to the calculated phase φ An FM demodulator (26) for performing an operation to demodulate the modulated signal, wherein the signal to be measured is measured based on the output of the quadrature demodulator and the output of the FM demodulator.

According to a second aspect of the present invention, there is provided an FM signal measuring apparatus according to the first aspect.
DC extraction means (32a, 32b) for extracting a DC component from the output of the FM demodulator; and a carrier frequency of the signal under measurement and a local oscillation signal of a quadrature demodulator based on the DC component extracted by the DC extraction means. Difference from the frequency f _C of
The A / D is calculated from the frequency deviation calculating means (32c) for calculating f, the frequency deviation calculated by the frequency deviation calculating means, and the frequency f _C of the local oscillation signal of the quadrature demodulator.
Calculating means (32d) for calculating the carrier frequency of the signal under measurement input to the converter.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a configuration of an FM signal measurement device 20 according to one embodiment.

In FIG. 1, an input terminal 20a of an FM signal measuring device 20 has a signal under test S expressed by the following equation.
(T) is input from a wireless device or the like to be measured. S (t) = A cos [2πft + 2π∫M (t) dt]

Here, A is the amplitude of the signal under test S (t),
f is the carrier frequency of the signal under test S (t), M (t) is the modulation signal, the frequency of the modulation signal is f _M , the maximum frequency shift (amplitude) is f _FM , and the noise and distortion components are ε.
If (t) is used, it is expressed by the following equation. M (t) = f _FM · cos (2πf _M t) + ε (t)

On the other hand, the local signal generator 21 is, for example, a direct digital synthesizer (DDS) or a PLL.
A local oscillation signal L ₀ having high frequency stability (referred to as frequency f ₀ ) is output to the frequency conversion unit 22 using a circuit. The frequency f ₀ of the local oscillation signal L ₀ output from the local signal generation unit 21 is converted to a lower intermediate frequency band having a predetermined frequency f _IF as a center frequency. The carrier frequency f and the frequency f of the measurement signal S (t)
It is set to be almost equal to the difference from _IF .

The frequency converter 22 comprises a mixer 22a and a band-pass filter 22b having the frequency f _IF as a center frequency. The signal S (t) to be measured and the local signal L ₀ of the local signal generator 21 are combined with each other. Are mixed (integrated) by the mixer 22a, and the carrier frequency of the mixed component is (f)
_0- f), a component substantially equal to the pass center frequency f _IF of the band-pass filter 22b, that is, a signal under measurement S ′ (t) expressed by the following equation is output. S ′ (t) = A cos [2πf′t + 2π∫M (t) d
t] where the amplitude of the local oscillation signal L ₀ is 1, and f ′ is the signal under measurement S
It is equal to (f ₀ −f) at the carrier frequency of (t).

The signal under test S '(t) output from the frequency converter 22 is input to an A / D converter 23 and converted into a digital signal. The A / D converter 23 outputs the signal under measurement S ′ at a predetermined sampling period Δt (= 1 / f _SP ).
(T) is sampled, and data corresponding to each sampling value is sequentially output to the quadrature demodulator 24.

The quadrature demodulator 24 includes a memory 25 described later,
It is constituted by a microprocessor such as a digital signal processor (DSP) together with the FM demodulator 26 and the measuring unit 30. As shown in FIG.
A local oscillator circuit 24a for outputting a local oscillation signal L _C of the frequency f _C that matches the _IF, the phase shifter 24b outputs the local oscillation signal L local oscillation signal is delayed 90 ° the phase of _C L _C ', A mixer 24c for mixing (integrating) the local oscillation signal L _C and the signal under measurement S ′ (t), and a component having a lower frequency (baseband component) among the mixed components of the mixer 24c is converted into the signal under measurement S (t )
, A low-pass filter 24d that outputs the real part (I component) of the phase modulation component, a local signal L _C ′ output from the phase shifter 24b, and the signal under measurement S ′ (t) are mixed (integrated). And a low-pass filter 24f that outputs the lower frequency component (baseband component) of the mixed output of the mixer 24e as the imaginary part (Q component) of the phase modulation component included in the signal under measurement S (t). And

That is, the local oscillation signal L _C of the quadrature demodulator 24 is c
os (2πf _C t) and the local oscillation signal L _C ′ is sin (2πf _C t).
_C t), the local oscillation signal L _C and the signal under measurement S ′ (t)
Product of the, S '(t) · cos (2πf C t) = A cos [4πf _C t
+ Φ (t)] + A cos [φ (t)], and the baseband component I (t) output from the low-pass filter 24d is I (t) = A cos [φ (t)]. Here, φ (t) = 2πΔft + 2π∫M (t) dt, Δf =
f′−f _C.

Similarly, the local oscillation signal L _C ′ and the signal under measurement S ′
The product of the (t) is, S '(t) · sin (2πf C t) = A sin [4πf _C t
+ Φ (t)] + A sin [φ (t)], and the baseband component Q (t) output from the low-pass filter 24f is Q (t) = A sin [φ (t)]. Here, if n is an integer, (Δf + nf
_It is necessary to satisfy the condition that the absolute value of _M ) is sufficiently smaller than the frequency f _C. n is the modulation index β (= f _FM /
f _M ), if β ≦ 1, then n ≦ 4. FIG. 3 shows the spectrum of the signal under measurement S (t) when β = 1 and f _M = 1 kHz.

The signal to be measured S 'thus obtained
(T) two orthogonal baseband components I (t), Q
(T) are sequentially stored in the memory 25.

The FM demodulator 26 has an inverse tangent conversion means 26a for obtaining the phase from the baseband components I (t) and Q (t) stored in the memory 25, and an inverse tangent conversion means 26a
And a correction means 26c for correcting the displacement of the phase obtained by the differential calculation means 26b.

The inverse tangent converter 26a obtains the phase φ '(t) by the operation of the following equation (1). φ ′ (t) = tan ⁻¹ [Q (t) / I (t)] (1)

Further, the differential operation means 26b obtains the displacement of the phase by the differential operation of the following equation (2) with respect to the phase obtained by the arctangent conversion means 26a. dφ ′ (t) / dt = φ ′ (t + Δt) −φ ′ (t) (2)

As shown in FIG. 4, the value of φ '(t) can be obtained only in the range of -π to π even if the signs of Q (t) and I (t) are considered. , Φ are not limited to the range of, for example, -π to π, the solution of the calculation by the above equation (1) cannot be specified.

However, when the range of the solution of equation (1) is limited to the range from -π to π, for example, the phase increases from φa by Δφ and exceeds π as shown in FIG. In this case, the calculation result of Expression (1) does not become φa + Δφ, but becomes −π + Δφ− (π−φa) = φa + Δφ−2π, and the calculation of Expression (2) is performed using the calculation result. Then, a difference of 2π occurs between the actual phase displacement Δφ and the calculation result.

The correcting means 26c compares the operation result of the equation (2) with -π or π to correct the difference of ± 2π based on the comparison result, and corrects the true phase φ. The displacement of (t) is obtained. Here, the sampling cycle of the A / D converter 23 is set in advance so that the actual phase φ (t) does not change by 2π or more during Δt.

Hereinafter, based on the flowchart of FIG.
The operation of the M demodulator 26 will be described in detail. Note that the memory 2
The 5, the quadrature demodulator _{24 Δt (= 1 / f SP} ) baseband component at time intervals [I (0), Q (0 ) ], [I
(1), Q (1)], ..., [I (N-1), Q (N-
1)] is stored.

The FM demodulator 26 stores, for example, N
When the set of baseband components is stored, n = 0, Δ
Initially, φ ₀ = 0 is set, the baseband components [I (n), Q (n)] are read out from the memory 25, and the inverse tangent transformation of the equation (1) is performed to obtain the phase φ ₀ thereof. , The baseband components [I (n + 1), Q (n + 1)] are read and subjected to arctangent transformation to determine the phase φ, and the phase displacement Δ
The phi determined by calculating the φ-φ ₀ (S1~S4).

Then, the displacement Δφ is compared with ± π, and the displacement Δ
When φ is within the range of ± π, Δφ is updated as a result of adding Δφ ₀ to Δφ, and φ ₀ is updated with the value of φ (S5, S6).

When the displacement Δφ is equal to or larger than π, Δφ ₀
Is decreased and updated by 2π, and when the displacement Δφ is equal to or less than −π, Δφ ₀ is increased and updated by 2π and the process proceeds to processing S6 (S6
7, S8).

Then, Δφ updated in step S6 is output as a demodulated output, n is incremented and updated by 1, and the operation of obtaining the next phase displacement is performed until n becomes equal to N (S9, S10).

By this series of processing, the FM demodulator 26
From the phase change Δφ of the signal under measurement S every Δt time,
That is, dφ (t) / dt is output as a demodulated signal. Also, as described above, φ (t) and M (t)
Is because it is φ (t) = 2πΔft + 2π∫M (t) dt M (t) = f FM · cos (2πf M t) + ε (t), the demodulated output of the FM demodulator 26, a right side of the formula Are equivalent. dφ (t) / dt = 2π [Δf + f _FM · cos (2πf _M
t) + ε (t)]

That is, the demodulated output of the FM demodulator 26 includes:
A DC component (2πΔf) proportional to the frequency deviation Δf and a component [2πf _FM · cos (2πf) of the modulation signal M (t)
And _M t)], are included and the noise component [2πε (t)].

The demodulated output of the FM demodulator 26 is sent to the measuring section 30. The measuring unit 30 includes a first frequency measuring unit 3
First, second frequency measuring means 32 and amplitude measuring means 33 are provided.

The first frequency measuring means 31 obtains the frequency f _M of the modulation signal M (t) by counting the AC component of the demodulated output of the FM demodulator 26 for a reference time.

The second frequency measuring means 32 removes the modulated signal component from the demodulated output of the FM demodulator 26 by the low-pass filter 32a, and further removes the noise component by the averaging means 32b, and is included in the demodulated output. Only the DC component 2πΔf is extracted, and the frequency component is divided by 2π by the frequency deviation calculating means 32c and further multiplied by a coefficient determined by the sampling period Δt of the A / D converter 23 to obtain a frequency deviation Δf. The frequency deviation Δf, the frequency f _{0 of} the local signal L ₀ of the local signal generator 21, and the frequency f _{C of} the local signal L _C of the quadrature demodulator 24
Are added by the adding means 32d to obtain the carrier frequency f (= f ₀ + f _C + Δf) of the signal under measurement S. Note that the frequency data of the local oscillation signals L ₀ and L _C are output from the local signal generator 21 and the quadrature demodulator 24.

As described above, in the FM signal measuring device 20, since the carrier frequency of the signal under test S is obtained based on the DC component included in the demodulated output of the FM demodulator 26, the signal under test S '(t ) And the frequency f _C of the local oscillation signal L _C of the quadrature demodulator 24 do not completely match, the carrier frequency of the signal under test S (t) can be accurately obtained. Also, the signal under measurement S (t),
Since the method is not a method of directly counting S '(t) by the counter, the frequency can be measured in a short time while the frequency is being modulated.

The amplitude measuring means 33 calculates the following equation Ac (t) = [I (t) ² + Q (t) ^{2 for the} baseband components I (t) and Q (t) sequentially output from the quadrature demodulator 24. The amplitude of the modulation signal M (t) is obtained by performing an operation of ^1/2 and averaging the results.

The measurement results of these measuring means are displayed on the display 35.

As described above, the FM signal measuring device 20
The A / D converter 23 converts a signal under test frequency-modulated by an analog modulation signal into a digital signal, and the base signal is converted by a quadrature demodulator 24 constituted by a microprocessor such as a digital signal processor (DSP). The band components I (t) and Q (t) are obtained, the base band component is subjected to inverse tangent transformation to obtain a phase, and the displacement per phase of the phase is obtained by a differential operation to demodulate the signal under measurement. This eliminates the need for complicated adjustments as in a conventional demodulator using an analog circuit, and provides highly linear demodulation characteristics. Moreover, since the demodulation and measurement of the analog-modulated signal to be measured can be performed by digital processing, it can be easily incorporated into a digital modulation measuring device.

[0039]

[Other Embodiments] In the above embodiment, the signal under measurement S
(T) is temporarily converted to an intermediate frequency band, and then A /
Although the signal to be measured is input to the D converter 23, the signal to be measured may be directly input to the A / D converter 23 when a signal to be measured having a low carrier frequency is measured. In this case, the frequency of the local oscillation signal L _C of the quadrature demodulator 24 can be changed according to the carrier frequency of the signal under measurement.

When the carrier frequency of the signal to be measured is high, a frequency converter and a local signal generator are further provided between the input terminal 20a and the frequency converter 22 to input the signal to the input terminal 20a. The signal to be measured may be converted into a frequency band that can be converted by the frequency conversion unit 22.

In the above embodiment, the frequency-modulated signal to be measured is measured. However, as in the FM signal measuring device 20 'shown in FIG. 6, an averaging process is performed from the output of the correcting means 26c of the FM demodulator 26. If the output (DC component) of the means 32b is subtracted by the subtracting means 40 and the subtracted output (AC component) is integrated by the integrating means 41, demodulation of the phase-modulated signal to be measured becomes possible. Can be measured by the first frequency measuring means 31, for example.

In this embodiment, the measuring apparatus for measuring the input signal to be measured has been described.
The present invention can be similarly applied to a measuring apparatus having a function of outputting a measurement signal to a device under measurement in addition to the function of measuring a signal.

[0043]

As described above, the FM signal measuring apparatus according to the present invention converts a signal under test, whose frequency or phase is modulated by an analog modulated signal, into a digital signal by an A / D converter, The baseband components I (t) and Q (t) are obtained by a demodulator, the inverse tangent conversion of the baseband components is performed by an FM demodulator to obtain a phase, and the signal to be measured is obtained by a differential operation of the phase. Since demodulation is performed, troublesome adjustment unlike a demodulator using a conventional analog circuit is not required, and a highly linear demodulation characteristic can be obtained. Moreover, since the demodulation and measurement of the analog-modulated signal to be measured can be performed by digital processing, it can be easily incorporated into a digital modulation measuring device.

[Brief description of the drawings]

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

FIG. 2 is a block diagram of a main part of one embodiment.

FIG. 3 is a diagram showing a spectrum of a signal under measurement.

FIG. 4 is a diagram for explaining an operation of a main part of the embodiment.

FIG. 5 is a flowchart illustrating a processing procedure of a main part of the embodiment;

FIG. 6 is a block diagram showing another embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 20 FM signal measuring device 21 Local signal generating unit 22 Frequency converting unit 23 A / D converter 24 Quadrature demodulator 25 Memory 26 FM demodulator 30 Measurement unit 31 First frequency measuring unit 32 Second frequency measuring unit 33 Amplitude measurement Means 35 Display

[Procedure amendment]

[Submission date] January 7, 1997

[Procedure amendment 1]

[Document name to be amended] Drawing

[Correction target item name] All figures

[Correction method] Change

[Correction contents]

FIG. 2

[Fig. 3]

[Fig. 4]

[Fig. 1]

[Fig. 5]

[Fig. 6]

Claims (2)

[Claims] An A / D converter (23) for converting a signal to be measured whose frequency or phase is modulated by an analog modulation signal into a digital signal and outputting the digital signal, and a signal output from the A / D converter Is frequency-converted into baseband components I (t) and Q (t) whose phases are orthogonal to each other by a local oscillation signal having a frequency f _C substantially equal to the carrier frequency of the signal under measurement input to the A / D converter. Demodulator (24) that outputs the baseband component I output from the quadrature demodulator
(T) and Q (t) are subjected to inverse tangent transformation of the following equation φ (t) = tan ^-1 [Q (t) / I (t)] to calculate the phase φ of the signal under measurement. An FM demodulator (26) for performing a differential operation on the calculated phase φ to demodulate the modulated signal, and based on an output of the quadrature demodulator and an output of the FM demodulator, An FM signal measurement device that performs measurements. 2. A DC extracting means (32a, 32b) for extracting a DC component from an output of the FM demodulator, and a carrier frequency and a quadrature demodulation of the signal under measurement based on the DC component extracted by the DC extracting means. a frequency deviation calculation means for calculating a difference Δf between the frequency f _C of the local oscillation signal of the vessel (32c), a frequency deviation obtained by the frequency deviation calculation unit, and the frequency f _C of the local oscillation signal of the quadrature demodulator 2. An FM signal measuring apparatus according to claim 1, further comprising: a calculating means (32d) for calculating a carrier frequency of the signal under measurement inputted to the A / D converter.