JPH05206965A - A/d converter for fdm signal - Google Patents

Abstract

PURPOSE:To simplify and miniaturize an A/D converter and to save power consumption. CONSTITUTION:This converter is provided with a first conversion system which frequency-converts an FDM signal into a base-band signal by a frequency mixer 2 and A/D converts it by an A/D conversion circuit 12 and a second conversion system which frequency-converts an FDM signal into a base-band signal orthogonally crossing the base-band of the first conversion system by a frequency mixer 3 and A/D converts it by an A/D conversion circuit 13. Besides, at the same time, pi/2 phase shifters 5 and 6 which respectively rotates the shifts of base-bands concerning the corresponding systems and adders 8 and 9 adding the outputs the pi/2 shape shifters 5, 6 and the base-band signals orthogonally crossing each other, which concerns the other systems are provided before the A/D conversion circuits 12 and 13 of the respective systems so as to set the sampling frequency of the A/D converter to be needed into a small value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an A / D converter for FDM signals, and in particular, it uses a simple A / D converter to
The present invention relates to an A / D conversion device for an FDM signal that constitutes an A / D conversion circuit.

[0002]

2. Description of the Related Art Conventionally, in the FDM signal A / D converter, the following method has been adopted in order to make all the output signals of the A / D converter into data on the real axis.

That is, when an N-channel FDM signal is shown on the frequency axis, it can be represented by a spectrum as shown in FIG. 5. In order to A / D convert this FDM signal, the maximum frequency (f _n It is necessary to perform sampling at a frequency having a value larger than twice the frequency (2 × f _n ).

This sampling frequency (f _S ≧ 2 × f _n
Is called the sampling frequency. ) Is the upper limit of A
Since the performance of the / D converter is limited, the FDM signal is usually subjected to A / D conversion after frequency conversion into the baseband band, and a circuit configuration is adopted in which the sampling frequency is as small as possible.

FIG. 7 shows a circuit configuration of a conventional FDM signal A / D converter, and FIG. 6 shows the number of channels being four.
2 shows the spectrum of the FDM signal under the assumption. In FIG. 7, reference numeral 7 is a local oscillator for frequency conversion, and its oscillation frequency is set to f _L1 shown in FIG. 2 is a frequency mixer, and the FDM signal is a frequency mixer 2
Then, the frequency is converted to a baseband band, and the signal is input to the A / D converter 12 through the reduction pass filter 10.

In the A / D converter 12, as described above, it is digitized at the sampling frequency of f _S (≧ 2 × f _n ) and output from the output terminal 14. Here, the purpose of the reduction pass filter 10 is to generate a folded spectrum at a frequency of m × f _S (m: 1, 2, 3, ...) In the baseband by the A / D conversion. Is inserted to prevent such unwanted waves from being combined.

[0007]

By the way, in the above-mentioned conventional A / D converter for FDM signals, the FD
Since the frequency of the local oscillator is selected as the minimum frequency of the M signal (f _{L1 in} FIG. 6), the sampling frequency (f _S )
Where N is the number of channels of the FDM signal and f _{0 is} the channel interval of the FDM signal, it is necessary to satisfy the condition of f _S ≧ 2 × N × f ₀ (1).

Therefore, if this system is adopted, it is necessary to increase the sampling frequency at the rate of the proportional constant (2 × f ₀ ) as the number of channels increases, and the circuit configuration of the A / D converter becomes complicated. It has the disadvantages of large size and high power requirements.

The present invention has been made in view of the above problems, and provides an A / D converter for an FDM signal in which the A / D converter can be simplified and downsized, and power consumption can be suppressed. With the goal.

[0010]

In order to achieve the above object, an FDM signal A / D converter according to the present invention frequency-converts an FDM signal into a baseband signal and performs A / D conversion by an A / D conversion circuit. The first conversion system and the FDM signal are frequency-converted into a baseband signal that is orthogonal to the baseband signal of the first conversion system, and the A / D conversion circuit performs A / D conversion.
A second conversion system for converting, and in front of the A / D conversion circuit of each system, a π / 2 phase shifter for rotating the phase of the baseband signal related to the system by 90 °, and this π The configuration is provided with an adder that adds the output of the / 2 phase shifter and the mutually orthogonal baseband signals according to another system.

[0011]

According to the FDM signal A / D converter having the above structure, the phase of one of the baseband signals orthogonal to each other is rotated by 90 ° and added to the other signal, so that the frequency conversion is performed. Since the frequency of the local signal is set to the center of the FDM signal, the sampling frequency of the required A / D converter can be set to a small value, and all signals output from the A / D converter are set. Data on the real axis is output, and digital processing of the latter stage of the A / D converter is easily performed.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing an embodiment of an A / D converter for FDM signals according to the present invention.

In the embodiment shown in the figure, one FDM signal distributed from the input terminal 1 via the power distributor 16 is A / D.
A first conversion system for converting and outputting to the output terminal 14 and a second conversion system for A / D converting the other FDM signal and outputting to the output terminal 15 are provided.

The first conversion system frequency-converts the FDM signal from the input terminal 1 into a baseband signal based on the output from the power divider 17 of the local oscillator 7 and the π / 2 phase shifter 4. Frequency mixer 2 and π / 2 phase shifter 5 for rotating the phase of the output of this frequency mixer 2 by 90 °
And an adder 8 for adding the output of the π / 2 phase shifter 5 and mutually orthogonal baseband signals of the second conversion system, which will be described later, and the output of the adder 8 via the reduction pass filter 10. And an A / D converter 12 for D / D conversion.

The second conversion system is based on the output of the local oscillator 7 through the power distributor 17, and the FDM signal from the input terminal 1 is orthogonal to the baseband signal of the first conversion system. The frequency mixer 3 for frequency-converting to a band signal and the phase of the output of this frequency mixer 3 are 90 °
The π / 2 phase shifter 6 for rotating, the adder 9 for adding the output of the π / 2 phase shifter 6 and the mutually orthogonal baseband signals of the first conversion system, and the output of the adder 9 A / D converter 13 that performs A / D conversion via the reduction pass filter 11
It has and.

Next, the operation of the A / D converter according to the embodiment will be described in detail. Now, it is assumed that the number of channels of the input FDM signal is N = 4 and that the signal represented by the spectrum of FIG. 6 is input. Further, in order to simplify the consideration given below, if the spectrum of each channel of the FDM signal is considered as a sine wave, the spectrum of the input signal can be represented by FIG. 2, and the signal of each channel can be represented by the following equation. ..

[0017]

[Equation 1]

The above spectrum is divided into two by the divider 16 and input to the frequency converters 2 and 3, respectively. Here, the oscillation frequency of the local oscillation signal input to the frequency converters 2 and 3 is selected as f _L2, and the signals after the two divisions are made orthogonal to each other by the phase shifter 4, and the frequency converter Two
Input to the local terminal of 3. The local signals of the frequency converters 2 and 3 in FIG. 1 can be expressed by the following equation.

[0019]

[Equation 2]

Therefore, the baseband signal after frequency conversion is expressed as follows. The output signal of the frequency converter 2 can be represented by the product of the equations (2) to (5) and the equation (6). That is, the signal of channel 1 is

[0021]

[Equation 3]

Here, the expression of the first term is a component of about twice the frequency of the local signal, and is removed by the reduction pass filter.

[0023]

[Equation 4]

Similarly, the signals of channel 2 to channel 4 are as follows.

[0025]

[Equation 5]

Next, the signal output from the frequency converter 3 can be expressed by the product of equations (2) to (5) and equation (7),
The channel 1 signal is

[0027]

[Equation 6]

Here, the expression of the first term is a component of about twice the frequency of the local signal, and is removed by the reduction pass filter.

[0029]

[Equation 7]

Similarly, the signals of channel 2 to channel 4 are as follows.

[0031]

[Equation 8]

Equations (8) to (11) and Equations (12) to
As can be seen from (15), the frequencies of the signals of channel 1 and channel 4 and channel 2 and channel 3 are equal, respectively. This is because by converting the spectrum signal of FIG. 2 by the local frequency f _L2 , the spectrum of the signal in the baseband band becomes a composite wave by superimposing the respective channels as shown in FIG. ..

Therefore, in this state, A / D conversion cannot be performed independently for each channel, and it is necessary to separate the signals of each channel.

As a method of separating the signals into the above channels, as shown in FIG. 1, one of the signals output from each frequency converter is rotated by π / 2 through π / 2 phase shifters 5 and 6, When added with the other signal, the output signals of the adders 8 and 9 are represented as follows.

The signal output from the adder 8 is expressed by the equations (8) to (1)
A signal obtained by rotating the signal of 1) by π / 2 phase and the equation (12)
Is the sum of the signals from (15),

[0036]

[Equation 9]

Similarly, the output signal of the adder 9 is the sum of the signals of the equations (8) to (11) and the signals obtained by rotating the signals of the equations (12) to (15) by π / 2,

[0038]

[Equation 10]

Here, in the induction process of the above equations (16) to (23), the amplitudes of the local oscillation signals input to the local terminals 2 and 3 of the frequency converter are equal (B ₁ = B ₂ =
B ₀ ).

As can be seen from the results of the equations (16) to (23), the output of the adders 8 and 9 is a composite wave due to the superposition of channel signals, as shown in FIGS. Not appearing, each channel signal is output independently on the frequency axis.

The above discussion has been made assuming that the spectrum of each channel of the FDM signal is a line spectrum (sine wave), but for a modulated wave signal having a certain frequency band such as FM, PM multi-level PSK, AM, etc. However, it can be considered as a set of the above-mentioned line spectra, and similar results can be easily obtained.

That is, in the circuit for rotating the phase of one of the mutually orthogonal baseband signals by 90 ° and adding it to the other signal, the frequency of the local signal for the frequency converter is set to the central frequency (f _L2 ) (However, if the number of channels N is odd, [center frequency] +
f _0/2, or, by setting [center frequency] -f _0/2), each channel baseband signal can be extracted independently on the real axis.

At this time, the output signal of each adder passes through the low pass filter and is input to the A / D converter. The spectrum of this input signal is as follows.

The spectrum of the input signal of the A / D converter 13 is UPPE of the local oscillation frequency f _L2 in the FDM input signal.
It is equal to the spectrum of R SIDE.

The spectrum of the input signal of the A / D converter 12 is the LOWER S of f _{L2 in} the FDM input signal.
It is equal to the folded spectrum of IDE. However, the band is limited by each low pass filter 10 and 11.

Therefore, the sampling frequency (f _S ) in each A / D converter is

[0047]

[Equation 11]

It suffices if the condition (1) is satisfied.

In the conventional example, as shown in equation (1),
Although it was necessary to increase the sampling frequency at the rate of the proportional constant (2 × f ₀ ) with the increase in the number of channels N, in the example of the present invention, as shown by the equation (24), The constant is f ₀ , which is half that of the conventional example. Therefore, it is possible to suppress an increase in the circuit configuration, shape, and power consumption of the A / D converter to be used, and since all signals output from the A / D converter are data on the real axis, digital processing in the subsequent stage It will be simple.

[0050]

As described above, according to the A / D converter of the FDM signal of the present invention, the rate of increase in the sampling frequency of the A / D converter with the increase in the number of channels is halved. It is possible to suppress an increase in the circuit configuration, shape, and power consumption of the D converter, and since all the data is on the real axis, the digital circuit in the subsequent stage is simple.

[Brief description of drawings]

FIG. 1 is a block diagram showing an A / D conversion device for an FDM signal according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a state of separating each channel signal when an FDM signal is considered as a line spectrum.

FIG. 3 is a diagram for explaining how to separate each channel signal when the FDM signal is considered as a line spectrum.

FIG. 4 is a diagram for explaining how to separate each channel signal when the FDM signal is considered as a line spectrum;
It is a figure which shows the output spectrum of an adder.

FIG. 5 is a diagram showing a spectrum of an FDM signal.

FIG. 6 is a diagram showing an example of a spectrum of an FDM signal.

FIG. 7 is a block diagram showing a conventional FDM signal A / D converter.

[Explanation of symbols]

 1 Input terminal 2,3 Frequency mixer 4,5,6 π / 2 Phase shifter 7 Local oscillator 8,9 Adder 10,11 Reduction pass filter 12,13 A / D converter 14,15 Output terminal 16,17 Power Distributor

Claims (1)

[Claims] 1. A first conversion system in which an FDM signal is frequency-converted into a baseband signal and A / D converted by an A / D conversion circuit, and an FDM signal is orthogonal to a baseband signal in the first conversion system. A second conversion system for performing frequency conversion into a baseband signal and A / D conversion by an A / D conversion circuit, and each of the baseband signals related to the system before the A / D conversion circuit of each system. The phase of 90
An FDM, which is provided with a π / 2 phase shifter that rotates by ° and an adder that adds the output of the π / 2 phase shifter and the mutually orthogonal baseband signals according to another system.
Signal A / D converter.