JPH02142248A - Dc off-set correcting system - Google Patents

Abstract

PURPOSE:To faithfully reproduce a base band signal by obtaining frequency difference between an input central frequency and a local oscillating frequency when the base band signal is reproduced, removing the DC component of a detector output, after that, correcting this frequency difference by digital signal processing and taking out the prescribed base band signal. CONSTITUTION:A local oscillating part 3 is provided to output an oscillating frequency signal to be separated from the central frequency of an input signal, which has a fixed band, only by a prescribed frequency. Then, a detector 2 is provided to mix the output of this local oscillating part 2 and the input signal. A capacitor 4 is provided to remove DC off-set from the output of the detector 2 and a digital signal processing part 5 is provided to correct the frequency difference between the central frequency of the input signal and the output of the local oscillating part 3 to the output of this capacitor 5 and to take out the prescribed base band signal. In the digital signal processing part 5, at first, the sample hold of an input base band signal (d) is executed by a sampling hold circuit 6 and digital conversion is executed by an analog/ digital converter 7. Thus, the base band signal can be faithfully reproduced.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a DC offset correction method, and in particular, for a signal whose baseband signal band is defined by DC, the baseband signal is corrected by a detector using a synchronous detection method or the like. The present invention relates to a DC offset correction method using digital signal processing to remove the DC offset of a detector that occurs when reproducing a waveform.

[Conventional technology]

Conventionally, in baseband regeneration methods that regenerate baseband signals using phase-locked detection methods, etc., a signal with the same frequency as the input center frequency is generated from a local oscillator, and this local oscillation signal and input signal are input to a phase detector. Then, phase detection was performed to directly obtain the baseband signal.

[Problem to be solved by the invention]

In the conventional baseband regeneration method described above, when the band of the baseband signal is specified from DC, the DC offset generated by the detector itself due to factors such as unbalance of the detector is combined with the baseband signal. It will be output as is. In particular, when the input signal is weak and the detection output of the detector becomes small, the DC offset of the detector can reach a level that cannot be ignored with respect to the DC component of the reproduced signal, resulting in a significant deterioration of signal quality and The disadvantage is that faithful reproduction of the signal becomes impossible.

[Means to solve the problem]

The DC offset correction method of the present invention includes a local oscillation unit that outputs an oscillation frequency signal that is separated by a predetermined frequency from the center frequency of an input signal having a certain band, and mixing the output of this local oscillation unit with the input signal. A detector, a capacitor for removing DC offset from the output of the detector, and a digital device for correcting the frequency difference between the center frequency of the input signal and the output of the local oscillator with respect to the output of the capacitor to extract a predetermined baseband signal. This digital signal processing section includes a sampling hold circuit,
It is characterized by comprising an analog-to-digital converter, a sampling timing generator, and a digital phase shifter.

〔Example〕 Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a diagram showing an example of the signal spectrum of each part in FIG.
(a) shows the input signal spectrum and the local oscillation spectrum, (b) shows the detector output spectrum, and (C) shows the baseband signal to be reproduced.

In Fig. 1, the input signal a is input to the detector 2 from the input terminal 1.
is input. This input signal a is, for example, as shown in FIG. 2(a).
As shown in , center frequency fo + bandwidth B ("f2 f
s) signal.

Assuming that this embodiment is a phase detection unit for performing phase synchronized detection, for example, if the phase of the local oscillation signal output from the local oscillation unit 3 and the input signal a are compared by the detector 2, the baseband signal C is obtained. can get. Here, the local oscillation signal S is not selected according to the center frequency fO of the input signal a, but for example, as shown in FIG. 2(a), from the center frequency f to B/2
In this case, the detector 2 converts it into a baseband signal C and outputs it as shown in FIG. A baseband signal d from which the component has been removed is obtained. However, this baseband signal d is different from the baseband signal to be originally reproduced (shown in FIG. 2(c)). This is because there is a frequency difference between the input signal a and the local oscillation signal, and this frequency difference (fo fs
) in order to reproduce the baseband signal shown in FIG. This is achieved by performing calculations.

g (t)=f (t)exp (-j 2πt)Hg
(n)=f (n)exp (-j 2rn/N)・
・・・・・・・・・・・・・・・・・・・・・(1)(1
) The left side of the arrow in the equation is a frequency shift equation with time t as a variable, and the right side is a frequency shift equation using discrete representation. Note that the internal sign of exp changes depending on the polarity of the frequency difference.

In the digital signal processing section 5. Input bass punt signal d
is first sampled and held by a sampling hold circuit (hereinafter referred to as SMP) 6, and the held signal e is converted into a digital signal f by an analog-to-digital converter (hereinafter referred to as AD) 7. A zero frequency shift in which this digital signal f corresponds to f (n) on the right side of equation (1) is realized by multiplying the digital signal f by exp (-j2πn/N), and this is achieved by using a digital phase shifter (hereinafter referred to as P
H8) Performed by 8. n described in equation (1) is the order of each sampling, and is defined as 0≦n≦N-1. In addition, N is determined from the relationship between the sampling frequency fs and the difference between the input center frequency and the local oscillation frequency (fo fs), and as a result, the frequency shift corresponding to the frequency difference (fo fs) and N
The frequency shift by the calculation of f(n)-exp (-j 2rn/N) is made to correspond.

(At this time, n is obtained by counting up from 0 to N-1 in the digital phase shifter.) Note that SMP6. A
D? The sampling signal g used in the PH98 is generated by the sampling timing generator 9.

The digital baseband signal whose frequency has been shifted in this way has a spectrum distribution shown in FIG. 2(C), and is outputted from the output terminal 10 to the outside. Since the digital baseband signal is a digital signal, for example, when performing digital signal processing for phase synchronized detection, it is possible to input this signal as is to a digital filter, and depending on the processing conditions, the digital baseband signal can be converted into a digital signal. It can be used by converting it into an analog signal using an analog converter.

〔Effect of the invention〕

As explained above, the present invention provides a frequency difference between the input center frequency and the local oscillation frequency during baseband signal reproduction, removes the DC component of the detector output, and then corrects the frequency difference by digital signal processing to a predetermined value. According to the present invention, the DC offset of the detector can be removed and the baseband signal containing the DC component can be faithfully reproduced. The effects of the present invention are particularly significant in situations where the input signal is weak.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a diagram showing an example of a signal spectrum of each part in FIG. 1. l...Input terminal, 2...Detector, 3...Local oscillation section, 4...Capacitor, 5...Digital signal processing section, 6...Sampling hold circuit (SMP), 7
...Analog-digital converter (AD), 8...
Digital phase shifter (PHS), 9... sampling timing generation circuit, 10... output terminal.

Claims (1)

[Claims] a local oscillation unit that outputs an oscillation frequency signal that is separated by a predetermined frequency from the center frequency of the input signal having a certain band;
a detector for mixing the output of the local oscillator and the input signal; a capacitor for removing DC offset from the output of the detector; and a center frequency of the input signal and a frequency of the output of the local oscillator for the output of the capacitor and a digital signal processing section that corrects the difference and extracts a predetermined baseband signal, and the digital signal processing section is composed of a sampling hold circuit, an analog-to-digital converter, a sampling timing generation section, and a digital phase shifter. A DC offset correction method featuring: