JP2769637B2 - Waveform digitizer - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveform digitizer, and more particularly, to improvement in deterioration of group delay characteristics when an anti-aliasing filter is added.

<Prior Art> Conventionally, there is a waveform digitizer that takes in a waveform as an input, performs analog-to-digital conversion (hereinafter referred to as AD conversion), and captures the data as digital data in a memory.

<Problems to be Solved by the Invention> By the way, in this type of waveform digitizer, in a device represented by a digital oscilloscope that emphasizes the fidelity of a waveform displayed on a time axis, a waveform is sampled at a constant cycle. There is a problem that the aliasing phenomenon (occurrence of a beat due to waveform folding) when a frequency component that is twice or more than the above is input is unavoidable.

Also, in order to prevent a frequency component more than twice the sampling frequency from being input, as shown in FIG. 5, between the amplifier 1 for amplification and impedance conversion and the AD converter 3, FIG. When an analog filter (anti-aliasing filter) 2 having a steep cutoff characteristic as shown is inserted, the problem of aliasing is improved. However, there is a problem that the group delay characteristic is deteriorated due to the phase characteristic of the filter as shown in FIG. 6B, and the fidelity of the waveform is impaired when the time axis is displayed.

An object of the present invention has been made in view of the above points, and it is an object of the present invention to provide a waveform digitizer that can correct the deterioration of the group delay characteristic when a normal analog anti-aliasing filter is added. is there.

<Means for Solving the Problems> According to the present invention for achieving the object, in a waveform digitizer, an anti-aliasing filter and an output of the anti-aliasing filter are digitally converted and data to be stored in a memory are stored. An AD converter to send out,
DA for converting data read from the memory to analog
A converter and a switch for selecting one of a waveform input and an output of the DA converter and inputting the selected signal to the anti-aliasing filter are provided.

<Operation> The time-series data that has been AD-converted through the anti-aliasing filter is temporarily stored in a memory, and then the stored data is read out in the reverse time-series, analog-converted, and AD-converted again through the anti-aliasing filter.

By doing so, it is possible to correct the deterioration of the group delay characteristic caused by using the anti-aliasing filter.

<Example> Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a main part configuration diagram showing one embodiment of a waveform digitizer according to the present invention. In the figure, 1 is an amplifier for amplifying and impedance-converting an input signal, 2 is an anti-aliasing filter, 3 is an AD converter for digitally converting the output of the anti-aliasing filter 2, and 4 is the output data of the AD converter 3 5 is a digital-to-analog converter (hereinafter referred to as DA) that converts data read from the memory into analog.
A switch 6 switches between the output signal of the amplifier 1 and the output signal of the DA converter 5 as a signal to be input to the anti-aliasing filter 2.

Reference numeral 7 denotes a control device, which controls the switch 6 and controls the order of writing and reading in the memory 4.

The operation in such a configuration will now be described. The input signal is amplified and impedance-converted by the amplifier 1 as in the operation of the conventional waveform digitizer, and is input to the anti-aliasing filter 2 via the switch 6. The sampling frequency is output to the output of the anti-aliasing filter 2.
A signal in which a frequency component equal to or higher than half the frequency of fs (fs / 2) is cut off is output.

As shown in FIG. 2A, the anti-aliasing filter 2 has a very steep cutoff characteristic (gain characteristic: this gain characteristic is represented by h (t)). As shown, a large phase delay occurs even in the pass band, and the output signal has an output waveform having group delay distortion.

This digital data is temporarily stored in the memory 4. The data sequence at this time is represented as time-series data sampled at t = 0, 1, 2,... N, and V (t = 0), V (t = 1), V (t = 2),. ..V (t =
n).

When the storage of the digital data in the memory 4 is completed, the switch 6 is then switched to change the digital data in the memory 4 at a sampling rate of 1 / fs at V (t = n),.
(T = 2), V (t = 1), V (t = 0) in reverse order of the time series, and output them, which are input to the DA converter 5.

The DA converter 5 converts the digital data into an analog signal at a sampling rate of fs. The gain and phase characteristics of this signal are shown in FIGS. 3 (a) and (b).

The DA-converted signal is input to the anti-aliasing filter 2 again via the switch 6.

Accordingly, as shown in FIG. 4, the aliasing component is removed by sampling, and the phase characteristic becomes a characteristic h (-t) obtained by making the time of the transfer characteristic h (t) of the anti-aliasing filter 2 negative.

Now, the Fourier transform H (ω) of h (t) is On the other hand, the Fourier transform H (ω ′) of h (−t) is Where -t = τ in the above equation, Since H (ω) · ▲ == H (ω) | ² = real number, h (t) * h (−t) is a real number.

For this reason, the transfer characteristic is an even function not including the imaginary part, and the group delay distortion is corrected.

The data corrected in this manner is digitized by the AD converter 3 and stored in the memory 4 again. Since the time series of this stored data is reversed, when the data is read out for waveform display, the time series is reversed and V (t =
0), V (t = 1), V (t = 2),... V (t = n).

<Effects of the Invention> As described above in detail, according to the present invention, an anti-aliasing filter can be added without deteriorating the group delay characteristic indispensable for waveform measurement on the time axis, and the problem of aliasing can be solved. A resolved waveform digitizer can be realized.

[Brief description of the drawings]

FIG. 1 is a diagram showing a main part of an embodiment of a waveform digitizer according to the present invention, FIG. 2 is a diagram showing cutoff characteristics and phase characteristics of an anti-aliasing filter, and FIGS. 3 and 4 are FIG. FIG. 5 is a diagram showing a gain characteristic and a phase characteristic in a circuit, FIG. 5 is a main part configuration diagram showing an example of a conventional waveform digitizer,
FIG. 6 is a diagram showing gain characteristics and phase characteristics in FIG. DESCRIPTION OF SYMBOLS 1 ... Amplifier 2 ... Anti-aliasing filter 3 ... AD converter, 4 ... Memory 5 ... DA converter, 6 ... Switch 7 ... Control device

Claims (1)

(57) [Claims] A waveform digitizer which takes in a waveform as an input, converts the data into an analog signal, and captures the data as digital data in a memory. An anti-aliasing filter, and converts the output of the anti-aliasing filter into digital data and stores the data stored in the memory. An AD converter to send out, and a DA to convert data read from the memory to analog
And a switch for selecting one of a waveform input and an output of the DA converter and inputting the selected signal to the anti-aliasing filter.First, the waveform input is AD-converted through the anti-aliasing filter and time-series. The data is stored in the memory, and then the stored data is read out in reverse time series, converted into an analog signal by the DA converter, and then input again to the anti-aliasing filter via a switch. A waveform digitizer wherein the group delay characteristic is prevented from deteriorating by storing it again in a memory.