JP3070809B2 - Function Generator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital type function generator in which arbitrary waveform data is read from a memory in which arbitrary waveform data is stored, converted into analog data, and output. And an improvement for generating a high-resolution and high-precision DC voltage.

[0002]

2. Description of the Related Art Conventionally, function generators using digital circuits and DA converters are well known. FIG. 2 is a main part configuration diagram showing an example of this type of function generator. In the figure, reference numeral 1 denotes an address generator (also referred to as a phase calculator), which is composed of an adder 2 and a latch 3 and has a clock (the source of the clock is not shown here because it has no direct relationship here).
Every time is input, a value obtained by cumulatively adding a predetermined value (N) is output. This output is used as an address for the memory 4. Time-series data such as a sine wave and a triangular wave are stored in the memory 4 in the order of addresses, and the waveform data read by addressing is input to a digital / analog converter (hereinafter, referred to as a DA converter) 5.

In general, a high-speed DA converter 5 is used, which converts an input signal into an analog signal and outputs it every time a clock is applied. 6 is an unnecessary frequency component of the output signal of the DA converter 5 (DA
This is a low-pass filter that removes a waveform disturbance caused by the conversion.

[0004] A portion composed of an address generator 1, a memory 4, a DA converter 5, and a low-pass filter 6 is a so-called digital direct synthesizer (D / D).
DS).

[0005] The output of the low-pass filter 6 is added to the offset voltage Voff by the adder 7 and sent to the output terminal 10 via the resistor 9. The offset voltage Voff is variable and is output from the voltage generator 8. Usually, a resistor of 50Ω is used as the resistor 9.

According to such a configuration, the contents (waveform data) of the waveform memory can be read from the address which increases by the set value N each time the clock is input, and the analog waveform can be transmitted from the output terminal 10. . A desired analog waveform can be generated by appropriately setting the waveform data stored in the waveform memory, and the frequency of the output signal can be easily changed by changing the value of the set value N. By setting the waveform data in the waveform memory 4 to a constant value, a DC voltage can be generated.

[0007]

In such a conventional function generator, when the D / A converter is operated with a high-speed clock, the change in the DC level of the output of the D / A converter increases as the clock frequency increases. In other words, the so-called DC stability deteriorates. For this reason, there is a disadvantage that the offset of the signal appearing at the output terminal 10 becomes inaccurate even though the correct offset voltage Voff is superimposed. Also,
When this function generator is used as a DC voltage generator, the output impedance is 50Ω (resistance 9
And the output impedance is too high for a voltage generator.

[0008] In view of the above, an object of the present invention is to provide:
A function that can be used as an arbitrary waveform generator that can set a high-precision offset voltage when generating an arbitrary waveform, and a DC voltage generator that generates a high-precision DC voltage with low output impedance when generating a DC voltage. The realization of a generator.

[0009]

According to the present invention, there is provided a function generator capable of generating an arbitrary analog waveform at an appropriate frequency according to the DDS method. A control operation circuit including a function capable of setting an offset value corrected for a DC change of the DDS caused in relation to the frequency, a DA converter for converting the offset value into an analog signal, and a DDS output through a switch. And an amplifier capable of appropriately amplifying and outputting the output of the DA converter and switching the output impedance. In the case of generating an arbitrary waveform, the DDS output and the output of the DA converter are added and amplified. And send it out,
In the case of DC voltage generation, the output of the amplifier is switched to low impedance,
It is characterized in that only the offset voltage from the converter is amplified and output.

[0010]

The offset voltage set by the control operation circuit is generated by a high-resolution and high-precision DA converter. In the control operation circuit, an offset in which a change in the DC component generated in relation to the frequency in the DDS is corrected is given to the DA converter. As a result, a stable arbitrary waveform of the DC component can be generated regardless of the frequency. On the other hand, when a DC voltage is output, the DDS output is disconnected, and only the output (offset voltage) of the DA converter is amplified and output.
Further, in this case, the circuit connection is switched by a switch so that the output impedance of the amplifier becomes low.

[0011]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.
FIG. 1 is a main part configuration diagram showing one embodiment of a function generator according to the present invention. In the figure, parts that are the same as those in FIG. 2 are given the same reference numerals, and descriptions thereof are omitted. 11
Is a first switch, which is a switch for selecting between offset addition and DC voltage generation. Numeral 12 denotes a high-precision amplifier, which adds two inputs, that is, both the waveform signal output from the low-pass filter 6 and the two inputs of the DC voltage from the DA converter 20, or amplifies only the DC voltage, and further outputs The impedance can be switched in two stages.

The configuration of the amplifier 12 will be described in detail below. An operational amplifier 13 has a non-inverting input terminal connected to a common line, and an inverting input terminal connected to one end of a first input resistor 14, a second resistor 15, and a feedback resistor 16, respectively. . The output of the low-pass filter 6 via the switch 11 is applied to the other end of the input resistor 14, and the second D
The output of the A converter 20 is added. Operational amplifier 13
Is connected to the output terminal 10 via a resistor 18 and to the other end of the feedback resistor 16 via a second switch 17. A third switch 19 is connected between a common connection point A between the resistor 18 and the output terminal 10 and a common connection point B between the feedback resistor 16 and the second switch 17. The second and third switches are switches for switching impedance and are normally turned on / off complementarily.

The second DA converter 20 is a DA converter that generates a high-resolution and high-precision DC voltage. Note that the operation speed may be low. Reference numeral 21 denotes a control operation circuit, which sets a specified value N to be applied to the address generator 1, sets an offset voltage value to be applied to the DA converter 20,
It has functions such as on / off control of the switches 11, 17, and 19.

The operation in such a configuration will be described below. (1) When outputting an analog waveform on which an offset voltage is superimposed The first switch 11 is turned on, an analog waveform signal from the DDS is applied to the amplifier 12, and a DC offset voltage is also applied from the DA converter 20. Second switch 17
Is turned on and the third switch 19 is turned off, a function generator having an output impedance of 50Ω can be realized.

In the function generator, a high-speed D signal is output so that a high-frequency analog waveform can be output.
The A-converter 5 is used, but as described above, the high-speed D / A converter usually has a problem that the DC level is unstable and the DC level changes in relation to the frequency. However, in the present invention, the control arithmetic circuit 21 corrects the change in the DC level caused by the frequency (the frequency is determined by the clock frequency and the specified value N given to the address generator) to set the offset value. Therefore, the offset (DC component) superimposed on the waveform is stable without being affected by the frequency.

On the other hand, the DA converter 20 may be a low-speed DA converter for handling a DC level. The fact that a low-speed D / A converter is sufficient means that a high-resolution and high-precision D / A converter can be easily created or obtained.
It can be easily superimposed on the output waveform.

(2) In the case of using a high-accuracy DC voltage generator: The first switch 11 and the second switch 17 are turned off.
The offset value is set with only the third switch 19 turned on. The output of the D / A converter 20 is DC-amplified and output by the amplifier 12. Since the amplifier 12 also has high precision, a high-precision DC voltage is generated from the terminal 10. In this case, the output becomes low impedance by turning on the third switch 19.

[0018]

As described above, according to the present invention, by using a low-speed but high-resolution and high-precision DA converter for generating an offset voltage, a high-speed signal output from the DDS can be used at a high speed. Accurate DC components can be easily superimposed. Also, the first stage is provided after the output of the high-speed DA converter.
By inserting this switch and turning off the switch, it is possible to quickly change to a high-precision DC voltage generator that amplifies and outputs only a high-precision DC voltage. Second
Also, there is an advantage that the output impedance can be easily switched by turning on / off the third switch.

[Brief description of the drawings]

FIG. 1 is a main part configuration diagram showing one embodiment of a function generator according to the present invention.

FIG. 2 is a main part configuration diagram showing an example of a conventional function generator.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Address generator 2 Adder 3 Latch 4 Waveform memory 5 DA converter 6 Low-pass filter 10 Output terminal 11 First switch 12 Amplifier 13 Operational amplifier 14, 15 Input resistance 16 Feedback resistance 17 Second switch 18 Resistance 19 Third Switch 20 DA converter 21 Control operation circuit

Claims (1)

(57) [Claims] A function generator capable of generating an arbitrary analog waveform at an appropriate frequency by a DDS method, wherein an offset value is set, and an offset value corrected for a DC change of the DDS caused by the frequency. A control operation circuit including a function capable of setting the offset value, a D / A converter for converting the offset value into an analog signal, and receiving the DDS output via a switch,
It has an amplifier that receives the output of the A converter, amplifies and outputs the output as appropriate, and can switch the output impedance. In the case of generating an arbitrary waveform, the DDS output and the output of the DA converter are added and amplified and transmitted. A function generator configured to switch the output of the amplifier to a low impedance when a DC voltage is generated and to turn off the switch to amplify and output only the offset voltage from the DA converter.