function generator

Implementation

(1) Function generator composed of discrete components: usually a single function generator with low frequency, its work is not very stable, and it is not easy to debug

(2) It can be made by general-purpose devices such as transistors and op amp ICs, and more is produced by a special function signal generator IC. Early function signal generator ICs, such as L8038, BA205, XR2207/2209, etc., have fewer functions, lower accuracy, and the upper frequency limit is only 300kHz, which cannot generate higher frequency signals, and the adjustment method is not flexible enough. The air ratio cannot be adjusted independently, and the two affect each other.

(3) Function generator using monolithic integrated chip: It can generate multiple waveforms, reach a higher frequency, and is easy to debug. In view of this, American Maxim has developed a new generation of function signal generator ICMAX038, which overcomes the shortcomings of the chip in (2) and can reach higher technical indicators, which is beyond the reach of the above chips. MAX038 has high frequency and good accuracy, so it is called high-frequency precision function signal generator IC. In the design of circuits such as phase-locked loops, voltage-controlled oscillators, frequency synthesizers, and pulse width modulators, the MAX038 is the preferred device.

(4) Function generator using dedicated direct digital synthesis DDS chip: can generate arbitrary waveforms and reach very high frequencies. But the cost is higher.

An electrical test signal instrument that produces the required parameters. According to its signal waveform, it is divided into four categories: ①Sinusoidal signal generator. Mainly used to measure the frequency characteristics, nonlinear distortion, gain and sensitivity of circuits and systems. According to its different performance and usage, it can be further subdivided into low frequency (20 Hz to 10 MHz) signal generator, high frequency (100 kHz to 300 MHz) signal generator, microwave signal generator, frequency sweep and program-controlled signal generator, Frequency synthesis signal generator, etc. ② Function (waveform) signal generator. It can generate some specific periodic time function waveforms (sine wave, square wave, triangle wave, sawtooth wave and pulse wave, etc.) signals, and the frequency range can be from several microhertz to tens of megahertz. In addition to being used for communication, instrumentation and automatic control system testing, it is also widely used in other non-electrical measurement fields. ③Pulse signal generator. A generator that can generate rectangular pulses with adjustable width, amplitude, and repetition frequency can be used to test the transient response of linear systems, or used as analog signals to test the performance of radar, multiplex communications, and other pulsed digital systems. ④ Random signal generator. Usually divided into noise signal generator and pseudo-random signal generator. The main purpose of the noise signal generator is to introduce a random signal into the system under test to simulate the noise in the actual operating conditions to determine the system performance; add a known noise signal to compare the internal noise of the system to determine the noise coefficient; use a random signal Instead of sinusoidal or pulse signals, to determine the dynamic characteristics of the system. When measuring the correlation function with a noise signal, if the average measurement time is not long enough, a statistical error will occur, which can be solved with a pseudo-random signal.

Generator

The signal generator is generally divided into a function signal generator and an arbitrary waveform generator, and the function waveform generator distinguishes analog and digital synthesis types in design. As we all know, the digital synthesis function signal source is better than analog in terms of frequency, amplitude, and signal-to-noise ratio (S/N). The design of its phase-locked loop (PLL) makes the output signal not only accurate in frequency, but also phase jitter ( phase jitter) and frequency drift can reach a fairly stable state, but after all, it is a digital signal source. The interference between digital circuits and analog circuits is always difficult to overcome effectively, and it is also inferior to analog functions in the output of small signals. Signal generator.

Talking about the analog function signal source, the structure diagram is as follows:

This is the structure of a general analog function signal generator, which is based on a triangular wave generating circuit and a sine wave shaping circuit formed by a diode to generate a sine wave, and at the same time, a square wave is generated by comparison of a comparator.

And how the triangle wave is generated, the formula is as follows:

In other words, if the capacitor is charged with a constant current source, a ramp wave with a positive slope can be generated. Similarly, the discharge of the charge stored in the capacitor with a constant current source on the right generates a ramp wave with a negative slope. The circuit structure is as follows:

When I1 = I2, a symmetrical triangle wave can be generated. If I1> >I2, a negative slope sawtooth wave is generated at this time. Similarly, I1 <<I2 generates a positive slope sawtooth wave.

As shown in Figure 2, the selection of switch SW1 can change the charging speed by multiples, that is, change the frequency of the signal, which is also the selection switch of the frequency file on the signal source panel. Changing I1 and I2 in the same way can also change the frequency. This is the potentiometer that adjusts the frequency on the signal source. It only needs to simply convert the voltage signal into a current.

The design of the duty cycle adjustment has the following two ideas:

1. The frequency (period) does not change and the pulse width changes. The method is as follows:

Changing the amplitude of the level, that is, changing the reference amplitude of the comparator of the square wave generating circuit, can achieve the characteristics of changing the pulse width without changing the frequency, but its main disadvantage is that the duty cycle cannot be adjusted to 20[%] The following results in the sampling circuit experiment, the signal collected by the instantaneous signal changes, if you want to use this signal for analog-to-digital (A/D) conversion, then the resulting digital signal changes and is at a loss. But it is undeniable that it is better in use.

2. The duty ratio changes and the frequency changes accordingly. The method is as follows:

It can be achieved by fixing the reference amplitude of the comparator of the square wave generating circuit (positive and negative can be switched by the circuit), and changing the charge and discharge slope. [NextPage]

In this way, the response of the general user is "difficult to adjust", which is a big disadvantage, but it can produce a duty ratio of less than 10 [%] but it is a necessary condition when sampling.

The above two kinds of duty cycle adjustment circuit design ideas have their own advantages and disadvantages. Of course, the associated ones also affect whether they can produce "decent" sawtooth waves.

Next, the design of PA (power amplifier). The first is to use an operational amplifier (OP), and then use a push-pull amplifier (pay attention to the prevention of cross-distortion Cross-distortion) to send the signal to the attenuation network. This part involves the indicator of the output signal of the signal source, including The signal-to-noise ratio, the rise time of the square wave and the frequency response of the signal source. A good signal source is of course a high signal-to-noise ratio of the sine wave, a fast rise time of the square wave, a good linearity of the triangle wave, and a good volt-frequency characteristic. Rise, the signal cannot be attenuated or cannot be reduced too much), this part of the circuit is more complicated, especially in addition to the use of capacitors for frequency compensation at high frequencies, it also involves the wiring of the PC board. If you are not careful, it may easily cause oscillation. In order to design this part of the circuit, in addition to the original simulation theory, practical experience is required, and the patience of "Try Error" is indispensable.

After the PA signal comes out, it will be attenuated by 10 times (20dB) or 100 times (40dB) through the π-type resistive attenuation network. At this time, a basic function waveform generator has been completed. (Note: The π-type attenuation network is used instead of the voltage divider circuit to keep the output impedance constant).

Other functions

A powerful function waveform generator, there are frequency sweep, VCG, TTL, TRIG, GATE and or LCD liquid crystal display frequency, which overlap with the frequency meter circuit.