Design of high-frequency precision function waveform generator

    Abstract: This article introduces a precision function waveform generator composed of MAX038 and MC145151. The generator can output sine wave, rectangular wave, and triangle wave signals. The output frequency can be adjusted in the range of 8kHz to 16MHz, and the adjustment increment is 1kHz. It can be used as Universal high-frequency precision function waveform generator.

    Keywords: function waveform generator MAX038 MC145151 signal source

Integrated function waveform generators generally use ICL8038 or 5G8038, but they can only generate medium and low-frequency sine waves, rectangular waves and triangle waves below 300kHz, and the frequency and duty cycle cannot be adjusted independently, which brings great inconvenience to use. It cannot meet the requirements of high-frequency precision signal sources. This article introduces a high-frequency precision function waveform generator composed of MAX038 and MC145151. This circuit can generate accurate sine wave, rectangular wave and triangle wave signals, and the frequency can be adjusted in the range of 8kHz ~ 16.383MHz at 1kHz intervals.

1 MAX038 function

MAX038 is a single-chip high-frequency precision function waveform generator produced by MAXIM Company. It can generate sine waves, rectangular waves and triangle waves from 0.1Hz to 20MHz. The highest frequency can reach 40MHz. It has good high-frequency characteristics, wide frequency range, easy and flexible use specialty. The MAX038 chip is packaged in DIP-20. (See Figure 1), its pin functions are as follows:

V+, V-: Positive and negative power supply terminals, connected to +5V and -5V power supplies respectively;

GND: Pins 2, 6, 9, 11 and 18 are all analog grounds;

A0, A1: Waveform setting terminal, the setting method is shown in Table 1;

Table 1 Output waveform setting

COSC: external oscillation capacitor terminal;

FADJ, DADJ: frequency adjustment and duty cycle adjustment terminals respectively;

IIN: The power input terminal of the oscillation frequency controller;

PDI, PDO: are the input and output terminals of the phase comparator;

SYNC: synchronous output;

DV+, DGND: are the +5V power supply terminal and digital ground of the digital circuit respectively;

OUT: waveform output terminal

The internal structure of MAX038 is shown in Figure 2. After the capacitor CF is connected between COSC and GND, oscillation can be formed by charging and discharging the constant current CF, thereby generating a triangle wave and two rectangular waves. IIN is the current poured into the IIN terminal, which is controlled by the voltage at the FADJ terminal. The calculation formula of oscillation frequency is:

F=IIN/CF

In the formula, the allowed variation range of IIN is 2~750μA, and the performance is best when 10~400μA. The capacity range of CF is 20pF~100μF.

The frequency can be adjusted coarsely and finely by changing the voltage at the FADJ terminal. Assuming that VFADJ is 0V and the nominal output frequency is f, when VFADJ remains constant, the output frequency is determined by the following formula:

f1=f(1-0.2915VFADJ)

When frequency fine-tuning is not performed, a 12kΩ resistor must be connected between the FADJ terminal and ground.

Changing the voltage at the DADJ terminal can control the duty cycle of the waveform. When VDADJ is 0V, the duty cycle is 50%. When VDADJ changes from +2.3V to -2.3V, the duty cycle will change from 10% to 90%. When the duty cycle is strictly equal to 50%, waveform distortion can be eliminated.

2 MC145151 pin functions

MC145151 is a parallel input phase-locked loop frequency synthesizer with a single-mode front-end calculator interface produced by Motorola. The ÷N counter range is 3 to 16383, and there are 8 ÷R values ​​for user selection: 8, 128, 256, 512, 1024, 2048, 2410 or 8192. MC145151 is packaged in DIP-28 pins. The pin arrangement is shown in Figure 3. The pin functions are as follows:

Fin: frequency input;

RA0~RA2: Reference address input, determine ÷R value;

N0～N13: N counter programming input;

T/R: send/receive offset address input;

OSCin, OSCout: respectively the input and output of the reference oscillator;

PDout: phase detector A output;

PDR, PDV: phase detector B output;

FV: N counter output;

LD: lock detector output;

VDD: Connect to the positive power supply;

VSS: Connect to negative power supply.

3 High-frequency precision function waveform generator

Figure 4 shows the circuit schematic diagram of a high-frequency function wave generator, which can generate accurate sine wave, rectangular wave and triangle wave signals. The RA2, RA1 and RA0 terminals of MC145151 are open circuit, which is equivalent to the input high level, and the ÷R value is set to 8192. The oscillation frequency is 8.192MHz. After dividing by R, the phase identification frequency fR of 1kHz can be obtained. The 14-bit switching values ​​S13~S0 are sent to the N13~N0 terminals of MC145151 to set the N value. The synchronization signal SYNC of MAX038 is sent to the Fin terminal. After dividing by N, it is compared with fR to detect the phase. The signal related to the phase difference is output through PDV and PDR, and is converted into a ±2.5V signal by MAX427, and then sent to the FADJ terminal of MAX038. .

Since MAX038 and MC145151 form a frequency phase-locked loop PLL, the output frequency is locked at N×fR, so the output frequency of the function generator can be adjusted in the range of 8kHz to 16.383MHz, with an adjustment increment of 1kHz. For example: when S1, S3 and S8 are disconnected, the output frequency is 261kHz.

MX7541 is a 12-bit DAC circuit. Send S13~S2 to the BIT1~BIT12 terminals of MX7541 for D/A conversion. The output signal is then converted into a current signal of 2μA~750μA by MAX412, and then sent to the IIN terminal of MAC038 as a frequency coarse signal. Adjust the signal to adjust the output frequency of MAX038 to the capture range of the frequency phase-locked loop, and then the PLL loop locks the output frequency at the frequency set by S13~S0.

The A0 and A1 terminals of MAX038 are used to set the output waveform, and can be connected to different levels as needed.

Connecting a 50MHz low-pass filter to the output terminal OUT of MAX038 can block and attenuate high-frequency noise, so that sine wave, rectangular wave and triangle wave signals within 16MHz can be smoothly output through the filter.

4 Conclusion

The high-frequency precision function waveform generator composed of MAX038 and MC145151 can provide sine wave, rectangular wave and triangle wave signals from 8kHz to 16MHz. The frequency phase-locked loop technology can further improve the frequency stability of the system, so it is a signal generator with excellent performance and can be used as a precision universal signal source.