Sine wave generator is widely used, but the high stability sine wave circuit is suitable for low frequency range. Table 1 is a comparison of several main sine wave generation methods.
It can be seen from table 1 that it is difficult to achieve a stable low frequency sine wave generator using only one sine wave generation method. In this paper, a phase-locked loop is used to design a frequency stable low frequency sine wave generator, and a low-frequency PLL function generator based on MC145151-2 and ICL8038 chips is given. The stability and accuracy of this low frequency phase-locked loop function generator are equivalent to that of reference frequency without additional error.
Diagram 1: block diagram of phase-locked loop.
Voltage controlled waveform generator and phase-locked loop circuit
ICL8038 is a kind of monolithic integrated voltage controlled waveform generator which is widely used now. It can produce sine wave, rectangular wave and triangle wave at the same time in the range of 0.01 ~ 300kHz. Only a small number of external resistors and capacitor elements can be used as a voltage controlled oscillator, a FM signal generator or a single function generator. The output level characteristic is square wave 0.2Vsupply (2mA); the triangle wave amplitude is 0.33Vsupply, the output impedance is 200 omega (5mA); the sine wave amplitude is 0.22Vsupply, and the typical value of the output impedance is 1K Omega.
MC145151-2 is MC145151-1's performance improvement product with reduced power consumption and improved ESD and lock performance. MC145151-2 uses 14 and 3 parallel input data lines to realize N counter and R counter programming. This device integrates reference oscillator, optional reference frequency divider, digital phase detector and 14 bit programmable N counter.
The chip has the following characteristics: because of the low power consumption of the CMOS process, the voltage range is 3 to 9V; it has the operating conditions of the on-chip or off chip reference oscillator; there are two phase signal output, in which the PDout is the output of the phase discriminator A, and the R and V are the output of the phase discriminator B, the phase discrimination error signal and the LD used to output phase locks. A fixed signal; in addition to the N range of 3~16383; 8 user selectable R values are 8, 128, 256, 512, 1024, 2048, 4096 and 8192, respectively corresponding to the 8 states of the RA2~RA0 from 000 to 111, respectively.
Design of sine wave generator
The circuit designed in this paper is composed of two parts: a basic phase locked loop and a circuit that generates reference frequency. The schematic diagram is shown in Figure 1. The reference frequency is usually generated by a crystal oscillator with a relative frequency stability of 10-6, and an appropriate reference frequency is provided after R frequency division. The circuit in the dotted box is the core part of the frequency synthesizer, that is, the phase locked loop, which consists of a phase detector, a low-pass filter and a voltage controlled oscillator. The frequency of the system output signal is fout= (N/R) fin. Changing the frequency division ratio N can easily obtain a large number of discrete frequency output signals.
Figure 2: a sine wave generator.
There are two changes in the phase locked loop compared with the typical high frequency phase locked loop circuit. One is the addition of the crystal vibration shaping and frequency division circuit before the phase detector, which is used to obtain the low frequency reference signal; the two is that the voltage controlled oscillator selects the devices working in the low frequency range, which is required by the low frequency reference signal.
In the circuit shown in Fig. 2, the crystal oscillator uses the temperature compensation crystal of 2MHz, its frequency stability can reach 10-8, and the phase detector A or the phase detector B can be selected. In this paper, phase detector A and PDout output are connected to a RC filter. If the selection of the phase detector B, with R and V output connected with active low-pass filter. The LD of MC145151-2 is used to indicate that the loop is locked and an external LED is connected. The LED will flicker when there is no lock. The oscillator part of Figure 2 adopts the on-chip oscillation mode of MC145151-2, and the two capacitance is 39P and 27P respectively. The fixed frequency divider of MC145151-2 is selected to be 8192, and the 2M signal is 244.14Hz after frequency division. The signal is used as the reference frequency to input the phase detector. The oscillator can also select the off chip form and increase the frequency division coefficient to further reduce the reference frequency. The specific circuit is shown in Figure 2. The two resistors are all 1K capacitors and the capacitance is 0.01 F. The low frequency sine wave generation circuit based on PLL is shown in Figure 3.
Phase locked loop circuit debugging is very troublesome, it is easy to lose the lock. In the process of realizing this circuit, we should pay attention to the following problems:
The R divider in MC145151-2 is internally connected with a pull resistor, so RA0 to RA1 must be suspended when it is "1", and it must not be connected to high level.
The square wave output of the ICL8038 must be coupled with a pull resistor. If it is compatible with the TTL level, it must connect to the +5V. MC145151-2 requires the input level. Under the +5V power condition, the high level of the input square wave should not exceed +5V, and the low level should not be lower than +1.5V.
Fig. 3: low frequency sine wave generator based on PLL.
The matching between MC145151-2 phase output voltage and ICL8038 control level is an important problem in the whole circuit implementation. To solve this problem, ICL8038 is connected to a function generating circuit of DC power divider control. The voltage control range of ICL8038 is 2/3 (|VCC|+|-VEE|) +2V < V < |VCC|+|-VEE|. A variable resistor is used to change the divider ratio and observe the output frequency of ICL8038. It is found that the frequency of the output signal decreases with the increase of the control voltage. Under the conditions of + 6V power, CT=4700P and RT=4.7K, the output frequency is 11.48kHz when the control voltage is +2.6V (relative to -VEE 8.6V); the output frequency is 182Hz while the control voltage is +5.6V (relative -VEE 11.6V), and the high voltage will appear waveform distortion.
The phase detection output of MC145151-2 is +2V to +3V after filtering, which can control ICL8038, but the voltage range is too small.
Contact Person: Mrs. Admin/Abby/May/Tina//Aimee