Scheme 1: PLL frequency synthesizer is adopted. However, the output frequency coverage coefficient of a single PLL is small, which can not meet the change of frequency range. If multiple phase-locked loops are used for segmented locking, the hardware cost is large, the circuit is complex and debugging is difficult.
Scheme 2: using single chip microcomputer, sine wave is generated by software, stored in RA M, addressed by instructions, and output waveform. Although hardware resources can be saved, it is difficult to meet the design requirements due to the limitation of crystal oscillation frequency and inherent instruction cycle of single chip microcomputer.
Scheme 3: Single-chip voltage-controlled function generator ICL8038.8038 is used to generate sine wave, square wave and triangular wave, and frequency conversion control is carried out. However, the output frequency stability is only RC oscillator, so it is difficult to meet the requirements of frequency step control and synthesize waveform.
Scheme 4: The waveform data to be generated is processed and calculated by single chip microcomputer, and the waveform quantized data of sine wave and triangle wave are stored by ROM (there are only two kinds of rectangular wave data, which need not be stored). When inputting the waveform parameters to be generated, the single chip microcomputer processes the data of each scanning point of the waveform and sends it to the external RAM through table lookup and calculation. Then, the 12 bit binary counter is driven by a clock pulse with a certain frequency (such as 10MHz) to quickly scan this 4K RAM. After the output of RAM is converted by 8-bit D/A converter 0832, periodic signals will appear, and the frequency stability and measurement accuracy can meet the requirements. This scheme improves the accuracy and controllability of the output signal, and can realize the combination of arbitrary waveforms because of the full digital processing process.
After repeated argumentation, scheme 4 was finally adopted.
12
Choose one of the two
12 system
counter
single chip microcomputer
wave form
Random access storage
Waveform formation
Digital (word)-analog (analog) converter
Amplitude control
Digital (word)-analog (analog) converter
amplifier
show
keyboard
1/32
Frequency divider
Waveform writing:
12
Choose one of the two
wave form
Random access storage
show
keyboard
single chip microcomputer
address line
Data line; data cable
Frequency control:
When "128 waveform" is stored in RAM (32 points), the period of the waveform is output.
The address is generated by the 12 binary counter, and the scanning time is:
Select RAM with storage capacity of 4K.
The counter clock is F0 = 1mhz and t0 =1μ s.
t 1 =(2 12- 1)×T0 = 1023μS
FOUT= 1/ TOUT= 125KHZ
TOUT=T 1/ 128=7.99μS
When "1 waveform" is stored in RAM (1024 point), the period of the waveform is output.
If the address is generated by the binary counter of 12 divided by 32, the scanning time is:
T2=T 1 ×32= 32736μS
FOUT= 1/ TOUT=30HZ
TOUT=T2 =32736μS
Amplitude control:
The reference voltage of DAC is formed by the output control waveform of amplitude control DAC.
Waveform formation
Digital (word)-analog (analog) converter
Amplitude control
Digital (word)-analog (analog) converter
amplifier
DAC output = (digital quantity) × reference voltage