Homemade Simple Frequency Synthesis Signal Source

In order to obtain a frequency-stable square wave signal, the author has developed a simple frequency synthesis signal source with a frequency range of 1Hz to 3MHz.

Since the selected crystal oscillator temperature coefficient is about 0.2ppm/℃, the output frequency is extremely stable. When the temperature changes by ±10℃, the output frequency does not change by more than 2ppm.

The circuit of the signal source is shown in the attached figure.

The signal source is composed of three integrated circuits. The reference oscillator is composed of CD4060 and 8.192MHz crystal oscillator, and its pin ① is divided by 8192 internally to output 1kHz, which is used as the reference frequency of the phase-locked loop.

The integrated phase-locked loop circuit CD4046 forms a standard frequency synthesis circuit. Its internal voltage-controlled oscillator generally operates at a frequency of 1.2MHz, which is overclocked in this circuit. The capacitance between pins ⑥ and ⑦ is reduced to 6.2pF. In actual applications, when the power supply voltage is 12V, it can operate at 2.7MHz, and when it is 15V, it can operate at 3.2MHz.

Of course, when the operating frequency is high, the waveform is slightly worse, which is not a big deal for amateur use. CD4040 is a twelve-level binary counter, which is used here as an arbitrary frequency divider of 1 to 3000, and then a clear pulse is used as the frequency division output.

Since CD4040 is a serial counter, the flip-flops of each bit are not synchronous. When the operating frequency is high, the burr interference at the clearing end is large, so a 390pF capacitor is connected to the ground to filter it out, and the value of R5 should be reduced to 620Ω to make the divider work properly. The frequency division coefficient of this circuit is the sum of the numbers corresponding to each closed switch.

By using different combinations of the 12 switches, frequency division integers between 1 and 4095 can be obtained.

Using CD4040 as a frequency divider is simpler than using a preset decimal subtraction counter to divide the frequency, and the frequency division ratio is larger, but the disadvantage is that it needs to be converted when adjusting the frequency, which is not as intuitive as the latter. This is still applicable to occasions that do not require frequent adjustments. The fourth foot of CD4046 can output a square wave signal of 1kHz to 3MHz.

At the same time, a 1000 frequency divider composed of a CD4518 and a CD4017 can divide the output signal of the fourth foot of CD4046 to obtain a signal of 1Hz~3kHz with the same accuracy.

The CD4017 is used to obtain a square wave signal with a duty cycle of 50%, while the output of the CD4518 is a pulse with a duty cycle of 20%.

If a CD4518 frequency divider is inserted in the middle, a square wave signal as low as 0.01Hz can be obtained.

In addition, an emitter follower can be connected to the output end to increase the output driving capability.

By using the 5V voltage regulator at pin (15) of CD4046 and adding a first-stage emitter follower, the output can be converted to TTL level.