Production and debugging of self-made high-sensitivity JJY timing signal receiving system (Part 1)

In order to realize a high-sensitivity JJY signal receiving system, according to the design principle, it is necessary to produce and debug from both hardware and software aspects. In terms of hardware, it is mainly divided into the frequency band processing part and the baseband processing part. In terms of software, it mainly completes the reception and decoding function of the JJY signal and the function of the electronic clock.

Hardware Circuit

1. Receiving antenna

In order to receive the 40kHz low-frequency timing signal, the receiving antenna should use a magnetic rod antenna, which forms a parallel resonant circuit with a capacitor as the input circuit. The magnetic rod antenna can be made by winding an enameled coil on a ferrite magnetic rod. According to experience, its inductance should be in the mH level. The magnetic rod is made of manganese-zinc ferrite with a magnetic permeability of 2kH/m, a cylindrical magnetic rod with a diameter of 10mm and a length of 80mm. The coil uses a single-strand enameled wire with a diameter of 0.25mm, and is wound into a magnetic rod antenna with an inductance of about 4.8mH (as shown in Figure 1). By adjusting the position of the magnetic core in the coil, the inductance can be fine-tuned to make the resonant frequency 40kHz.

Figure 1 Magnetic rod antenna

Figure 2 Receiver demodulator circuit

Figure 3 Sampling comparison circuit Figure 4 Receiving status monitoring and display circuit

2. Receive demodulation circuit

The receiving chip LA1650 is used to receive and demodulate the JJY40 timing signal. This chip from SANYO is specially designed for receiving the JJY40 timing signal, and its receiving sensitivity reaches 10dBμV. The circuit is shown in Figure 2. The recommended operating voltage of LA1650 is 1.5V, with a chip selection control terminal PON and a JJY baseband inverted signal output terminal TCO'. Since the MCU level is 5V, the port connection needs to be level-converted. By using the OC gate characteristics of the P0 port of the 51 single-chip microcomputer, an external pull-up resistor and a 1.5V level can be used to convert the level of the PON end. For the demodulated signal output terminal TCO', the comparator LM311 can be used to convert the level and invert the signal. The input terminal 1.5V is a high level, and the comparison threshold is set to about 0.8V. Therefore, the voltage divider resistors for 5V are selected as 4.7kΩ and 1kΩ, and the circuit is shown in Figure 3.

3. Baseband processing circuit

According to the design principle, the STC89C52 microcontroller is used as the baseband signal processing core to realize the basic functions. Among them, the LCD display uses LCD1602, and the keyboard uses a 4-button independent keyboard for function setting. Its functions are: forced reception switch button; time menu selection button; unit increase button; unit decrease button.

In order to monitor the reception status of the signal, LED lights are used for display, as shown in Figure 4. The resistor is an external pull-up resistor of the P0 port. The functions of each light are as follows: second quasi-synchronous monitoring, lights up after second quasi-synchronous; second synchronization monitoring, lights up after second synchronization; minute synchronization monitoring, lights up after minute synchronization; receiving error monitoring, lights up after error occurs; parity monitoring, lights up after parity check judgment error; final conversion monitoring, lights up after data error is found during the final information conversion; JJY receives the demodulated baseband signal indication, displays the signal in real time, high level lights up and low level turns off.

4. Power circuit

The power supply needs to output 5V and 1.5V to power the microcontroller and the receiving chip respectively. The voltage regulator 7805 and LM317 are used to generate 5V and 1.5V, as shown in Figure 5. The 1.5V is generated according to

V0 = (1 + R _{30 ^/} R ₂₉ ) × 1.25 (V) + (R30 × 50 (μA)).
After calculation, R29 is selected as 1kΩ and R30 is selected as 200Ω.

Figure 6 Debugging the receiving antenna

Software Program

According to the program flow chart and the JJY signal receiving and decoding ideas, the C language program can be completed. The C language source code of the 8051 microcontroller can be downloaded from the website of "Radio" magazine.

Debugging Tests

According to the circuit diagram, make a PCB circuit board, then solder the components and write a program, and then you can carry out debugging and testing.

1. Receiving antenna

The debugging of the receiving antenna is mainly to adjust the center frequency of the receiving resonant loop. By adjusting the position of the magnetic core in the coil, using the signal source and oscilloscope to test, the center frequency is set to 40kHz. The test method is shown in Figure 6. Change the frequency of the signal source and use the oscilloscope to observe the signal amplitude. When it is the maximum, the output frequency of the signal source is the center frequency of the loop. This method can make the magnetic core in the position of resonant 40kHz, achieving a good reception frequency selection effect.

2. Receive demodulation circuit

Replace the input receiving antenna loop with a 51Ω resistor, which is equivalent to the state when there is no signal input, that is, the static working state. Supply 1.5V to the chip, measure the potential of each pin, refer to the chip data, the error between the measured value and the reference value is very small, indicating that the static working state is normal. Then switch the input back to the receiving antenna loop, and set the chip enable terminal high, use the signal source to generate a 2ASK signal (amplitude mV level) with a carrier frequency of 40kHz sine wave and a modulation signal of 1Hz square wave, and add the modulated signal to the chip through coupling (as shown in Figure 7), and use an oscilloscope to observe the waveform at the output end of the chip. If it is a 1Hz square wave, it means that the demodulated signal can be received.