Frequency Detection and Signal Generator Made with LM567

The circuit shown below is a frequency meter signal generator.

Since LM567 has the characteristics of high input sensitivity (20~200mV) and high input impedance (20kΩ), a pocket frequency meter can be constructed with fewer components. The working principle is: the detected frequency signal is sent from CK through LM567 pin ③, and multiple signals are sent as input signals at the same time. The external RC (W+R2 and C4~C6 in the figure below) of LM567 pin ⑤ and ⑥ and the internal oscillation signal are also sent to the detector to compare with the input signal frequency and phase. After comparison, the signal within the bandwidth range. After internal amplification, a low level is output from its pin ⑧. If there is no signal input, the pin ⑧ is high (same as VCC). The driving current of its output signal reaches 100mA. In the figure below, for frequency timing, the signal pin (self-made) is sent from CK, and W is adjusted (if necessary, K1 is used to convert the capacity of C) to make the oscillation frequency f0 generated by the internal oscillator of LM567 track the external input frequency fin. When fin=f0, the output of LM567C pin ⑧ changes from high level to low level, and the LED lights up. The frequency value indicated by the multi-turn potentiometer with a pointer is the measured frequency fin. The function of D1 and D2 is to limit the amplitude of fin and protect the safety of LM567.

When this circuit is used as a signal generator, the plug of the frequency meter can be unplugged, the left end of C1 is grounded through the socket CK, and the LM567 pin ③ is short-circuited to the ground (to prevent external interference). At this time, the circuit is a signal generator. The signal of the signal generator is output through the VT emitter and then through the CT. Since the circuit is simple, the printed circuit board can be designed and made according to the selected shell. In the figure below, it is best to use a non-polar capacitor for C1. If there is no or the volume is too large, two 100μF/6.3V electrolytic capacitors can be used to connect the two positive and negative electrodes respectively and then connect them according to the figure.

This signal generator is ready to use once installed. The key is to adjust, that is, to calibrate the frequency value of the multi-turn potentiometer pointer scale. With the help of a standard signal generator or a digital frequency meter, the scale can be adjusted. If K1 is allowed to change the frequency of the third gear and the scale cannot overlap, the C5 and C6 values ​​can be increased or decreased to achieve the purpose.

This instrument can also be used as a signal generator when measuring frequency, without affecting each other. Made according to the parameters shown in the figure, the sensitivity is 200mV, and the conversion switch K1 can change the frequency within 100Hz~500kHz, with a relative error within 1%~5%. The frequency timing error is less than 10%, which is large in the low frequency band and small in the high frequency band. This is a shortcoming of this frequency meter.