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The NAND gate functions as a switching inverter and can be used to build various pulse waveform generating circuits. The basic working principle involves the charging and discharging of a capacitor. When the input voltage reaches the threshold voltage (VT) of the NAND gate, the output state changes. Therefore, the pulse parameters of the circuit are directly influenced by the RC components in the circuit.
1. Asymmetric Multivibrator
As shown in Figure 12-1, NOT gate 3 is used for waveform shaping. The output waveform of an asymmetric multivibrator is not symmetrical. When using TTL NAND gates, the pulse widths are Tw1 = RC, Tw2 = 1.2RC, and the total period T = 2.2RC. Adjusting R and C values allows control over the oscillation frequency. Typically, C is used for coarse adjustment, while a potentiometer R is used for fine tuning.
2. Symmetric Multivibrator
As illustrated in Figure 12-2, the circuit is completely symmetrical, resulting in equal charge and discharge time constants, producing a symmetrical square wave. Changing R and C values affects the output frequency. Gate 3 is used for waveform shaping. Normally, R should be ≤ 1KΩ. When R = 1KΩ, C ranges from 100pF to 100μF, with frequencies ranging from nHz to nMHz. Pulse width is tw1 = tw2 = 0.7RC, and the total period T = 1.4RC.
3. Ring Oscillator with RC Circuit
Figure 12-3 shows a ring oscillator circuit where NOT gate 4 is used for waveform shaping. R acts as a current-limiting resistor, typically around 100Ω, and the potentiometer Rw should be ≤ 1KΩ. The capacitor C controls the voltage at point D, which determines the opening and closing of the NAND gate, creating a multi-resonant oscillation. The charging time (tw1), discharging time (tw2), and total period (T) are approximately: tw1 ≈ 0.94RC, tw2 ≈ 1.26RC, T ≈ 2.2RC. Adjusting R and C modifies the oscillation frequency.
The state transitions in these circuits occur when the input level reaches the threshold voltage VT of the NAND gate. However, the capacitor's charging and discharging near VT is slow, and the threshold voltage itself may not be stable due to factors like temperature, power supply fluctuations, and noise, leading to poor frequency stability.
4. Quartz Crystal Frequency-Stabilized Multivibrator
When high frequency stability is required, traditional multivibrators may not suffice. Quartz crystals are often used as a reference for signal frequency. A multivibrator built with a quartz crystal and logic gates is commonly used to provide clock signals in microcomputers and similar devices.
Figure 12-4 shows a typical crystal-stabilized multivibrator. (a) and (b) represent a crystal oscillator using TTL devices, while (c) and (d) show one using CMOS devices, commonly found in electronic watches operating at 32768 Hz. In Figure 12-4(c), gate 1 is used for oscillation, and gate 2 is for buffer shaping. Rf is a feedback resistor, usually around 22MΩ, and R is a stabilizing resistor, typically between 10kΩ and several hundred kΩ. C1 is a trimming capacitor for frequency adjustment, and C2 helps correct temperature effects.
**Second, Experiment Purpose**
1. Understand how to use gate circuits to create pulse signal generators.
2. Learn how to calculate timing component values that affect the output pulse waveform.
3. Study the principle of quartz crystal frequency stabilization and how to construct an oscillator using a crystal.
**Third, Experimental Equipment**
1. +5V DC power supply
2. Dual-trace oscilloscope
3. Digital frequency meter
4. 74LS00 (or CC4011) NAND gate, 32768Hz crystal oscillator, potentiometers, resistors, and capacitors.
**Fourth, Experimental Content**
1. Build an asymmetric multivibrator using 74LS00 according to Figure 12-1, with R as a 10KΩ potentiometer and C as 0.01μF.
- Observe and record the output waveform and the capacitor voltage.
- Adjust the potentiometer and measure the upper and lower frequency limits.
- Use a 100μF capacitor across pins 14 and 7 of the 74LS00 to observe any waveform distortion or power supply ripple.
2. Connect a symmetric multivibrator using 74LS00 as per Figure 12-2, with R = 1KΩ and C = 0.047μF. Observe and record the output waveform.
3. Construct a ring oscillator as shown in Figure 12-3. Use a 510Ω and 1KΩ potentiometer in series with RW, set R = 100Ω, and C = 0.1μF.
- When RW is at maximum, observe and record the voltages at points A, B, D, E, and v0. Measure the period T and compare it with theoretical calculations.
- Adjust RW and observe the changes in the output waveform v0.
4. Set up the circuit according to Figure 12-4(c). Use a 32768Hz crystal oscillator with a CC4011 NAND gate. Observe the output waveform with an oscilloscope and measure the frequency with a frequency meter.
**Fifth, Experimental Preparation**
1. Review the working principle of self-excited multivibrators.
2. Draw a detailed experimental circuit diagram.
3. Prepare data sheets and recording forms.
**Sixth, Experimental Report**
1. Draw the circuit and compare your measured results with theoretical values.
2. Plot the observed waveforms on graph paper and analyze the experimental outcomes.