QSpice (8) --Examples and Command Probe usage

Hello uu, based on the experience of learning Spice before, I believe it will not be a problem for us to build a circuit by ourselves now. The best way to learn is to get moving, use things, encounter some problems and solve them. I believe you will be able to master it quickly.

Today we will use the Probe by building a capacitance measurement circuit (changing the capacitance to change the 555 output frequency and the MCU detecting the 555 output frequency to determine the capacitance value) as an example to use other commands.

Although there are not many devices in QSpice, there is 555 (symbol&ip--analog--NE555). We can use 555 to build an astable circuit. The circuit is shown in Figure 1 below, referring to the TI NE555 manual. We just need to build the circuit according to this.

Figure 1: NE555 astable circuit

The frequency here is determined by RA|RB|C. Assuming that we keep the resistance unchanged and only change the capacitance to change the frequency, the calculation method is as follows:

Figure 2: 555 frequency capacitance calculation

CalcPad formula:

Now that we have the parameters and the circuit, we can build the circuit in Qspice as follows:

Figure 3: Qspice astable schematic

|Tips: QSpice can paste pictures directly into the schematic diagram|

When we simulate, we usually need to click on the port to add the simulated waveform to the simulation observation window. We can use .Probe to add the waveform. If it is a voltage waveform, it is V(xxx). xxx represents the network name. If it is a current waveform, it is I(xxx). If you want to see multiple waveforms, you need to use ", " to separate them. Don't be stupid enough to copy "". The above example observes the OUT voltage waveform and C1 current waveform. The simulation code is as follows, and the simulation result is shown in Figure 4:

Simulation code:

Figure 4: Simulation results

Zooming in on the waveform, we find that the result is not the frequency we calculated, and there is still a little gap:

Figure 4: Enlarged simulation result - frequency 73.945KHz

If we look at the NE555 manual, we can see how it takes values. We choose a value of RA+2RB=144Kohm to observe the output frequency under different capacitances. After all, we are just measuring the capacitance value. In order to make the frequency and capacitance correspond better, I chose the value of 144Kohm. 1nF is equal to 10Khz, 10nF=1Khz. Let's use the Step and param commands to simulate it.

Figure 5: NE555 frequency comparison table

Figure 6: Modified circuit

Figure 7: Steps to select simulation

After the adjustment in Figure 6, the simulation waveform can basically match the frequency. The smaller the capacitance, the higher the frequency and the greater the deviation. If you really want to do it, it is recommended to divide it into grades. When the capacitance is small, use a 144Kohm resistor. When the capacitance is large, you can use a 14.4Kohm resistor. Use Relay to cut it. The on-resistance of Relay will be smaller. However, if you pursue volume, you can consider using an analog switch. However, the resistance of the analog switch is not very linear. But the capacitance does not need to be very accurate anyway, just look at it roughly, and the analog switch should be more suitable.

The final simulation code is as follows:

Have a nice weekend~