Tinkering with the oscillator voltage

Tinkering with the oscillator voltage

Soil Moisture Sensor Project

Tinkering with the oscillator voltage

After experimenting with heat, it was time to change the voltage supplied to the oscillator to see if that would have much of an effect on the output.

This is not just for fun. The power supply to the entire system will run around the garden. Voltages can decrease over long runs of wire. This means that there may be oscillators that receive lower voltages simply because they are further away from the power supply.

This graph shows what happened when I changed the voltage. The voltage started at 5v which corresponded to 62.9MHz. Then I decreased the voltage to 4.5v which corresponded to 62.6MHz. Finally I increased the voltage up to 11.5v which corresponded to 63.7MHz.

So this means that a voltage change of even half a volt can shift the frequency.

Ripple in the power supply

This got me wondering whether there was an effect of ripple in the power supply.

If you connect a battery to a circuit, you will get a constant voltage coming out from that battery until it starts to run down. But when you start converting between voltages or between AC and DC, there will be some tiny fluctuations in the DC.

Taken from https://www.homemade-circuits.com/whats-ripple-current-in-power-supplies/

This picture shows the process of using a smoothing capacitor to try and flatten out the ripple in the converted voltage.

So how bad can it be?

This bad.

When I removed the smoothing capacitor from the circuit, this is what I saw. The receiver is picking up frequencies between 106.5MHz and 107MHz. I don’t know the terminology to describe this signal, but it doesn’t seem right.

In comparison, here is what I saw when the 0.1uF ceramic capacitor was in place. There is still some spread in the oscillator output across a small range of frequencies. Much better, but I think we can improve.

Now this looks much better. After adding a 0.15uF polyester capacitor I got a much narrower signal.

What does this mean for me?

This means that I need to pay a lot of attention to the power supply of the system. Particularly the power supply to the oscillator.

It appears that the different types of capacitors have different strengths. At the moment, I am limited to when I have left in my electronics box. The ceramic capacitor didn’t do as good a job as the polyester capacitor, but I will need to learn more about this.

I haven’t even looked at what a form of inductor called a RF choke. This is just an inductor that tries to prevent electromagnetic interference from being carried down a wire.

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