Building a prototype oscillator

Building a prototype oscillator

Soil Moisture Sensor Project

Building a prototype oscillator

My previous oscillators were built on top of bread board. This is bad for working at radio frequency because bread boards have lost of extra capacitance between tracks and inductance within the tracks. I promised that I would keep the math to a minimum so I won’t show you any extra equations.

Frequency is inversely proportional to the product of the capacitance and inductance of the oscillator. The frequency decreases as the capacitance or inductance increase. The extra capacitance and inductance of the breadboard greatly reduced the frequency from what I calculated that it would be (150MHz) to something  a bit lower (36MHz).

I have always been reluctant to solder something together because I don’t want to throw it away. But this is the time to bite the bullet.

In the picture below, you can see my first attempt at “Manhattan Style” construction. The ground plane is the large copper pad on the bottom. All of the connections between components are made on little islands of copper pad that are stuck on top of the ground plane with double sided tape.

Using this style of construction I was able to increase the frequency from 35-36MHZ to 85MHz. Closer, but not close enough.

After shortening the legs of some of the components, it went up to 91MHz. Better…

Shortening the legs of the transistor increased the frequency to 93MHz.

By this point I figured that I was hitting the Law of Diminishing Returns. I could keep trying to squeeze a bit more frequency out of the oscillator, or I could use it.

Adding the sensor

As soon as I added the sensor to the circuit, the frequency decreased to about 40MHz.

Then I remembered reading something about the fact that a straight length of wire would behave as an inductor. A small inductor, but one that was larger than the little yellow coil you can see in the photos. When I added the sensor, I was adding two of these inductors to the circuit and reducing the frequency of the oscillator.

I tested the effect on the frequency of the oscillator by seeing what the frequency would do as I touched a long and then a short piece of wire onto the circuit.

Firstly, the 10cm wire. It reduced the frequency from 93.5MHz to 87MHz, a drop of 6.5MHz.

Then the 5cm wire. It reduced the frequency from 93.5MHz to 88.6MHz, a drop of 4.9MHz.

So, we get a smaller drop in frequency using a shorter length of wire. This means that the circuit is definitely reacting to the increased inductance as we expected it would. It also means that we need to use as short a length of wire as possible for the sensors.

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