Science Buddies: "Ask an Expert"

We are able to design the following project for middle school Science fair.
https://www.sciencebuddies.org/science- ... #procedure
We used only 1Mhz oscillator.We are not using 1.20MHz Oscillator.We do not have Oscilloscope.We are able to measure longer distances of output signal with longer antenna wires.
The question is
what other parameters of out put signal can we measure?
If we send constant input signal( for example 440Hz input signal),is it reasonable to show that same output signal frequency is obtained using any frequency app?
If we decrease the input voltage ,will the output decrease?What should be the effect?
Where exactly carrier signal is given?transformer or Oscilloscope.
Can anyone please explain the circuits (including changes in amplitude voltage and frequency of the signal)?
We need to submit tomorrow

Take a look at the explanation of the circuit in the source web site http://sci-toys.com/scitoys/scitoys/rad ... itter.html (listed in the Summary page of the project).

The input signal has no effect on the transmitted frequency but it simply causes the oscillator to get more or less voltage, which makes the output stronger or weaker. These changes cause the output to have amplitude modulation which is what your AM radio is looking for. Without an oscilliscope there isn't much else you can see or measure except for the useful distance. If your AM radio has a signal strength meter on it, that will at least give you some measure of how strong your signal is.

Howard

i got your point.I understand that the receiver frequency depends on the oscillator(not on the input signal frequency).
I measured the frequency of output signal from the radio using the app to show that the output signal is same as input.
In AM modulation ,only the input signal amplitude is changed.Frequency of input signal remains the same.
I am trying to prove that.Please let me know ASAP.

This is how AM modulation works, and it's the same information you've probably seen from other sources.


If you had an oscilloscope you could see the output of the oscillator without any audio input looks like the top signal (although the waves might be square instead of smooth). The oscilloscope connected to the output of your audio source would also be a changing wave - a steady one like the second in that photo if you used a sine wave generator. More like

if it was music or speech. Imagine if the carrier wave was a piece fo clay you rolled into a long snake or cylinder. Grab one part of it and squeeze - leaving your finger indentations in the clay. If the smooth shape of the clay before represents the carrier wave, the finger indentations are the audio wave and the new shape is the amplitude modulated wave (the third part of the first image).

Without an oscilloscope you can't 'prove' that it's working. The mere fact that the AM radio (which requires that specific audio encoding method) can pick up the signal shows that it works.

Howard

Does it makes sense to draw the graph between radio frequencies and output signal showing the output zero at all frequencies ,maximum signal at 1MHz.Exactly what should be the parameter on Y - axis and what amplitude at 1MHz frequency .Can we measure output voltage (around 2.7V)at the antenna and show that as the output at 1MHz.Please help us.

You can only graph data that is the result of something else changing. If you were working with a slingshot and did tests where you pulled back on the rubber band varying amounts you could then plot the distance your shot traveled. In this case, if you could measure signal strength with a proper RF meter you could plot that as a function of supply voltage. You don't have the equipment to measure signal strength and you didn't change the 9V battery during your tests. Your frequency never changed because that was controlled by the crystal oscillator. If you had access to an oscilloscope you could have measured the amplitude of the output as a function of the amplitude of the audio tone input. Can you get access to an oscilliscope? The graph will simply be a straight line but is a measurement you could make.

"Can we measure output voltage (around 2.7V)at the antenna and show that as the output at 1MHz?"

Sorry, no. A multimeter won't generally tell you anything useful about a radio frequency signal unless it's a steady tone and you have an expensive meter. Given the tests you said you've done I don't think you really have any data to graph. Your comment earlier about the audio coming out of the radio being the same frequency as the input is really important though. It demonstrates that the transmitter successfully delivers the signal intact. You may be able to use spavce on your display board by including screen shots of the apps showing that your test tones got transmitted. You could also draw your own versions of the waveforms showing what you think an oscilloscope would have displayed.

Howard