I recently picked up a new tool for the workbench, a Rigol DS1102e oscilloscope! An oscilloscope has been on my wish list for a while, as it is an essential tool for electronics design and debugging. They are also extremely useful for measuring and tuning RF circuits.
To present how an oscilloscope is useful for RF transmitter design, I will measure some of the circuits we have previously explored. A mini review of the scope is also included below.
First up is the basic crystal oscillator transmitter. Crystal oscillators produce a square wave, which generates odd harmonics. To measure the oscillation, build the circuit, and connect your probe to pin 3 on the oscillator and the ground probe to ground or pin 2. You should see something like this:
The top reading in yellow is showing the signal generated by the crystal oscillator, while the bottom in purple is showing the Fast Fourier Transform (FFT) of the signal. The signal isn't a perfect square wave, but it's close enough. You can see the frequency measurement at the bottom of the screen showing 1.00 MHz. The FFT is useful for analyzing harmonics generated by the signal, and you can clearly see the odd harmonics generated by the square wave.
Next, we will look at some measurements from the Michigan Might Mite transmitter. For the first measurement we will look at the waveform inside the oscillator stage of the circuit. Hook the probe to one leg of the crystal and the ground probe to ground. The waveform will look like this:
You can see that the oscillator is generating a sine wave at 6.925 MHz, exactly as it should (because I'm using a 6.925 MHz crystal). The sine wave isn't perfect, but it's pretty good. You can also see that it produces a few harmonics, by referring to the purple FFT. By tuning the circuit with the variable capacitor you will see the sine wave change in amplitude and shape. When it moves out of the proper range, the radio signal will drop out. In general, you will want to tune the circuit to have the highest amplitude and/or the most clean waveform.
Hooking the probe to the transmitter output is where things get really messy. Be careful hooking up your scope to a transmitter antenna input! This is only recommended for low power transmitters, as you can blow up your scope if the voltages range too high!
Connect the transmitter to a dummy load, and connect your probe to the dummy load. For this test I'm just using a 50 ohm resistor as a dummy load (a 47 ohm resistor will work fine too). The wave form generated looks like this:
As you can see, the waveform isn't clean at all, and looking at the FFT we can see many powerful harmonics that are nearly as strong as our desired output frequency! These harmonics are a design limitation of the Michigan Mighty Mite that can be cleaned up with a low pass filter
For the final measurement, I connected the transmitter to my 40 meters low pass filter, connected the filter to a dummy load, and measured the waveform again, by connecting my probe to the dummy load resistor. The wave form generated is a very clean sine wave! Looking at the FFT there is one clean spike at 6.925 MHz, which means the low pass filter is working perfectly, eliminating all the harmonics generated in the AM signal!
Posted: Sep 09, 2013
Keyword tags: electronicsmeasurementRFoscilloscopeharmonicsessential equipmentcrystal oscillatortransmitter