Basic RF Oscillator

Posted on Feb 5, 2014

Make a basic RF oscillator. The circuit is ridiculously simple, so I will skip all the complicated maths and make component selection simple by using calculators. Let us assume I want an oscillator for 7MHz. The circuit looks like this: The first transistor to the left (TR1) is a simple emitter-follower. It has a voltage gain of just under 1. The two 27K resistors can be as high as 100K, but if you want to use a lower gain

Basic RF Oscillator
Click here to download the full size of the above Circuit.

transistor (2N2222 etc) then 27K is good. C2 and C3 work with L1 to form a voltage multiplier. The current is reduced, but TR1 is a current amplifier. In this way we have a total voltage/current gain of more than 1 between base and emitter. It will oscillate. TR2 (the other one) is a simple emitter follower that is used only as a buffer between the oscillator and the outside world. Here then are the brief specifications of the oscillator: I have included a few calculators here. Each time you see the text [Enter] you are being asked to enter data in the field. If you have previously used another calculator then there will already be default start values entered for you. These are the results of your previous calculaton. You may of course enter new values. C1, C2 and C3 are all in series and so form a single tuning capacitor of about 60pf. The ratios of C2:C3 determine the gain and their values may vary quite a lot. In general, C2 = 3x wavelength, C3 = 9x wavelength. C1 can be any value from 1x wavelength to 10x wavelength and is normally chosen to fine-set the frequency of the oscillator. Make C1 start value about the same as C2. The actual values can vary over quite a wide range. This calculator will calculate the total tuning capacitance for your own selected values. In the above calculator, enter a frequency and click the "SHOW C1, C2 & C3" button and the values of C1, C2 & C3 will be displayed. If you then click the...

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