16300 MHz Testing Colpitts oscillator 

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 RF Oscillator Circuits / Oscillator Circuits
To make a 50 Ohm output, so the generator can drive a 50 Ohm cable, I measured the output impedance of Q3 while using a 220 Ohm source resistor. This was done by measuring the Peaktopeak output voltage without a substantial load, at 20 MHz, then loading the source with an additional 470 Ohm resistor, and calculating the effective output resistance of the source follower based on the change in amplitude. This value was found to be about 139 Ohms. This is much simpler and more direct than trying to calculate the output impedance as a function of data sheet values, since the key parameters such as IDon vary too widely to allow design without trimming. The output impedance is about 50 Ohms at 20 MHz, but this design approach does not separate out the resistive from the reactive part of the impedance, and I expect the impedance to change as a function of frequency.
Using a program that seeks numerical solutions to formulae, I found the values of resistors for R11 and R12 that simultaneously satisfy the requirement that the total resistance be equal to 220 Ohms and the resistance from the the connection between R11 and R12 to ground, including the effects of the source follower, is equal to 50 Ohms. The values, R11 = 150 Ohms and R12 = 68 Ohms are the closest 5% resistor values.
The +24 volt power supply was made separately, and is described separately on this site. You can find it with this link: 5 Volt to 24 Volt DC...
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