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250MHz RF Generator

250MHz RF Generator

This project shows how you can build a RF signal generator with a frequency range of 0 to 250MHz.This is perfect to use for testing and tuning any radio equipment or filter.The frequency is PLL controlled and a 2 line display shows the exact frequency.The frequency is PLL controlled and a 2 line display shows the exact frequency.The output frequency is a combination of digital step-tuning 50kHz, and analogue fine-tuning (10 turn pot) to a precision of a few Hz.
Here is the basic principal of the project. (Take a look at the block diagram in the website)
It is impossible to build an oscillator that will work from 0 Hz to 250MHz in one sweep. The solution must be to use two oscillators and a mixer.
If you mix two frequencies you will get a lot of products. f1,f2, f2-f1, f2+f1 and lot of multiplier of these. The interesting product in this case is the f2-f1 product. f1 and f2 are produced by two 900MHz VCOs which are controlled by 2 PLL circuits.
f1 will be constant at 820 MHz and f2 will step from 820 MHz to 1070 MHz.
Since the output from the mixer will be f2 – f1, we get 1070 – 820 = 250 MHz as max difference..Great! How about the other frequencies?
Well, since they all will be quit high we can use a low pass filter to reject them.
As you know, a PLL regulating system need a reference frequency to be stable and work properly.
In this project I use a very stable and exact 12.8MHz TCXO to control PLL 1 and VCO 1. This TCXO will also feed the PIC16F870.
PLL 2 and VCO 2 has a different reference frequency feed from a VCXO (Voltage controlled Crystal oscillator).
This oscillator is also very stable but the nice thing is that the frequency can be varied by a controlling voltage (Tune).


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