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Crystal Controlled RF Source

circuit xtal osc I wanted a simple RF source that would give a reasonable output into 50 ohms in order to check things like HF transformers and my growing collection of anonymous ferrite toroids. The circuit is very straightforward, the oscillator I built uses a 12 MHz crystal but the circuit should work over a wide range. It would be a simple matter to arrange for the crystal to be plugged and the oscillator also used for testing crystals.

There was no attempt to ensure high stability and I'm sure it could be improved upon by stabilising the oscillator supply and separating it from the buffer supply. (Click on the images for a larger version.)

xtal osc Construction: As can be seen, the oscillator is built 'dead bug' style on a piece of single sided copper clad board approximately 55 x 60 mm. The aluminium box cost me 50 pence at a rally and I kick myself for only buying a couple! The long sides are folded in and have 4 holes for taking small self-tapping screws, which makes assembly very simple, two screws in the sides accept slots in the lid and the whole is a nicely screened box.

The transformer, T1, is wound on an unmarked ferrite toroid removed from a defunct switch mode lighting PSU. Twenty turns are wound evenly over the ferrite and five turns are then wound over the top of the first winding. The output socket is a standard phono which is good up to at least 48 MHz in this case.

The output is approximately 8 mW into 50 ohms (I don't have an accurate means of measurement low power at RF), I see 0.63 volts RMS from my home made probe into a DVM and around 0.8 V peak to peak on my 10 MHz oscilloscope. The output looks very clean. Loosely coupled to my RX (50 ohm resistor in the oscillator output socket and dipole in the attic) the level is 5-9+40 db and is approximately 30 db down on that at 24, 36 & 48 MHz.