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Radio Broadcast Technical Consulting and Sales
 10032 Island Drive, Brainerd, MN  56401

 

Technical Tips from Mark W. Persons

AM Transmitter RF Oscillator
Older AM broadcast transmitters have RF oscillator sections that require a "fundamental" crystal to generate the frequency of operation.  That means a station on 540 KHz needs a 540 KHz crystal.  Those crystals are expensive, difficult to make, and are almost unavailable today if the required frequency is below 1000 KHz. 

To work around the problem, I have custom built oscillators that use crystals which run at two or four times the required frequency and then divide that crystal frequency down to the required AM frequency.  This one is 5" x 4" x 3" in size.     

Here is the inside of one that I built years ago and then reused in another transmitter more recently.  For 540 KHz, the crystal runs at 2160 KHz and is divided by four.  The output of the oscillator module goes the the tube grid of the original oscillator stage in the transmitter involved.  Ten or fifteen volts peak to peak of RF is all that is needed to get the job done.

The newer crystals are able to hold frequency within a few Hertz over a relatively wide temperature range without requiring a temperature controlled oven.  The crystal in this one is the silver colored can on the lower left.  It was purchased from Classic Frequency Control in Oklahoma City, OK. 

For those who know something about tubes, the crystal is plugged into a 9-pin miniature tube socket.  This oscillator module is a very good approach for updating, repairing, or retuning old AM transmitters.   

The stories go on and on.  Stop in again sometime.  I'll leave the soldering iron on for you. 
Mark W. Persons   Ham WØMH      

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page last edited 01/31/2010