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Oscillator (for FM radio)

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mstechca

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What is the best oscillator to use for FM, and how do I make it using any combination of the following: transistors, resistors, capacitors, inductors, diodes, and power?

Someone here mentioned that an oscillator in an FM radio circuit is set to about 10.7Mhz. Why does it need to be 10.7Mhz?
 

stevez

Active Member
The 10.7 mHz is probably the IF or Intermediate Frequency. In most radios the receiver mixes the desired incoming signal with a stable RF signal so that the sum or difference is equal to the desired IF.

If you wanted to listen to 98.7 FM and had a 10.7 mHz IF you'd want to generate an 88.0 mHz signal so that the difference was 10.7 mHz. You'd probably generate the 88.0 mHz signal with an LC or crystal at some lower frequency then multiply it several times to get 88. A similar scheme might apply to a synthesized oscillator.

Find the schematic for this section of a transistorized FM radio and you are all set. It's been done a million times by manufacturers of radios. Not something that the average hobbyist would mess with although I am sure it's been done with great success.

While this isn't exactly the answer to your question it might help you look for info that suits your needs.
 

crashsite

Banned
NEWS FLASH! FM Oscillator reply gets totally out of control!

Hi,

I'm assuming that you are interested in building an oscillator that will transmit to an FM radio. I used to build these things for a number of purposes and frequencies. The circuit is very simple and can be easily built on a small piece of perf board (and other ways as well).

But, it's really more practical to just buy a cheap FM wireless microphone if you want to have a practical device. It will typically have similar performance and range as a simple home made oscillator. Unfortunately, it would be cheaper to buy one than to buy the parts to build one (it kind of takes the joy out of building when you can just buy one cheaper).

If you're trying to use this as a learning opportunity, you might try to find an older FM transistor radio and cut out the local oscillator (the one that mixes with the incoming radio signal to create the IF as described in a previous message). But, the new FM radios are all on an IC and you can't isolate the different parts anymore. I good source for older radios is thrift stores and garage sales.

The AFC (automatic frequency control) diode can be used as an FM modulator with some modifications to properly bias it. Wow! This is really getting pretty heavy, huh? But, this was the kind of stuff folks knew and did back in the olden days before computers made techno-idiots of us.

As an educational exercise it's excellent as it requires you to learn about the different sections of a radio and how they work. These same principles are still used in just about any radio or microwave equipment and a solid, practical knowledge of radio is still a desirable and salable skill.

As to the values of the components for the FM band, you should expect the coils to be a few micro henries and the capacitors to be a few pico farads. Just about any modern small signal type transistor should work at aroun 100 MHz (the FM band). There are formulas for figuring out the values but, it's easier to use something called a nomograph which allows you to quickly and easily find various things (like the values needed for this purpose).

I've scanned a couple of pages from an old Radio Shack data book that includes the frequency range you need (http://www.madras.net/sala/xfer/reactance_nomograph.jpg). Print the picture. Then, pick any two values you have (or want) and draw a straight line through them. Read the other values along the line. Hint: At resonance, XL = XC (capacitive and inductive reactance are equal) and should be about 1000 Ohms for a small oscillator. That will suggest typical values for your coil and capacitor.

The capacitor will usually be factory-made but, other nomographs can be used to give you coil winding data (coils for this frequencey are typically just a few turns and are very easy to make).

Well anyway, it was kind of fun dredging up some of my old electronics knowledge. Kind of refreshing after all the time spent at the computer lately.
 
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