radio.html

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Since my first crystal receiver so long ago, there's been something about grabbing a signal out of the air and listening to it for free that has intrigued me in such a way as to keep me building simple radio receivers. In the fifteen or so years of my electronics hobby, I've built several simple radio receiver projects. Most of them AM for the broadcast band shortwave receivers. They all seem to share the same dismal performance characteristics with poor selectivity and low scratchy audio.

Not so with the 'One Transistor FM Receiver'. This radio is sensitive enough to tune 20 stations across the FM band, some with volume high enough to drive a small PM speaker. The ability to tune 88.9 MHz and 89.1 MHz is testimony of its selectivity. The signal-to-noise ratio rivals that of some of the better walkman type radios.

I recently built an SCA decoder to listen in on the FM sub-carrier stations. It's perfectly legal if not used for making money. The input of the SCA decoder required that I modify an existing FM radio in order to pick-off the FM carrier at the detector. It was during the modification that I became interested in building my own FM receiver. I wanted to keep the parts count low while maintaining adequate performance and build it into the same cabinet as the decoder.

Pouring over dozens of schematics and kits, I elected to experiment with the TDA7000. Though its performance is very good (with a five foot antenna), I was unable to access the detector circuitry buried deep within the IC. Back to the drawing board...

While searching for another alternate approach (still wanting to avoid the complexity of the common heterodyne design), I found the one transistor FM receiver called the "Radio Shack Special" by Patrick Cambre.

The simplicity of the circuit piqued the interest of this 'keep-it-simple-Simon' hobbyist and I set out to build it. Patrick has put together an excellent construction article for the "Radio Shack Special" and anyone wishing to build one should have no difficulty following his instructions. I've never been really good at following instructions, so I'll detail the few differences in my circuit.

The picture shows that I stayed pretty close to Patrick's recommendations with the component layout, though I opted for the 'through-hole' approach. The two rules I find most helpful in RF layout are:

(2) Keep the ground path short and direct so that it doesn't wind all over the place creating stray capacitances.

I easily become cross-eyed while counting windings on a coil and I ended up with seven instead of six. By my caluculations, it has about 164nH of inductance before stretching it out to about a 1/2 inch in length. The extra winding gives this radio a broader tuning range without affecting the selectivity, but limits the Cmax/Cmin ratio using a standard 150pF variable capacitor. The entire band is tuned using less than a third of the capacitors' range. A steady hand and some patience are required while tuning this touchy arrangement.

The RF choke was built per the plan, on a 5/16 inch wooden dowel and glued to the circuit board.

The 10K ohm fixed resistor can be replaced with a potentiometer and set as low as 6K ohm to experiment with the regeneration voltage or driving a small PM speaker.

The amplifier section is straightforward with no deviations from the plan. This picture also gives a close-up of the antenna hook-up. The single copper trace runs the full length of the 4 and 1/2 inch circuit board. Using a bulldog clip and about 10 inches of 22 AWG magnet wire, I built an auxillary antenna to better receive the high end of the band; though I've found it to be unnecessary except for those far-away stations. I devised a handy thumbwheel for the regeneration voltage (gain potentiometer) because tweaking this pot while tuning across the band is very necessary.

The wooden stand holds this circuit right out front for all to see the genius of its design and to subject it to any and all EMI and RFI noise however, except for the secondary oscillator images created by this simple circuit being imposed on the audio section, its fidelity is quite good. The back view shows the power switch mounting and the 9V battery clip.

That's all there is to building this excellent little FM receiver. I'm still trying to figure out the theory behind the works!

Oh yeah, the SCA decoder? I've been having so much fun with the Radio Shack Special that I haven't been working on it. Maybe if I buffer the filtered detector output...we'll see!

Hope this article has been of help to some, feel free to e-mail me if you have questions on it or can help me with the theory of operation.