May 5, 2004
NATE: Not Another TinEar! Click the image for a larger view.
If TinEars are made of tin, what do they make NATEs out of? Back in the days of yore, QST periodically ran a feature titled "Just like QST, except..." It was a warning to those who would deviate from published plans and expect equivalent results. I don't think QST meant to discourage experimentation, only to caution homebrewers against expecting miracles. For example, substituting a 6AQ5 in a circuit designed for a pair of 4-400s was asking for trouble.
Homebrewing from the junk box requires a modular systems approach. Break the circuit down into its component functions. Then you can substitute X for A or Z for B. Lest you say, Oh, but you have to really know your 'trons to do this, I assure you, this ain't rocket science. This OT has never studied engineering; I majored in English Lit and Drama, and have never even taken a calculus course. All I know about designing radios I got from books -- mostly ARRL publications -- and by burning my fingers and the midnight oil.
A friend of mine is building a TinEar kit. I got interested in the project and decided to build something like it. I didn't have a soda straw handy, so the PTO was out. So I made the VFO a VCO instead. If you click the image above you can view a larger version in a new window.
NATE runs from right to left. The VCO is on the right, and the tuning diode is the black 1N4007 in the lower right corner. Next to it is a 1N4148 for temp compensation. I used the TinEar VFO but put a 0.1-µF cap between the bottom of the inductor and the tuning caps. There's a 9-35 pF N650 trimmer, and a combination of silvered-mica and NPØ caps in parallel with the 1N4007.
The 1N4007 is actually a PIN diode and works very well for tuning. As the circuit is now, coverage is wider than the CW subband. I'm going to fix that by reducing the voltage across the tuning pot. The oscillator inductor is about 3 µH. I used a T50-2 core. A T50-6 would be better but I'm running low on those. I wound the RF choke on an FT37-43 core.
The RF amplifier and mixer are pretty much the same as the TinEar's, but I had to substitute a few components. By the way, I used J310 FETs for the VCO, RF amp and mixer, because that's all I had.
I also copied the TinEar audio preamp, but I used a 2N2222A instead of a 2N3904. Because I wanted to get the receiver finished and try it out, I used an LM386 instead of discrete components for the audio output stage. With the 386 bypassed for gain=50 the audio is more than adequate for headphone listening.
The single-sided board is 1-3/8 x 4 inches (35 x 102 mm), cut to fit in a plastic project box.