I remember one time when I was young where we had to take shelter during a tornado warning. As a Midwesterner, you get used to stuff like this. When we were huddled in the basement waiting for the storm to pass, I remember my dad having a black box in his hand, with a grey top and two antennas sticking out from it. It looked like an old walkie-talkie, but in fact, this box would end up changing my life forever.
Old Technology, New Ideas
That radio turned out to be an early 1980s radio scanner. It had 4 channels, each controlled by a crystal. These crystals drove me to research all I could about crystal oscillators and their function, and now that I'm a ham radio operator, I understand them even more.
Crystals fascinated me and captivated my interests. How could a single disk made from one of the most common materials here on Earth create such a pure, accurate signal source? I had to find out. So, I pulled the crystals from the radio. Channel 1 was 42.060, Channel 2 was 156.210, Channel 3 was 155.100, and Channel 4 was 155.730 - all common public safety frequencies in the city of Maryville (well, except Channel 1 - not sure why that was there...). I immediately took to Google to find more information and more crystals.
Sadly, crystals are an old, bygone technology. Synthesized frequency generators and DDS modules have since replaced the crystal, as well as PLL circuitry and VCOs. I was saddened to find out that the crystals that did exist only went up to a certain frequency. After that, the quartz wafers would be too fragile to handle. So the science behind crystals festered in my mind. I had given up hope.
Or Did I?
A few years before I was a licensed ham, I had learned about overtones. No, not from electronics - from none other than music! Each instrument has its own timbre that contributes to the sound of the orchestra. Sometimes, when instruments are so in tune, they can produce overtones, or tones that are even multiples of its fundamental frequency. It hit me as hard as a train - crystals also use this concept! Of course, a woodwind/clarinet player like me had missed the point: my reed acted as the crystal's wafer! I had my first eureka moment ever.
So I took to Google again, and this time, I learned about frequency mixing. We know as hams that some radios use intermediate frequencies, or IF. The IF of the 856, I soon found out, was 10.7MHz. Right away I knew something wasn't right. The crystals in the scanners were operating at a different frequency.
Recently, I went back over my calculations, and I concurred just as I had expected. The way to find a crystal's frequency is as follows:
Fund. = (Marked freq. - 10.7) / 3
Let's do an example. Say I wanted to tune at or near the local NOAA weather station on 162.425MHz. I can plug in the values like this. We take 162.425 multiplied by 3 (the harmonic) and add 10.7. As a result, we'd get somewhere near 50MHz!
A Job for the Si5351???
Besides my Pixie adventure, I want to see if I can get this scanner to scan much more than the inactive public safety channels in our area. The old technology is gone, so why not use a generator like the Si5351? It's cheap, readily available, and relatively small!
I think it works like this: Fund. freq. >>> Multiplier circuitry (Fund. x 3) >>> IF (10.7MHz). Please correct me if I'm wrong! If we can generate a fundamental frequency, say 16 or 17MHz, we should be able to get a product of 162.425! But that's not all...
This scanner can pick up UHF signals as well, including GMRS and FRS. We'll need to generate a higher frequency to accomplish this, only a few MHz above. I think an Si5351 will do just the trick, and will hold a frequency as stable as a crystal!
I did some experimentation last night with an LC circuit, comprised of an axial 10uH inductor and a 10pF trimmer capacitor. I can't say it failed, but I can't say it was a success either. It was a little difficult to tune to the weather frequency but there were times where I was positioned just right and could receive it. I might have to experiment a bit more, but if all else fails, the Si5351 might do just the trick!
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