More CB Goodies: AM to CB converter

 Another impulse buy, but one that has lots of historical significance and one that sparked my curiosity. What came in the mail this time?

Product Description

Nothing more than an upconverter for CB radio. That said, how does it work?

If you recall one of my previous posts, I built a up/downconverter for my old crystal-controlled scanners using an Si5351 and a basic single Schottky (or germanium) diode mixer. It was about as passive as things could get save for the Arduino board running the 5351. I can confirm that it works in theory and actuality, provided that you know basic math and how mixing works. It's not overly sensitive, but that could be changed with more bandpass filtering and an added LNA on the antenna input side. Regardless, it was more proof-of-concept than anything and it worked for what I needed it to do - bring a crystal radio scanner back from the dead to receive NOAA weather broadcasts.

The device above and many like it came out in the early to mid-1970s during the heyday of CB radio. It allows you to tune across the 23 channels CB originally had before 1977. I imagine this would tune up to channel 40 but I won't know until I bust into it and reverse engineer it. This works better with an analog tuning AM radio in comparison to a digital AM tuner. It will still work if your vehicle only has digital tuning but you'll miss out on the fine tuning aspect.

I mostly bought this because it was $10. And it was old. I like old things. And I wanted to see what kind of circuitry it dealt with. My guess is that this is going to be much more complex than my single diode converter. It does run off of 12VDC so there's got to be active components in here like ICs and transistors. 

I imagine with some creativity some folks could have gotten these to receive 10 meter signals or really any signal, as long as it was AM-modulated. With an added BFO, your car's AM radio could turn into a decent HF SSB/CW receiver. Again, it's all about mixing. Isn't RF circuitry wonderful?

Remix Review

Okay, so just to recap, a mixer is a wonderful device. A mixer takes a frequency and coverts it to another frequency. Mixers can either add or subtract frequencies, thereby transforming them into signals that are easier to filter out. An AM CB, for example, will usually downconvert 27MHz all the way down to 455KHz. The Midland 13-857 downconverts 27MHz to 10.7MHz and then to 455KHz. We call this frequency the intermediate frequency. After that we can filter or form the received signal into any way, shape or form we desire.

Most mixers are constructed using diodes. The simplest mixer you can make is made with a single low-voltage diode, either Schottky or germanium type. Common (and excellent) diodes include the 1N5711, the 1N34A, and the OA95. Any Schottky or germanium type will work, but make sure it has a super low turn-on/operating voltage, somewhere on the order of tenths or hundredths of volts. 0.2V is a great starting point.

From looking at the box, this converter claims to have "Mos-fet technology," which is most likely true. I've found a schematic to a similar converter ("Roadmate CB Converter" from Popular Electronics, Oct 1976) and sure enough, it uses a popular and readily-available MPF102 FET.

I'm still trying to figure out why the mixed crystal is 8MHz or so. That said I think it has to do with using the crystal in overtone mode.

Not many active components, as the main mixing element is the MPF102 and the NPN transistor is most likely a part of the oscillator section.