How to Build a Spectrum Analyzer out of a CB

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NOTE: The original ramp generator schematic had an error that would have prevented it from working. The schematic shown at the bottom of the page is correct.

Here's the general idea behind the project.

A spectrum analyzer (for our purposes) is basically a device which displays radio activity within a certain range of frequencies. It does this by synchronizing the horizontal sweep of an o-scope with the swept tuning of a radio receiver. In this project's case, the synchronizing signal is a ramp signal generator, and the receiver is the VCO-controlled receive circuit of a CB. The horizontal trace of the scope is controlled by the ramp signal, which is also controlling the frequency of the VCO in the CB (The VCO is what actually "tunes" the CB radio). When the vertical trace of the o-scope is tied to the CB's detector output, we have a DC voltage generated for every signal present, and those voltages are spaced equally for each chanel that has activity.

As for accuracy and specific measurements, this setup is probably not going to do what you need, but for seeing what activity might be present in the 10 or 11 meter bands, this is just dandy. Besides, its a real kick to know that you built it yourself out of basically junk. Another big plus is that it can be built in less than an hour if you're good at breadboarding, reading schematics, and tracking down specific parts in a CB.

For those of you who like to tinker, here's how to do it. You will need an o-scope with X-Y capability, and any AM-mode, PLL-controlled CB with a working receiver. It would help to have a CB to which you can get the schematic, because you will need to be able to identify the VCO control point, as well as the detector circuit.

Basic steps are as follows:

1. Build a ramp generator capable of at least 4 volts peak-to-peak output (more than 4 is ok, but at least 4 is recommended to give the VCO a decent range) .

2. Tie the output of the ramp generator to the X input of the scope.

3. Tie the output of the generator to the control voltage input of the CB's VCO, after unhooking the VCO from the PLL chip, and eliminate any filtering caps on the VCO control voltage line. The filter caps need to be removed to allow the fast tuning needed here.

4. Tie the Y input of the scope to the output of the AM detector. You will need to invert the scope input if the DC level out of the detector is negative, or your signal levels will be showing upside down on the scope (kinda neat, actually, if you like the strange and unusual. I set mine up to show frequencies in reverse order AND upside down just for kicks. You can't do that with those high-dollar analyzers I'll bet!).

You will need to play with the ramp generator frequency to get the best results. I found that from 10 to 60 hz did pretty well. The faster the ramp, the more the individual signals tended to run together on the display. A cb with a faster-responding VCO will make for a sharper display and better resolution, which will allow faster ramp speeds without loss of display sharpness.

You will also need to play with the settings of the two scope inputs in order to get a usable display. I found that the X input did well at anywhere from 1 VPD to .2VPD, and the Y input I ran at 1VPD, with variations in the calibration on both inputs. The horizontal position control ended up being maxed out on my scope by the time I was happy with the results.

Here are a couple pictures of the one I threw together at the project's start:

Below is a schematic of the basic signal generator used in the project.

Feel free to email with any questions you have.