Re: For what is a crystal spotter used?

Jim, N5IB

The question asked was, "why not just power the Bayou Jumper's oscillator, but not the PA, and just listen for the signal in the receiver?"

The answer is - The Bayou Jumper does in fact work exactly that way. When you press the key while in the receive mode, the oscillator is activated, but not the PA. It was designed to do that. The signal can be heard in the receiver. Great, just what's needed, right ?!

Well, not so much....

Remember, the receiver is a regen. That means that any strong signal will pull the receiver onto the signal frequency. And remember that the source of that signal (many many millivolts) is right on the same PC board, just a few cm away from the detector. The zero beat "point" on the dial ends up being a broad region, with no some strong over a fair amount of dial in the middle and squeals on either side.

Now, if you run the regeneration control all the way to max, run the attenuation all the way to max attenuation, and turn the AF gain way down, you can sometimes just about get away with it. But then you have to reset everything back to normal receiving. Much simpler to have a little oscillator sitting next to the rig. Touch the crystal to it, and a clearly discernible, but not overly strong, signal can be heard. Tune to zero beat, pop the crystal back in the rig (maybe mark the dial) and you're ready to roll. You'll hear everything with three of four kHz of the zero beat point.

Bottom line, there's just not an easy way to knock down the signal strength of the internal oscillator to the point where it will play nice with a regen detector. BTW - the original Paraset couldn't spot it's own signal either. The B+ was switched off the receiver during transmit. But, they didn't have to. They knew where to listen for London (definitely QRO!) , and London knew what crystals they had.

BTW - When David designed the Soup'er Up'er he had to fight that problem to get a working sidetone. The audio amp had to still be powered during Tx, but he had to knock down the Rx output with a MOSFET switch.

Jim, N5IB

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