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|marconi||Posted - 19 September 2012 11:6 |
I have seen several types of early detectors for "undamped" or continuous waves in early wireless literature. One consisted of a steel razor blade mounted on a steel spring contacting a chunk of iron pyrite in a cup with an electromagnet under the spring. The electromagnet was connected to an AC voltage supply. The cup and the spring arm were each connected to the tuning circuit. Another consisted of a telegraph relay with the contact terminals connected to the tuning circuit. The coils of the relay went to an AC supply. Both circuits caused an "interupption" of the detector circuit. Anyone have experience with these? Ever tested anything like this?
I can hear the wobbling sound of (most likely) hams on my shortwave crystal set. It would be great to try and "decipher" their CW or SSB signals using a "period-correct" technique.
|gzimmer||Posted - 19 September 2012 11:44 |
As you know, Continuous Waves have no Amplitude Modulation so they can't be detected by a simple diode. You would only hear a click as the signal began and ended.
Because of this, various methods were used in the receiver to impart some artificial amplitude modulation. Besides the mechanical interrupters you have described, there was the "Ticker" and various other tone wheels, as well as a motor-driven variable capacitor which rapidly brought the tuned circuit in and out of resonance.
After the Hetrodyne detector was invented these mechanical CW detectors rapidly went out of use.
Decoding a SSB signal is exactly as you describe. The signal is mixed with a "period correct" local oscillator (eg BFO) so as to generate mixing products in the audio range. Simply placing a transistor radio near your Xtal set will do the job if its Local Oscillator (or a harmonic) will tune across the required frequency.
Interesting stuff indeed.
|gzimmer||Posted - 19 September 2012 13:26 |
After thinking about it for a while, I think my initial explanation above is a bit weak.
Certainly if you Amplitude Modulate the incoming signal (via a spinning capacitor, etc), you could then use a diode to detect the resultant AM signal.
But most Ticker circuits I have seen don't use a diode: The ticker somehow manages to directly detect the incoming RF.
I am guessing that by randomly chopping the RF (abrupt on/off), the Ticker is creating wide-band modulation products (sidebands), some of which extend down into the audio range, and it is these products which can be heard in the headphones.
Edited by - gzimmer on 9/20/2012 5:30:19 AM
|gzimmer||Posted - 20 September 2012 3:7 |
Phll Anderson has an article titled "The Ticker Revisited" at:
a google search on "Poulsen Tikker" or "Poulsen Ticker" turns up many other references, but I haven't yet seen a good explanation of how they actually work.
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