[ARC5] Pre-WWII product detector
Bill Fuqua
wlfuqu00 at uky.edu
Sat Jul 7 22:40:32 EDT 2012
Somewhere I heard a statement that the first product detectors were not
developed until after WWII.
However, there was an interesting one used long before WWII. It was the
Marconi Balanced Crystal
Detector. It used two biased detectors connected in such a way that their
detected signals would
cancel out. However, there is another RF tank circuit driven by a buzzer
that provides the BFO signal.
The following has a schematic of the detector.
http://californiahistoricalradio.com/CHRSPix/10LeeDunwoodyCoal2009.pdf
Here is a description of it, however in this drawing one diode is reversed.
<http://books.google.com/books?id=Myt_AAAAMAAJ&pg=PA238&lpg=PA238&dq=marconi+balanced+detector+Undamped&source=bl&ots=xKFIq6YSc4&sig=AgyS65nR2cXSHkK3902IzyszmZU&hl=en&sa=X&ei=2-_4T4DkAYG29QT3goWJBw&ved=0CE4Q6AEwAA#v=onepage&q=marconi%20balanced%20detector%20Undamped&f=false>
This is indeed a product detector an behaves as one. With out the BFO
signals cancel out but adding the buzzer and the tank circuit the diodes
are driven out of balance by a damped wave and beat with the incoming
signal. This also increases the apparent gain, or reduced the loss, of the
detector
by around 2 orders of magnitudes.
73
Bill wa4lav
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