BTW, Bill, amplitude modulation and "mixing" are the same process mathematically. In a "mixer" we just select one of the "sidebands" -depending on whether we are doing "up-conversion" or "down-conversion" mixing. So, in an optical fiber, passing 2 MHz 160m RF, there would be two sidebands at 2 MHz above and below the optical carrier frequency. The photo-diode would see the optical carrier and the +/- 2 MHz sidebands as an amptltude modulate optical carrier and we would select the lower sideband to recover the 2 MHz "baseband" signal.
73, Charlie, K4OTV 73. Charlie Cunningham -----Original Message----- From: Topband [mailto:[email protected]] On Behalf Of Bill Wichers Sent: Thursday, October 24, 2013 1:39 PM To: Tom W8JI; Shoppa, Tim; Bruce; [email protected] Subject: Re: Topband: Light fiber question I see your point, since the signal is undergoing a conversion. My thinking was modulator = baseband to some kind of RF or digital signal, i.e. something very different from the original signal in terms of content of the waveform. I wasn't thinking of using a band-limited section of spectrum being converted to an amplitude-modulated light source as a "modulator" in this case. What I had meant was that the electrical->optical conversion doesn't have to be a particularly fancy system when you're only trying to run about 200kHz of spectrum over the fiber in the 2(ish)MHz range. The basics I mentioned before and some op amps are all that are needed. The op amps will likely be the limiting factor for dynamic range. I do agree with you that a simple run of decent coax is likely to be the better option though. -Bill > Sorry, but by definition that is a modulator and demodulator. > > You have to convert the radio signal to light, and that involves modulating > a light beam of some type. At the other end, you have to demodulate the > light into the original baseband. > > You may not recognize it as a modulator/demodulator system, but that's > what > it is. You cannot just hook the coax to the fiber optic cable at each end. > > 73 Tom _________________ Topband Reflector _________________ Topband Reflector
