brian@sdcsvax.UCSD.EDU (Brian Kantor) (08/02/87)
In the middle 70's, the FM transmitter at the college radio station where I did some of my time was a wonderful old tube widget. It was a very reliable device, except for the time the fan motor unbolted itself and fell into the final cage, causing much arcing and breakage of glass. I think it was made by G.E. It featured a VERY unusual FM modulator using what was termed a "Phantastron" tube (or maybe it was the circuit). Unfortunately, the section of the manual that explained how it worked was klepto'd by somebody before I got there, so I never really understood how the thing worked. It was explained to me that the phantastron tube was basically set up so that an electron beam travelled from cathode to anode as in most tubes, but that there was a large magnet clamped around the tube to force the beam into a helical path to make it longer. (Yup, you could see the magnets or something clamped around the tube!) Apparently there were a set of electrodes that stretched or compressed the length of the helix, thus changing the time it took for the electron beam to traverse the tube, thus phase modulating the signal. I believed it at the time. I think I still do. Is that really how it worked? Brian Kantor UC San Diego "You're listening to Jazz-88, KSDS in San Diego... 250 watts of power, with an effective radiated power of 185 watts. Someday we're gonna get a new antenna, folks...."
haynes@ucscc.UCSC.EDU.ucsc.edu (99700000) (08/03/87)
Believe you're talking about the phasitron(?) which was made by Sylvania. Phantastron is a timing circuit that uses the constant-current property of a pentode to charge a capacitor linearly - hence a linear rather than exponential sawtooth generator. haynes@ucscc.ucsc.edu haynes@ucscc.bitnet ..ucbvax!ucscc!haynes
larry@kitty.UUCP (Larry Lippman) (08/05/87)
In article <580@saturn.ucsc.edu>, haynes@ucscc.UCSC.EDU.ucsc.edu (99700000) writes: > Phantastron is a timing circuit that uses the constant-current property of > a pentode to charge a capacitor linearly - hence a linear rather than > exponential sawtooth generator. Ah yes, the phantastron circuit. Believe it or not, the name originated from some British engineering group who thought the circuit operation was "fantastic". While many phantastron circuits used the 6SA7 pentode, the circuit could be implemented with any pentode which had a sharp suppressor grid. The phantastron was a circuit, and not a particular tube. For those of you who remember their ubiquitous "Terman", the phantastron was a variation of the Miller sweep circuit. The phantastron should not be confused with the "phanotron", which was a cold-cathode diode rectifier tube. Anyone remember the "aerotron"? :-) <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|ames|boulder|decvax|rocksanne|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {hplabs|ihnp4|mtune|seismo|utzoo}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"
peter@imtec.co.uk (Peter Rushworth) (08/10/87)
In article <3574@sdcsvax.UCSD.EDU> brian@sdcsvax.UCSD.EDU (Brian Kantor) writes: > >It was explained to me that the phantastron tube was basically set up >so that an electron beam travelled from cathode to anode as in most >tubes, but that there was a large magnet clamped around the tube to >force the beam into a helical path to make it longer. Apparently >there were a set of electrodes that stretched or compressed the length >of the helix, thus changing the time it took for the electron beam to >traverse the tube, thus phase modulating the signal. > What you describe sounds similar to what I think is called the "serrodyne" effect (spelling from memory). I once worked on a system which used a travelling wave tube to modulate a (GHz) signal like this. I don't recall magnets, but there was a sawtooth input (<10Khz) to the helix, as you increased the amplitude, the signal freqency reduced. ( My instructor gave strict instructions that the helix was never to be disconnected or the TWT would blow. I subsequently found out that the TWiT's cost over 1K pounds. :-) )
larry@kitty.UUCP (Larry Lippman) (08/11/87)
In article <600@imtec.co.uk>, peter@imtec.co.uk (Peter Rushworth) writes: I think we were discussing means of FM modulation... > >It was explained to me that the phantastron tube was basically set up > >so that an electron beam travelled from cathode to anode as in most > >tubes, but that there was a large magnet clamped around the tube to > >force the beam into a helical path to make it longer. Apparently > >there were a set of electrodes that stretched or compressed the length > >of the helix, thus changing the time it took for the electron beam to > >traverse the tube, thus phase modulating the signal. > > > What you describe sounds similar to what I think is called the "serrodyne" > effect (spelling from memory). I once worked on a system which used > a travelling wave tube to modulate a (GHz) signal like this. I don't recall > magnets, but there was a sawtooth input (<10Khz) to the helix, as you > increased the amplitude, the signal freqency reduced. You may be referring to a "serrasoid" modulator, which is a circuit and not a specific device. In this circuit, a reference frequency in a sawtooth waveform is fed to one input of a comparator; the modulating voltage is fed to a voltage-to-quadratic function converter whose output is fed to the other input of the comparator. The output of the comparator goes through a filter, and a frequency multiplier if necessary. The circuit works because the output of the comparator generates short pulses whenever the quadratic modulating function crosses the reference sawtooth with a positive slope. The spacing of these pulses varies with time, and when _filtered_ form a linear FM waveform. The degree of frequency variation is limited with this basic circuit, which is why a frequency multiplier is often employed after the filter to achieve the desired FM deviation. <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {hplabs|ihnp4|mtune|seismo|utzoo}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"