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22054Parametric amplifiers(was Re: Checkerboard Billboard)

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  • OZOB99
    Jan 1, 2012
    • 0 Attachment
      ok thanks; it's probably what I read years ago but each sentence/statement was followed by 2-5 pages of supporting math so could'nt connect the dots.

      --- In coldwarcomms@yahoogroups.com, "widebandit" <widebandit@...> wrote:
      >
      > --- In coldwarcomms@yahoogroups.com, "OZOB99" <ozob99@> wrote:
      > >
      > > Besides dual/quad diversity it seems tropo(& earth stations) always used parametric amps in the front end for lower noise figure, but I never fully understood this circuit from technical descriptions(too much math); something about non linear or variable diodes kicking in. Can someone give us a simplified explanation on how they operated?
      > >
      >
      > Ok... I'll try to do this without going off the deep-end...
      > No guarantees...
      >
      > A parametric amplifier is actually a specialized form of a lower-sideband up-converter that uses a variable-capacitance varactor diode as the non-linear device...
      >
      > Circuit components are as follows:
      > 1. Input port
      > 2. 3-port Circulator
      > 3. Varactor modulator
      > 4. Tuned reflection circuit
      > 5. Pump oscillator (beat-oscillator, or BFO for you hams)
      > 6. Output port
      >
      > The signal S to be amplified enters circulator input port one and exits port two which is connected to the varactor up-converter circuit...
      >
      > At the varactor, signal S mixes with the much higher frequency pump oscillator signal P producing a lower-sideband or difference frequency P-S, which is called the idler signal because it remains within the circuit...
      >
      > The variable capacitance of the varactor produces an energy transfer from the pump signal P into the idler-signal P-S, which becomes an amplified image of the original input signal S...
      >
      > The lower sideband is reflected back into the varactor modulator by the tuned reflection circuit where it is down-converted back to the original signal frequency S but at a much higher power...
      >
      > The down-converted, amplified signal S then passes back to circulator port two, is transfered to output port three, and an amplified version of the original input signal S emerges...
      >
      > Parametric amplifiers are low noise devices because the varactor diode is never forward biased and only minority-carrier electrons cross the diode junction. Since thermal noise in active devices derives from the random movement of electrons (sometimes referred to as shot-noise), the fewer electrons moving through the circuit the lower the noise figure...
      >
      > Varactor diodes tend to self-resonate around 8-GHz, so the pump signal is in the 20 GHz range to prevent exciting the varactor into oscillation. This may be achieved by means of an 11-GHz klystron and a frequency doubler. The klystron circuit must be very stable in both amplitude and frequency to minimize distortion of the output signal...
      >
      > For satellite research in the sixties, to further reduce noise figure, BTL developed a parametric amplifier that operated within a 2-1/2 gallon liquid nitrogen Dewar flask. It was an expensive device but still much cheaper than the alternative liquid hydrogen MASER...
      >
      > BTL also considered incorporating a parametric amp into the TD-3 radio receiver but then discovered that it would not handle up-fades very well. Fortunately a low-noise Schottky-Barrier diode receiver modulator was developed in the nick of time - which was then promptly back-fitted into TD-2 front-ends...
      >
      > Neat... Huh?...
      >
      > - waw -
      >
      > >
      > > --- In coldwarcomms@yahoogroups.com, Sheldon Daitch <sheldondaitch@> wrote:
      > > >
      > > > David,
      > > >  
      > > > Way back, right after the Army discovered tropo systems, a fair
      > > > number were installed in Vietnam for the older Back Porch tactical
      > > > tropo systems and the ICS/IWCS fixed plant systems and almost
      > > > all of them were quad diversity.
      > > >  
      > > > I don't have acces to my ICS/IWCS manuals, to back up my memory,
      > > > so if I have it wrong, team, feel free to correct me.
      > > >  
      > > > Typically quad diversity did use two TX frequencies, and
      > > > generally two antennas at each end.
      > > >  
      > > >  
      > > > TX F1 is on antenna 1, TX F2 is on antenna 2 at the transmit end.
      > > >  
      > > > At the other end of the circuit, Antenna 1 feeds a receiver on F1 and
      > > > a receiver on F2.  Antenna 2 also has a receiver on F1 and a receiver
      > > > on F2. 
      > > >  
      > > > Since we have both frequency diversity and two sets of space diversity,
      > > > the chances of all path combinations fading simultaneously are much
      > > > lower.
      > > >  
      > > > Bear in mind a full duplex circuit running quad diversity would require
      > > > four frequencies. 
      > > >  
      > > > IIRC, all of the tropo shots in Vietnam running 120 foot billboards had
      > > > the feedhorn at the focal point mounted on a separate tower roughly
      > > > 60 feet tall.
      > > >  
      > > > The 60 foot billboards were mostly offset feeds, with the feed horn mounted
      > > > at ground level.   I don't remember any 60 foot antenna systems which were
      > > > a full parabolic antenna.
      > > >  
      > > > The shots running 30 foot or smaller antennas were simple
      > > > parabolic antennas back mounted, and the feedhorn mounted on a frame
      > > > attached to the parabolic.
      > > >
      > > > The photos at:
      > > >
      > > > http://phulam.com/photos.htm
      > > >
      > > > are typical of the tropo era in Vietnam.
      > > >
      > > > 73
      > > > Sheldon
      > > >
      > > > On 12/31/11 12:37 AM, widebandit wrote:
      > > >
      > > > > The system ran quad diversity - two transmitters and four receivers at each end.
      > > >
      > > > I know how RX diversity works; but how does TX diversity work? ISTM the
      > > > transmitters must be on separate freq. or they will hetrodyne -
      > > > constructively or destructively. Even if there were a splitter in the
      > > > waveguide so both were fed from one oscillator/transmitter; I'd think the
      > > > same was true.
      > > >
      >
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