Update and comments on the DC-DC converter for PLO bricks
        
                N1BWT Nov. 1996
                
        All but one of the PLO bricks that I've ever seen used a 
        negative supply voltage, typically -19 or -20 volts. Since
        most of us prefer to run everything from +12 volts for 
        battery operation, a DC to DC converter is required.
        
        While it is possible to build a circuit to do this, and
        several have been published, DC to DC converter modules
        frequently appear as surplus.  One common variety provides 
        + and - 12 volts out with a nominal 12 volt input, at output
        currents of 0.5 amps or more.  Floating the centertap provides
        24 volts, so it is a simple matter of regulating the output
        voltage down to the required level with a 3-terminal regulator.
                
        Additional 3-terminal regulators can provide a negative 
        voltage for GaAsFET bias, and for switching 28 volt coax
        relays.  I connect the relay between a -15 volt output and
        the +12 volt battery to get roughly 27 volts on the relay.

        The other problem is eliminating the high-frequency noise
        generated by switching inside the module, with switching 
        frequencies in the 30 to 100 KHz range.  I do this as shown
        in the schematic, with common-mode filtering on both sides
        of the module so that noise cannot sneak down the ground 
        path.  I've seen as much as 5 volts of switching noise on 
        the +12 volt input side, so filtering both sides is important.
        The whole circuit, including the 3-terminal regulators,
        is packaged inside a metal box with RFI filters on all leads.
        
        If not filtered, the switching noise will modulate the PLO
        brick output.  One local ham who doesn't believe this has
        S9 birdies 30 KHz on each side of his 10 GHz signal.  I 
        don't know how much of his transmit power is wasted, but
        I've called him on the birdie frequency on more than one
        occasion.
        
        CAUTION:  Some DC-DC converter modules do not have all leads
        isolated from the case.  One model has the output centertap 
        connected to the case, and gets quite hot if the case is
        in contact with ground, shorting out one side of the output.