[Elecraft] Keying Waveform Measurement

[G3VVT at aol.com]

In a message dated 09/02/05 20:33:27 GMT Standard Time,  
kercel1 at suscom-maine.net writes:

As for  the matter of running a 50 Ohm source into a high Z scope input, Tek 
has a  slick solution. They use these 50-Ohm 2-Watt terminators that you 
apply  right at the BNC connector at the scope input


I picked up some of those with an ex military scope probe kit.  Previously 
used a BNC T adapter with a 75 ohm termination on the spare port  for CATV 
testing with a scope where a terminated line was required. A 50 ohm  termination 
could be used in the same way if only one has the facility to  dissipate the 10 
or 100W as needed.
 
What I did for this purpose for 2 way VHF radio testing in earlier  times was 
to borrow an idea from the CATV industry where close into the line  
amplifier, the tap off unit from the coax feeder line (T unit) is constructed  from a 
resistive divider to provide the necessary attenuation. These can contain  as 
little as a single resistor or can be of the terminated type. For my  purpose 
an attenuator was made up in a small in line filter box  with a straight 
through connection which goes from the TX to the dummy  load. Off this a resistor or 
a string of resistors is arranged from the center  conductor to a socket for 
feeding into the test equipment. The test equipment  socket was also 
terminated to achieve optimum attenuation. Once the voltage  level has been reduced to 
reasonable levels cheaper low power coaxial  attenuators can be used if 
needed. The resistors have to be carbon or other  non inductive types and the divide 
ratio/resistor wattage can easily be  work out by math. Made this up many 
years ago and still have the unit  today.
 
It is amazing what one can do with bits of junk when you have hardly two  
cents to rub together!
 
The old UK made, KW Electronics PEP meter which can measure up to 300W  
worked in a similar fashion I found years later was using a resistive  divider to 
bring down the voltage levels from the through RF to a level that can  be used 
by the metering circuit.
 
Bob, G3VVT