[Heathkit] IG-4505

[k2cby]

The RG-58 50-Ohm coax leading from the IG-4505 is a transmission line. You
want to terminate any transmission line in its characteristic impedance. As
an oscilloscope calibrator, you will be connecting the IG-4505 to the input
of the scope's vertical amplifier, which has a characteristic impedance of
(typically) 10 Meg. with a parallel capacitance of 10 pF or so. If you don't
include a 51 (or so) Ohm shunt resistor at the far end of the coax, the
transmission line is going to see what amounts to an open circuit. This
means that the voltage at the end of the line is going to depend on the
frequency and the length of the line. This is the last thing you want in a
scope calibrator! If, on the other hand, the transmission line is terminated
in its characteristic impedance, the length of the line becomes irrelevant,
and the voltage at the end of the line will equal the source voltage,
regardless of frequency (and regardless of the length of the coax). The only
reason to leave the coax unterminated would be if you were to connect it to
a low-impedance load - ideally 50 Ohms. If, for example, the load impedance
were 200 Ohms, you would want to shunt it with 66.6 Ohms at the load end so
the coax transmission line would "see" a 50-Ohm load. 

 

Miles B. Anderson, K2CBY
16 Round Pond Lane
Sag Harbor, NY 11963
Tel.: (631) 725-4400

Fax.: (631) 725-2223
e-mail: k2cby at optonline.net