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RE:Receiver impedance mismatch





One example is a receiver input that presents a high input impedance,
yet requires a low source impedance for best noise matching.

QEX July August 2002 has a worked design using the obsolete MC13055
WBFM receiver chip.

The filter is lightly loaded by the high impedance, so the response is
typically narrower (higher Q) than one might expect from a matched system.
Such light loading may result in a double humped response if you are using
a two resonator system.  Two resonator systems are popular in ham literature
because they offer much better performance than single resonators, yet can 
be tuned easily by skilled hams. 

In real life, the actual impedance is altered by the length of cable between
the transmission line between the filter and the receiver--a quarter wavelength
cable transforms that high Z to a low Z.

There are techniques for obtaining a receiver that has both a good noise figure
and a well matched input receiver.  One is to use hybrid combiners and a pair of
amplifiers--I have a 6GHz preamp that uses this technique. You are probably looking
at a 3x increase in cost versus a normal preamp. It is housed in an aluminum block 
with waveguide input/output that must have cost a small fortune to machine.  

Zack Lau W1VT

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