On Fri, 11 Jan 2008 17:16:22 CET, shristov@ptt.yu wrote:
>Receiver's input impedance is irrelevant for sensitivity (and noise figure)
>measurements, which are defined in terms of the "available power" from the
>generator. The generator output impedance should be close to 50 Ohms.
>Receiver input impedance can be very different from 50 Ohms.
No, the input impedance is NOT irrelevant -- it loads the generator and
reduces the voltage at the receiver input. The difference between a 50 ohm
load and no load is 6 dB. A lower impedance than 50 ohms would cause an error
in the opposite direction.
On the other hand, it is not uncommon for the input impedance of a receiver
to be significantly different from the "assumed" or "nominal" 50 ohms. To
measure this, you would need to measure the voltage of the generator using a
bridging (high Z) RF voltmeter or scope with and without the receiver
connected, and do so in a manner (and at frequencies) where transmission line
effects were not contributing errors.
73,
Jim Brown K9YC
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